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Global Warming: A Very Short Introduction

Very Short Introductions are for anyone wanting a stimulating
and accessible way in to a new subject. They are written by experts, and have
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Mark Maslin

A Very Short Introduction



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Acknowledgements ix
Abbreviations xi
List of illustrations




What is global warming? 4
A brief history of the global warming hypothesis
Your viewpoint determines the future

What is the evidence for climate change? 43
How do you model the future?


What are the possible future impacts of global
warming? 83



Surprises 102
Politics 118
What are the alternatives? 134


Further reading 151



This page intentionally left blank


The author would like to thank the following people: Johanna and
Alexandra Maslin for being there; Emma Simmons and Marsha Filion
for their excellent editing and skill of finally extracting the book
from me; Catherine D’Alton and Elanor McBay of the Department of
Geography Drawing Office UCL; John Adams for helping me develop
my critical view of this debate; Richard Betts and Eric Wolff for their
insightful and extremely helpful reviews; and all my colleagues in
climatology, palaeoclimatology, social science, and economics who
continue to strive to understand and predict our influence on climate.

This page intentionally left blank



Antarctic Bottom Water
Arctic Oscillation
Atmosphere–Ocean General Circulation Models
Alliance of Small Island States
Business and Industry Non-Governmental Organization
Conference of the Parties
Environmental Non-Governmental Organization
~o-Southern Oscillation
El Nin
general circulation model
galactic cosmic ray
Global Historical Climate Network
Intergovernmental Panel on Climate Change
Japan, USA, Switzerland, Canada, Australia,
Norway and New Zealand
marine air temperature
North Atlantic deep water
North Atlantic Oscillation
Non-Governmental Organization
National Research Council
Organization for Economic Cooperation and Development
Organization of Petroleum Exporting Countries
parts per billion by volume
parts per million by volume


sea-surface temperature
Thermohaline Circulation
United Nations Conference on Trade and
United Nations Framework Convention on Climate
vector-borne disease

List of illustrations


The earth’s annual
global mean energy

6 Variation of the earth’s
surface temperature 30

2 Greenhouse gases and
temperature for the last
four glacial cycles
recorded in the Vostok ice
3 Indicators of the human
influence on the
atmosphere composition
during the industrial
4a CO2 emissions from
industrial processes


4b CO2 emissions from
land-use change


7 Global warming
and the media


8 The four myths of


9 Four myths of human

Four rationalities



Combined global
warming scenarios
with myths of human


The anatomy of past
climatic changes



5 Possible climate system
responses to a


Northern Hemisphere
reconstruction for the
last thousand years







Global distribution
of meteorological


Changes in precipitation
over land

24 Global, annual-mean
radiative forcings

Estimated sea-level
rise 1910–1990


Mozambique floods
of 2000


Ice core records showing
CO 2 in phase with
Antarctic warming
Simulated annual
global mean surface


Locations at which
systematic long-term
studies meet stringent
criteria documenting
recent climate change
impacts on physical
and biological systems 66

22 The development of
climate models, past,
present, and future

The global climate
of the 21st century



26 Flooding of Bangladesh
in 1998

~o – Southern
El Nin


28 The deep circulation
of the ocean

20 Schematic of observed
variations of the
a) temperature indicators
and b) hydrological and

23 A simplified version of
the present carbon


29 Different possible
circulation of the
deep ocean


30 Future sea level



Bifurcation of the
climate system


32 Met office model of CO2
concentration and mean
temperature over time 115
33 Five different cost


34 Climate change risks
with increasing global


Global warming is one of the most controversial science issues of
the 21st century, challenging the very structure of our global society.
The problem is that global warming is not just a scientific concern,
but encompasses economics, sociology, geopolitics, local politics,
and individuals’ choice of lifestyle. Global warming is caused by the
massive increase of greenhouse gases, such as carbon dioxide, in
the atmosphere, resulting from the burning of fossil fuels and
deforestation. There is clear evidence that we have already elevated
concentrations of atmospheric carbon dioxide to their highest level
for the last half million years and maybe even longer. Scientists
believe that this is causing the Earth to warm faster than at any
other time during, at the very least, the past one thousand years.
The most recent report by the Intergovernmental Panel on Climate
Change (IPCC), amounting to 2,600 pages of detailed review and
analysis of published research, declares that the scientific
uncertainties of global warming are essentially resolved. This report
states that there is clear evidence for a 0.6°C rise in global
temperatures and 20 cm rise in sea level during the 20th century.
The IPCC synthesis also predicts that global temperatures could
rise by between 1.4°C and 5.8°C and sea level could rise by between
20 cm and 88 cm by the year 2100. In addition, weather patterns
will become less predictable and the occurrence of extreme climate
events, such as storms, floods, and droughts, will increase.
This book tries to unpick the controversies that surround the global
warming hypothesis and hopefully provides an incentive to read

Global Warming

more on the subject. It starts with an explanation of global warming
and climate change and is followed by a review of how the global
warming hypothesis was developed. The book will also investigate
why people have such extreme views about global warming, views
which reflect both how people view nature and their own political
The second half of the book examines the evidence showing that
global warming has already occurred and the science of predicting
climate change in the future. The potentially devastating effects of
global warming on human society are examined, including drastic
changes in health, agriculture, the economy, water resources,
coastal regions, storms and other extreme climate events, and
biodiversity. For each of these areas scientists and social scientists
have made estimates of the potential direct impacts; for example, it
is predicted that by 2025 five billion people will experience water
stress. The most important impacts are discussed in this book,
along with plans to mitigate the worst of them.
There are also potential surprises that the global climate system
might have in store for us, exacerbating future climate change.
These include the very real possibility that global deep-ocean
circulation could alter, plunging Europe into a succession of
extremely cold winters or causing unprecedented global rise in sea
level. There are predictions that global warming may cause vast
areas of the Amazon rainforest to burn, adding extra carbon to
the atmosphere and thus accelerating global warming. Finally,
there is a deadly threat lurking underneath the oceans: huge
reserves of methane which could be released if the oceans warm up
sufficiently – again accelerating global warming. The final chapters
look at global politics and potential adaptations to global warming.
It should be realized that the cost of significantly cutting fossil-fuel
emissions may be too expensive in the short term and hence the
global economy will have to become more flexible and thus adapt to
climate change. We will also have to prioritize which parts of our
global environment to protect. The theory of global warming thus

challenges our current concepts of the nation-state versus global
responsibility, and the short-term vision of our political leaders,
both of which must be overcome if global warming is to be
dealt with effectively. Be under no illusion: if global warming
is not taken seriously, it will be the poorest people in our global
community, as usual, that suffer most.



Chapter 1
What is global warming?

The Earth’s natural greenhouse
The temperature of the Earth is controlled by the balance between
the input from energy of the sun and the loss of this back into space.
Certain atmospheric gases are critical to this temperature balance
and are known as greenhouse gases. The energy received from
the sun is in the form of short-wave radiation, i.e. in the visible
spectrum and ultraviolet radiation. On average, about one-third of
this solar radiation that hits the Earth is reflected back to space.
Of the remainder, some is absorbed by the atmosphere, but most
is absorbed by the land and oceans. The Earth’s surface becomes
warm and as a result emits long-wave ‘infrared’ radiation. The
greenhouse gases trap and re-emit some of this long-wave
radiation, and warm the atmosphere. Naturally occurring
greenhouse gases include water vapour, carbon dioxide, ozone,
methane, and nitrous oxide, and together they create a natural
greenhouse or blanket effect, warming the Earth by 35°C. Despite
the greenhouse gases often being depicted in diagrams as one layer,
this is only to demonstrate their ‘blanket effect’, as they are in fact
mixed throughout the atmosphere (see Figure 1).
Another way to understand the Earth’s natural ‘greenhouse’ is by
comparing it to its two nearest neighbours. A planet’s climate is
decided by several factors: its mass, its distance from the sun, and of
course the composition of its atmosphere and in particular the

1. The Earth’s annual global mean energy balance

Global Warming

amount of greenhouse gases. For example, the planet Mars is very
small, and therefore its gravity is too small to retain a dense
atmosphere; its atmosphere is about a hundred times thinner
than Earth’s and consists mainly of carbon dioxide. Mars’s average
surface temperature is about −50°C, so what little carbon dioxide
exists is frozen in the ground. In comparison, Venus has almost the
same mass as the Earth but a much denser atmosphere, which is
composed of 96% carbon dioxide. This high percentage of carbon
dioxide produces intense global warming and so Venus has a
surface temperature of over + 460°C.
The Earth’s atmosphere is composed of 78% nitrogen, 21% oxygen,
and 1% other gases. It is these other gases that we are interested
in, as they include the so-called greenhouse gases. The two most
important greenhouse gases are carbon dioxide and water vapour.
Currently, carbon dioxide accounts for just 0.03–0.04% of the
atmosphere, while water vapour varies from 0 to 2%. Without the
natural greenhouse effect that these two gases produce, the Earth’s
average temperature would be roughly −20°C. The comparison
with the climates on Mars and Venus is very stark because of the
different thicknesses of their atmospheres and the relative amounts
of greenhouse gases. However, because the amount of carbon
dioxide and water vapour can vary on Earth, we know that this
natural greenhouse effect has produced a climate system which is
naturally unstable and rather unpredictable in comparison to those
of Mars and Venus.

Past climate and the role of carbon dioxide
One of the ways in which we know that atmospheric carbon dioxide
is important in controlling global climate is through the study of
our past climate. Over the last two and half million years the Earth’s
climate has cycled between the great ice ages, with ice sheets over 3
km thick over North America and Europe, to conditions that were
even milder than they are today. These changes are extremely rapid
if compared to other geological variations, such as the movement of

2. Greenhouse gases and temperature for the last four glacial cycles
recorded in the Vostok ice core

What is global warming?

continents around the globe, where we are looking at a time period
of millions of years. But how do we know about these massive ice
ages and the role of carbon dioxide? The evidence mainly comes
from ice cores drilled in both Antarctica and Greenland. As snow
falls, it is light and fluffy and contains a lot of air. When this is
slowly compacted to form ice, some of this air is trapped. By
extracting these air bubbles trapped in the ancient ice, scientists can
measure the percentage of greenhouse gases that were present in
the past atmosphere. Scientists have drilled over two miles down
into both the Greenland and Antarctic ice sheets, which has enabled
them to reconstruct the amount of greenhouse gases that occurred
in the atmosphere over the last half a million years. By examining
the oxygen and hydrogen isotopes in the ice core, it is possible to
estimate the temperature at which the ice was formed. The results
are striking, as greenhouse gases such as atmospheric carbon
dioxide (CO2) and methane (CH4) co-vary with temperatures over
the last 400,000 years (see Figure 2). This strongly supports the

idea that the carbon dioxide content in the atmosphere and global
temperature are closely linked, i.e. when CO2 and CH4 increase, the
temperature is found to increase and vice versa. This is our greatest
concern for future climate: if levels of greenhouse gases continue to
rise, so will the temperature of our atmosphere. The study of past
climate, as we will see throughout this book, provides many clues
about what could happen in the future. One of the most worrying
results from the study of ice cores, and lake and deep-sea sediments,
is that past climate has varied regionally by at least 5°C in a few
decades, suggesting that climate follows a non-linear path. Hence
we should expect sudden and dramatic surprises when greenhouse
gas levels reach an as yet unknown trigger point in the future.

Global Warming

The rise in atmospheric carbon dioxide during the
industrial period
One of the few areas of the global warming debate which seems to
be universally accepted is that there is clear proof that levels of
atmospheric carbon dioxide have been rising ever since the
beginning of the industrial revolution. The first measurements of
CO2 concentrations in the atmosphere started in 1958 at an altitude
of about 4,000 metres on the summit of Mauna Loa mountain in
Hawaii. The measurements were made here to be remote from
local sources of pollution. What they have clearly shown is that
atmospheric concentrations of CO2 have increased every single year
since 1958. The mean concentration of approximately 316 parts
per million by volume (ppmv) in 1958 rose to approximately 369
ppmv in 1998 (see Figure 3). The annual variations in the Mauna
Loa observatory are mostly due to CO2 uptake by growing plants.
The uptake is highest in the northern hemisphere springtime;
hence every spring there is a drop in atmospheric carbon dioxide
which unfortunately does nothing to the overall trend towards ever
higher values.
This carbon dioxide data from the Mauna Loa observatory can be
combined with the detailed work on ice cores to produce a complete

3. Indicators of the human influence on the atmosphere composition
during the industrial era

Global Warming

record of atmospheric carbon dioxide since the beginning of the
industrial revolution. What this shows is that atmospheric CO2
has increased from a pre-industrial concentration of about 280
ppmv to over 370 ppmv at present, which is an increase of 160
billion tonnes, representing an overall 30% increase. To put this
increase into context, we can look at the changes between
the last ice age, when temperatures were much lower, and the
pre-industrial period. According to evidence from ice cores,
atmospheric CO2 levels during the ice age were about 200 ppmv
compared to pre-industrial levels of 280 ppmv – an increase of
over 160 billion tonnes – almost the same CO2 pollution that we
have put into the atmosphere over the last 100 years. This carbon
dioxide increase was accompanied by a global warming of 6°C as
the world freed itself from the grips of the last ice age. Though
the ultimate cause of the end of the last ice age was changes in
the Earth’s orbit around the sun, scientists studying past climates
have realized the central role atmospheric carbon dioxide has
as a climate feedback translating these external variations into
the waxing and waning of ice ages. It demonstrates that the
level of pollution that we have already caused in one century
is comparable to the natural variations which took thousands
of years.

The enhanced greenhouse effect
The debate surrounding the global warming hypothesis is whether
the additional greenhouse gases being added to the atmosphere will
enhance the natural greenhouse effect. Global warming sceptics
argue that though levels of carbon dioxide in the atmosphere are
rising, this will not cause global warming, as either the effects are
too small or there are other natural feedbacks which will counter
major warming. Even if one takes the view of the majority of
scientists and accepts that burning fossil fuels will cause warming,
there is a different debate over exactly how much temperatures will
increase. Then there is the discussion about whether global climate
will respond in a linear manner to the extra greenhouse gases or

whether there is a climate threshold waiting for us. These issues are
tackled later in the book.

Who produces the pollution?

The second major source of carbon dioxide emissions is as a result
of land-use changes. These emissions come primarily from the
cutting down of forests for the purposes of agriculture,
urbanization, or roads. When large areas of rainforests are cut
down, the land often turns into less productive grasslands with
considerably less capacity for storing CO2. Here the pattern of
carbon dioxide emissions is different, with South America, Asia,
and Africa being responsible for over 90% of present-day land-use
change emissions, about 4 billion tonnes of carbon per year (see
Figure 4b). This, though, should be viewed against the historical
fact that North America and Europe had already changed their own
landscape by the beginning of the 20th century. In terms of the
amount of carbon dioxide released, industrial processes still
significantly outweigh land-use changes.

What is global warming?

The United Nations Framework Convention on Climate Change
was created to produce the first international agreement on
reducing global greenhouse gas emissions. However, this task is not
as simple as it first appears, as carbon dioxide emissions are not
evenly produced by countries. The first major source of carbon
dioxide is the burning of fossil fuels, since a significant part of
carbon dioxide emissions comes from energy production, industrial
processes, and transport. These are not evenly distributed around
the world because of the unequal distribution of industry; hence,
any agreement would affect certain countries’ economies more than
others. Consequently, at the moment, the industrialized countries
must bear the main responsibility for reducing emissions of carbon
dioxide to about 22 billion tonnes of carbon per year (see Figure
4a). North America, Europe, and Asia emit over 90% of the global
industrially produced carbon dioxide. Moreover, historically they
have emitted much more than less-developed countries.

4a. CO2 emissions from industrial processes

4b. CO2 emissions from land-use change

What is the IPCC?
The Intergovernmental Panel on Climate Change (IPCC) was
established in 1988 jointly by the United Nations Environmental
Panel and World Meteorological Organization because of worries
about the possibility of global warming. The purpose of the IPCC is
the continued assessment of the state of knowledge on the various
aspects of climate change, including scientific, environmental, and
socio-economic impacts and response strategies. The IPCC is

What is global warming?

So who are the bad guys in causing this increase in atmospheric
carbon dioxide? Of course, it is the developed countries who
historically have emitted most of the anthropogenic (man-made)
greenhouse gases, as they have been emitting since the start of the
industrial revolution in the latter half of the 1700s. Moreover, a
mature industrialized economy is energy-hungry and burns vast
quantities of fossil fuels. A major issue in the continuing debate is
the sharing of responsibility. Non-industrialized countries are
striving to increase their population’s standard of living, thereby
also increasing their emissions of greenhouse gases, since
economic development is closely associated with energy
production. The volume of carbon dioxide thus will probably
increase, despite the efforts to reduce emissions in industrialized
countries. For example, China has the second biggest emissions
of carbon dioxide in the world. However, per capita the Chinese
emissions are ten times lower than those of the USA, who are
top of the list. So this means that in the USA every person is
responsible for producing ten times more carbon dioxide
pollution than in China. So all the draft international agreements
concerning cutting emissions since the Rio Earth Summit in 1992
have for moral reasons not included the developing world, as this
is seen as an unfair brake on its economic development. However,
this is a significant issue because, for example, both China and
India are rapidly industrializing, and with a combined population
of over 2.3 billion people they will produce a huge amount
of pollution.

Global Warming

recognized as the most authoritative scientific and technical voice
on climate change, and its assessments have had a profound
influence on the negotiators of the United Nations Framework
Convention on Climate Change (UNFCCC) and its Kyoto Protocol.
The meetings in The Hague in November 2000 and in Bonn in
July 2001 were the second and third attempts to ratify (i.e. to make
legal) the Protocols laid out in Kyoto in 1998. Unfortunately,
President Bush pulled the USA out of the negotiations in March
2001. However, 186 other countries made history in July 2001 by
agreeing the most far-reaching and comprehensive environmental
treaty the world has ever seen. But the Kyoto Protocol has yet to
be ratified. What is required for this to happen is discussed in
Chapter 8.
The IPCC is organized into three working groups plus a task force
to calculate the amount of greenhouse gases produced by each
country. Each of these four bodies has two co-chairmen (one from a
developed and one from a developing country) and a technical
support unit. Working Group I assesses the scientific aspects of the
climate system and climate change; Working Group II addresses
the vulnerability of human and natural systems to climate change,
the negative and positive consequences of climate change, and
options for adapting to them; and Working Group III assesses
options for limiting greenhouse gas emissions and otherwise
mitigating climate change, as well as economic issues. Hence the
IPCC also provides governments with scientific, technical, and
socio-economic information relevant to evaluating the risks and to
developing a response to global climate change. The latest reports
from these three working groups were published in 2001 and
approximately 400 experts from some 120 countries were directly
involved in drafting, revising, and finalizing the IPCC reports and
another 2,500 experts participated in the review process. The IPCC
authors are always nominated by governments and by international
organizations including Non-Governmental Organizations. These
reports are essential reading for anyone interested in global
warming and are listed in the Further Reading section.

What is climate change?
Many scientists believe that the human-induced or anthropogenicenhanced greenhouse effect will cause climate change in the near
future. Even some of the global warming sceptics argue that though
global warming may be a minor influence, natural climate change
does occur on human timescales and we should be prepared to
adapt to it. But what is climate change and how does it occur?
Climate change can manifest itself in a number of ways, for example
changes in regional and global temperatures, changing rainfall
patterns, expansion and contraction of ice sheets, and sea-level
variations. These regional and global climate changes are responses
to external and/or internal forcing mechanisms. An example of an
internal forcing mechanism is the variations in the carbon dioxide
content of the atmosphere modulating the greenhouse effect, while
a good example of an external forcing mechanism is the long-term
variations in the Earth’s orbits around the sun, which alter the

What is global warming?

The IPCC also compiles research on the main greenhouse gases:
where they come from, and the current consensus concerning their
warming potential (see below). The warming potential is calculated
in comparison with carbon dioxide, which is allocated a warming
potential of one. This way the different greenhouse gases can be
compared with each other relatively instead of in absolute terms.
The Global Warming potential is calculated over a 20- and 100-year
period. This is because different greenhouse gases have different
residence times in the atmosphere because of how long they take to
break down in the atmosphere or be absorbed in the ocean or
terrestrial biosphere. Most other greenhouse gases are more
effective at warming the atmosphere than carbon dioxide but are
still in very low concentrations in the atmosphere. As you can see
from Table 1 there are other greenhouse gases which are much more
dangerous mass for mass than carbon dioxide but these exist in very
low concentrations in the atmosphere, and therefore most of the
debate concerning global warming still centres on the role and
control of atmospheric carbon dioxide.

Table 1: Main greenhouse gases and their comparative ability to warm the atmosphere










278 ppmv


Human source

358 ppmv


(30% increase)








20 years

100 years







Land-use changes
Cement production


700 ppbv

1721 ppbv

Fossil fuels

(240% increase)

Rice paddies
Waste dumps

Nitrous oxide


275 ppbv

311 ppbv


(15% increase)

Industrial processes




0.503 ppbv

Does not exist

Liquid coolants/




naturally and is
human generated



0.105 ppbv

Liquid coolants



0.070 ppbv

Production of





Does not exist
naturally and is
human generated



Does not exist


naturally and is
human generated
Sulphur hexafluoride


Does not exist
naturally and is
human generated

ppmv = part per million by volume
ppbv = parts per billion by volume

0.032 ppbv

Dielectric fluid

5. Possible climate system responses to a linear-forcing

regional distribution of solar radiation to the Earth. This is thought
to cause the waxing and waning of the ice ages. So in terms of
looking for the evidence for global warming and predicting the
future, we need to take account of all the natural external and
internal forcing mechanisms. For example, until recently the
cooling that occurred globally during the 1970s was unexplained
until the ‘external’ and cyclic variations every 11 years in the sun’s
energy output, the so-called sunspot cycle, was taken into

(a) Linear and synchronous response (Figure 5a). In this case the
forcing produces a direct response in the climate system whose
magnitude is in proportion to the forcing. In terms of global
warming an extra million tonnes of carbon dioxide would cause a
certain predictable temperature increase. This can be equated to
pushing a car along a flat road: most of the energy put into pushing
is used to move the car forward.
(b) Muted or limited response (Figure 5b). In this case the forcing may
be strong, but the climate system is in some way buffered and
therefore gives very little response. Many global warming sceptics
and politicians argue that the climate system is very insensitive to
changes in atmospheric carbon dioxide so very little will happen in
the future. This is the ‘pushing the car up the hill’ analogy: you can

What is global warming?

We can also try to abstract the way the global climate system
responds to an internal or external forcing agent by examining
different scenarios (see Figure 5). In these scenarios I am
assuming that there is only one forcing mechanism which is trying
to change the global climate. What is important is how the global
climate system will react. For example, is the relationship like a
person trying to push a car up a hill which, strangely enough, gets
very little response? Or is it more like a person pushing a car
downhill, which, once the car starts to move, it is very difficult to
stop. There are four possible relationships and this is the central
question in the global warming debate, which is most applicable to
the future.

Global Warming

spend as much energy as you like trying to push the car but it will
not move very far.
(c) Delayed or non-linear response (Figure 5c). In this case, the climate
system may have a slow response to the forcing thanks to being
buffered in some way. After an initial period the climate system
responds to the forcing but in a non-linear way. This is a real
possibility when it comes to global warming and why it is argued
that as yet only a small amount of warming has been observed over
the last 100 years. This scenario can be equated to the car on the
top of a hill: it takes some effort and thus time to push the car to the
edge of the hill; this is the buffering effect. Once the car has reached
the edge it takes very little to push the car over, and then it
accelerates down the hill with or without your help. Once it reaches
the bottom, the car then continues for some time, which is the
overshoot, and then slows down of its own accord and settles into a
new state.
(d) Threshold response (Figure 5d). In this case, initially, there is no or
very little response in the climate system to the forcing; however, all
the response takes place in a very short period of time in one large
step or threshold. In many cases the response may be much larger
than one would expect from the size of the forcing and this can be
referred to as a response overshoot. This is the scenario that most
worries us, as thresholds are very difficult to model and thus predict.
However, thresholds have been found to be very common in the
study of past climates, with rapid regional climate changes of over
5°C occurring within a few decades. This scenario equates to the
bus hanging off the cliff at the end of the original film The Italian
Job; as long as there are only very small changes, nothing happens
at all. However, a critical point (in this case weight) is reached and
the bus (and the gold) plunge off the cliff into the ravine below.

Though these are purely theoretical models of how the global
climate system can respond, they are important to keep in mind
when reviewing the possible scenarios for future climate change.
Moreover, they are important when we consider in Chapter 3 why
different people see different global warming futures despite all

having access to the same information. It depends on which of the
above scenarios they believe will happen. An added complication
when assessing climate change is the possibility that climate
thresholds contain bifurcations. This means the forcing required to
go one way through the threshold is different from the reverse (see
Figure 5e). This implies that once a climate threshold has occurred,
it is a lot more difficult to reverse it. The bifurcation of the climate
system has been inferred from ocean models which mimic the
impact of fresh water in the North Atlantic on the global deep-water
circulation, and we will discuss this can of worms in great detail in
Chapter 7.

Linking global warming with climate change


What is global warming?

We have seen that there is clear evidence that greenhouse gas
concentrations in the atmosphere have been rising since the
industrial revolution in the 18th century. The current scientific
consensus is that changes in greenhouse gas concentrations in the
atmosphere do cause global temperature change. However, the
biggest problem with the global warming hypothesis is
understanding how sensitive the global climate is to increased levels
of atmospheric carbon dioxide. Even if we establish this, predicting
climate change is complex because it encompasses many different
factors, which respond differently when the atmosphere warms up,
including regional temperature changes, melting glaciers and ice
sheets, relative sea-level change, precipitation changes, storm
~o, and even ocean circulation. This
intensity and tracks, El Nin
linkage between global warming and climate change is further
complicated by the fact that each part of the global climate system
has different response times. For example, the atmosphere can
respond to external or internal changes within a day, but the deep
ocean may take decades to respond, while vegetation can alter its
structure within a few weeks (e.g. change the amount of leaves) but
its composition (e.g. swapping plant types) can take up to a century
to change. Then, add to this the possibility of natural forcing which
may be cyclic; for example, there is good evidence that sunspot

Global Warming

cycles can affect climate on both a decadal and a century timescale.
There is also evidence that since the beginning of our present
interglacial period, the last 10,000 years, there have been climatic
coolings every 1,500 ±500 years, of which the Little Ice Age was the
last. The Little Ice Age began in the 17th and ended in the 18th
century and was characterized by a fall of 0.5–1°C in Greenland
temperatures, significant shift in the currents around Iceland, and a
sea-surface temperature fall of 4°C off the coast of West Africa, 2°C
off the Bermuda Rise, and of course ice fairs on the River Thames in
London, all of which were due to natural climate change. So we
need to disentangle natural climate variability from global
warming. We need to understand how the different parts of the
climate system interact, remembering that they all have different
response times. We need to understand what sort of climatic change
will be caused, and whether it will be gradual or catastrophic. We
also need to understand how different regions of the world will be
affected; for example, it is suggested that additional greenhouse
gases will warm up the poles more than the tropics. All these
themes concerning an understanding of the climate system and the
difficulty of future climate prediction are returned to in Chapters 4
and 5.
So if you are reading this book for the first time and are primarily
interested in the science of global warming then I would suggest
you read Chapters 4, 5, 6, and 7. However, I would encourage you
also to read Chapters 2, 3, 8, and 9, which look at the social, historic,
economic, and political aspects of global warming, since global
warming, as far as I am concerned, cannot be seen as a scientific
problem; rather, it is a problem for our global society.


Chapter 2
A brief history of the
global warming hypothesis

Historical background
Scientists are predicting that global warming could warm the
planet by between 1.4 and 5.8°C in the next 100 years, causing huge
problems for humanity. In the face of such a threat it is essential to
understand the history of the global warming theory and the
evidence that supports it. Can the future really be as bleak as
scientists are predicting? This whole debate over the global
warming theory and its possible impacts, more than any other
controversy in science, demonstrates the humanity of scientists
and the politics of new scientific ideas. This is because, despite the
Hollywood vision of scientists, we are not logical machines like
Mr Spock from Star Trek, nor mad scientists like Dr Frankenstein,
but highly driven individuals. Though I must admit I do like the
heroic portrayal of a ‘paleoclimatologist’ in the Day after Tomorrow;
if only we were really like that. So it must remembered that money
is not the main driving force of science; rather it is curiosity tainted
with ambition, ego, and the prospect of fame. So please divest
yourself of the image of scientists divorced from the world around
them. The history of the global warming hypothesis clearly shows
that science is deeply influenced by society and vice versa. So what
we discover is that the essential science of global warming was
carried out 50 years ago under the perceived necessity of
geosciences during the Cold War, but was not taken seriously

Global Warming

as a theory until the late 1980s. I hope to give you some insight into
why there was such a significant delay.
It is now over one hundred years ago that global warming was
officially discovered. The pioneering work in 1896 by the Swedish
scientist Svante Arrhenius, and the subsequent independent
confirmation by Thomas Chamberlin, calculated that human
activity could substantially warm the Earth by adding carbon
dioxide to the atmosphere. This conclusion was the by-product of
other research, its major aim being to offer a theory whereby
decreased carbon dioxide would explain the causes of the great ice
ages, a theory which still stands today but which had to wait until
1987 for the Antarctic Vostok ice-core results to confirm the pivotal
role of atmospheric CO2 in controlling past global climate.
However, no one else took up the research topic, so both Arrhenius
and Chamberlin turned to other challenges. This was because
scientists at that time felt there were so many other influences on
global climate, from sunspots to ocean circulation, that minor
human influences were thought insignificant in comparison to the
mighty forces of astronomy and geology. This idea was reinforced
by research during the 1940s, which developed the theory that
changes in the orbit of the Earth around the sun controlled the
waxing and waning of the great ice ages. A second line of argument
was that because there is 50 times more carbon dioxide in the
oceans than in the atmosphere, it was conjectured that ‘The sea
acts as a vast equalizer’, in other words the ocean would mop up
our pollution.
This dismissive view took its first blow when in the 1940s there was
a significant improvement in infrared spectroscopy, the technique
used to measure long-wave radiation. Up until the 1940s
experiments had shown that carbon dioxide did block the
transmission of infrared ‘long-wave’ radiation of the sort given off
by the Earth. However, the experiments showed there was very little
change in this interception if the amount of carbon dioxide was
doubled or halved. This meant that even small amounts of carbon

This still left the argument that the oceans would soak up the extra
anthropogenically produced carbon dioxide. The first new evidence
came in the 1950s and showed that the average lifetime of a carbon
dioxide molecule in the atmosphere before it dissolved in the sea
was about ten years. As the ocean overturning takes several
hundreds of years, it was assumed the extra carbon dioxide would
be safely locked in the oceans. But Roger Revelle (director of
Scripps Institute of Oceanography in California) realized that it was
necessary not only to know that a carbon dioxide molecule was
absorbed after ten years but to ask what happened to it after that.
Did it stay there or diffuse back into the atmosphere? How much
extra CO2 could the oceans hold? Revelle’s calculations showed that
the complexities of the surface ocean chemistry are such that it
returns much of the carbon dioxide that it absorbs. This was a great
revelation, and showed that because of the peculiarities of ocean

The global warming hypothesis

dioxide could block radiation so thoroughly that adding more gas
made very little difference. Moreover, water vapour, which is
much more abundant than carbon dioxide, was found to block
radiation in the same way and, therefore, was thought to be more
important. The Second World War saw a massive improvement in
technology and the old measurements of carbon dioxide radiation
interception were revisited. In the original experiments sea-level
pressure was used but it was found that at the rarefied upper
atmosphere pressures the general absorption did not occur
and, therefore, radiation was able to pass through the upper
atmosphere and into space. This proved that increasing the
amount of carbon dioxide did result in absorption of more
radiation. Moreover, it was found that water vapour absorbed
other types of radiation rather than carbon dioxide, and to
compound it all, it was also discovered that the stratosphere, the
upper atmosphere, was bone dry. This work was brought together
in 1955 by the calculations of Gilbert Plass, who concluded that
adding more carbon dioxide to the atmosphere would intercept
more infrared radiation, preventing it being lost to space and thus
warming the planet.

chemistry, the oceans would not be the complete sink for
anthropogenic carbon dioxide that was first thought. This principle
still holds true, although the exact amount of anthropogenic carbon
dioxide taken up per year by the oceans is still in debate. It is
thought to be about 2 gigatonnes, nearly a third of the annual total
anthropogenic production.

Global Warming

Charles Keeling, who was hired by Roger Revelle, produced the next
important step forward in the global warming debate. In the late
1950s and early 1960s Keeling used the most modern technology
available to measure the concentration of atmospheric CO2 in
Antarctica and Mauna Loa. The resulting Keeling CO2 curves
have continued to climb ominously each year since the first
measurement in 1958 and have become one of the major icons
of global warming.
Spencer Weart, the director of the Center of History of Physics at
the American Institute of Physics, argues that all the scientific facts
about enhanced atmospheric CO2 and potential global warming
were assembled by the late 1950s–early 1960s. He argues that it
was only due to the physical geosciences being favoured financially
in the Cold War environment that so much of the fundamental
work on global warming was completed. Gilbert Plass published
an article in 1959 in Scientific American declaring that the world’s
temperature would rise by 3°C by the end of the century. The
magazine editors published an accompanying photograph of coal
smoke belching from factories and the caption read, ‘Man upsets
the balance of natural processes by adding billions of tons of
carbon dioxide to the atmosphere each year’. This resembles
thousands of magazine articles, television news items, and
documentaries that we have all seen since the late 1980s. So why
was there a delay between the science of global warming being
accepted and in place in the late 1950s and early 1960s and the
sudden realization of the true threat of global warming during
the late 1980s?


Why did it take so long to recognize global

Since the 1940’s the northern half of our planet has been cooling
rapidly. Already the effect in the United States is the same as if every
city had been picked up by giant hands and set down more than 100
miles closer to the North Pole. If the cooling continues, warned the
National Academy of Sciences in 1975, we could possibly witness the
beginning of the next Great Ice Age. Conceivably, some of us might
live to see huge snow fields remaining year-round in northern
regions of the United States and Europe. Probably, we would see
mass global famine in our life times, perhaps even within a decade.
Since 1970, half a million human beings in northern Africa and Asia
have starved because of floods and droughts caused by the cooling

The global warming hypothesis

The key reasons for the delay in recognizing the global warming
threat were, first, the power of the global mean temperature
data set and, second, the need for the emergence of global
environmental awareness. The global mean temperature data
set is calculated using the land-air and sea-surface temperature.
From 1940 till the mid-1970s the global temperature curve
seems to have had a general downward trend. This provoked
many scientists to discuss whether the Earth was entering the
next great ice age. This fear developed in part because of
increased awareness in the 1970s of how variable global climate
had been in the past. The emerging subject of palaeoceanography
(study of past oceans) demonstrated from deep-sea sediments
that there were at least 32 glacial-interglacial (cold-warm)
cycles in the last two and a half million years, not four as had
been previously assumed. The time resolution of these studies
was low, so that there was no possibility of estimating how
quickly the ice ages came and went, only how regularly. It led
many scientists and the media to ignore the scientific revelations
of the 1950s and 1960s in favour of global cooling. As Ponte (1976)

Global Warming

It was not until the early 1980s, when the global annual mean
temperature curve started to increase, that the global cooling
scenario was questioned. By the late 1980s the global annual mean
temperature curve rose so steeply that all the dormant evidence
from the late 1950s and 1960s was given prominence and the global
warming theory was in full swing. What is intriguing is that some of
the most vocal advocates for the global warming theory were also
the ones responsible for creating concern over the impending ice
age. In The Genesis Strategy in 1976, Stephen Schneider stressed
that the global cooling trend had set in; he is now one of the leading
proponents of global warming. In 1990 he stated that ‘the rate of
change [warming] is so fast that I don’t hesitate to call that kind of
change potentially catastrophic for ecosystems’.
Why the hysteria? John Gribbin (1989) describes the transition very
neatly in his book In Hothouse Earth: the Greenhouse Effect and
In 1981 it was possible to stand back and take a leisurely look at the
record from 1880 to 1980 . . . . In 1987, the figures were updated to
1985, chiefly for neatness of adding another half a decade to the
records . . . . But by early 1988, even one more year’s worth of data
justified another publication in April, just four months after the last
1987 measurements were made, pointing out the record-breaking
warmth now being reached. Even there, Hansen [James Hansen,
head of the NASA team studying global temperature trends] and
Lebedeff were cautious about making the connection with the
greenhouse effect, merely saying that this was ‘a subject beyond the
scope of this paper’. But in four months it had taken to get the 1987
data in print, the world had changed again; just a few weeks later
Hansen was telling the US Senate that the first five months of 1988
had been warmer than any comparable period since 1880, and
greenhouse effect was upon us.

It seems, therefore, that the whole global warming issue was driven
by the upturn in the global annual mean temperature data set. This

The upturn in the global annual mean temperature data was not the
sole reason for the appearance of the global warming issue. During
the 1980s there was also an intense drive to understand past climate
change. Major advances were made in obtaining high-resolution
past climate records from deep-sea sediments and ice cores. It was,
thus, realized that glacial periods, or ice ages, take tens of thousand
years to occur, primarily because ice sheets are very slow to build up

The global warming hypothesis

in itself is interesting because some scientists in the early 1990s
believed that this was a flawed data set because: (1) many of the
land monitoring stations have subsequently been surrounded by
urban areas, thus increasing the temperature records because of the
urban heat island effect, (2) there have been changes in the ways
ships measure the sea-water temperature, (3) there was not an
adequate explanation for the cooling trend in the 1970s, (4) satellite
data did not show a warming trend from the 1970s to the 1990s,
and (5) the global warming models have overestimated the
warming that should have occurred in the northern hemisphere
over the last 100 years. Since the early 1990s the urban heat island
and variations in sea-temperature measurements have been taken
into account. We now know that the cooling trend of the 1970s is
due to the decadal influence of the sunspot cycle. It turns out that
the satellite results were spurious for a number of reasons and a
greater understanding of the system and recalibrated data shows a
significant warming trend. Lastly, it was discovered that other
pollutants, such as sulphur dioxide aerosols, have been cooling
industrial regions of the globe, and as the models of the early 1990s
did not take them into consideration, they were overestimating the
amount of warming. So the latest IPCC 2001 Science Report has
reviewed and synthesized a wide range of data sets and shows that,
essentially, the trend in the temperature data is correct, and that
this warming trend has continued unstopped until the present day
(see Figure 6). In fact we know that 1998 was globally the warmest
year on record, with 2002 the second, 2003 the third, 2001 the
fourth and 1997 the fifth warmest. Indeed the ten warmest years on
record have all occurred since 1990.

Global Warming

6. Variation of the Earth’s surface temperature

and are naturally unstable. In contrast, the transition to a warmer
period or interglacial, such as the present, is geologically very quick,
in the order of a couple of thousand years. This is because once the
ice sheets start to melt there are a number of positive feedbacks that
accelerate the process, such as sea-level rise, which can undercut
and destroy large ice sheets. The realization occurred in the
palaeoclimate community that global warming is much easier and
more rapid than cooling. It also put to rest the myth of the next
impending ice age. As the glacial-interglacial periods of the last two
and half million years have been shown to be forced by the changes
in the orbit of the Earth around the sun, it would be possible to
predict when the next glacial period will begin, if there were no
anthropogenic effects involved. According to the model predictions
by Berger and Loutre (2002) at the Université catholique de
Louvain in Belgium, we do not need to worry about another ice age
for at least 5,000 years. Indeed, if their model is correct and
atmospheric carbon dioxide concentrations double, then global
warming would postpone the next ice age for another 45,000 years.
Palaeoclimate work has also provided us with worrying insights into
how the climate system works. Recent work on the ice cores and

deep-sea sediments demonstrate that at least regional climate
changes of 5°C can occur in a matter of decades. This work on
reconstructing past climate seems to demonstrate that the global
climate system is not benign but highly dynamic and prone to
rapid changes.

It was the discovery in 1985 by the British Antarctic Survey of
depletion of ozone over Antarctica which demonstrated the global
connectivity of our environment. The ozone ‘hole’ also had a
tangible international cause, the use of CFCs, which led to a whole
new area of politics, the international management of the
environment. There followed a set of key agreements, the 1985
Vienna Convention for the Protection of the Ozone Layer, the 1987
Montreal Protocol on Substances that Deplete the Ozone layer, and
the 1990 London and 1992 Copenhagen Adjustments and
Amendments to the Protocol. These have been held up as examples
of successful environmental diplomacy. Climate change has had a
slower development in international politics and far less has been

The global warming hypothesis

The next change that occurred during the 1980s was a massive
grass-roots expansion in the environmental movement, particularly
in the USA, Canada, and the UK, partly as a backlash against the
right-wing governments of the 1980s and the expansion of the
consumer economy and partly because of the increasing number
of environmental-related stories in the media. This heralded a
new era of global environmental awareness and transnational
NGOs (Non-Governmental Organizations). The roots of this
growing environmental awareness can be traced back to a number
of key markers; these include the publication of Rachel Carson’s
Silent Spring in 1962, the image of Earth seen from the moon in
1969, the Club of Rome’s 1972 report on Limits to Growth, the
Three Mile Island nuclear reactor accident in 1979, the nuclear
accident at Chernobyl in 1986, and the Exxon Valdez oil spillage in
1989. But these environmental problems were all regional in effect,
i.e. limited geographically to the specific area in which they

Global Warming

7. Global warming and the media

achieved in terms of regulation and implementation. This is, at its
most simplistic level, because of the great inherent uncertainties of
the science and the greater economic costs involved.
The other reason for the acceptance of the global warming
hypothesis was the intense media interest throughout the late 1980s
and 1990s. This is because the global warming hypothesis was
perfect for the media: a dramatic story about the end of the world as
we know it, with important controversy about whether it was even
true. Anabela Carvalho, now at the University of Minho (Braga,
Portugal), has done a fascinating study of the British quality press
coverage of the global warming issue between 1985 and 1997. She


The global warming hypothesis

concentrated particularly on the Guardian and The Times and
found throughout this period that they promoted very different
world-views. Interestingly, despite their differing views, the number
of articles published per year by the British quality (broadsheet)
papers followed a similar pattern and peaked when there were key
IPCC reports published or international conferences on climate
change (see Figure 7). But it is the nature of these articles that
shows how the global warming debate was constructed in the
media. From the late 1980s The Times, which published most
articles on global warming in 1989, 1990, and 1992, cast doubt on
the claims of climate change. There was a recurrent attempt to
promote mistrust in science, through strategies of generalization,
of disagreement within the scientific community, and, most
importantly, discrediting scientists and scientific institutions. A
very similar viewpoint was taken by the majority of the American
media throughout much of the 1990s. In fact it has been claimed
that this approach in the American media has led to a barrier
between scientists and the public in the USA. In the UK the
Guardian newspaper took the opposite approach to that of The
Times. Although the Guardian gave space to the technical side of
the debate, it soon started to discuss scientific claims in the wider
context. As scientific uncertainty regarding the enhanced
greenhouse effect decreased during the 1990s, the Guardian
coherently advanced a strategy of building confidence in science,
with an emphasis on consensus as a means of enhancing the
reliability of knowledge. This was because the Guardian
understood and promoted one of the fundamental bases of science,
which is that a theory, such as global warming, can only be accepted
or rejected by the weight of evidence. So, as evidence from many
different areas of science continues to support the theory of global
warming, so correspondingly our confidence in the theory should
increase. Far from painting science as ‘pure’ or ‘correct’, instead the
Guardian politicized it to demonstrate the bias that is inherent in
all science. This clearly showed that many of the climate change
claims were being eroded by lobbying pressure, mainly associated
with the fossil-fuel industry. This politicizing of science allowed the

Global Warming

Guardian to strengthen their readers’ confidence in science.
Moreover, they clearly conveyed the uncertainties that the science
of the global warming hypothesis contains and were and still are in
favour of the precautionary principle. It was through this media
filter that scientists attempted to advance their particular global
warming view, by either making claims for more research or
promoting certain political options. From the late 1980s onwards,
scientists became very adept at staging their media performances,
and it is clear that the general acceptance of the global warming
hypothesis is in part due to their continued effort to communicate
their findings. Indeed, both the sceptical and the supportive stances
of The Times and Guardian, respectively, so legitimized the debate
over global warming that the public became aware that this was not
an overnight news story but something that has become part of the
very fabric of our society.
It seems that the media has also influenced our use of words. From
1988 onwards the use of the phrase ‘global warming’ and ‘climate
change’ gained support, while ‘greenhouse effect’ lost its appeal and
by 1997 was rarely mentioned. The change in terminology is
reflected in this book. The title is Global Warming, as everyone
knows what it means, and the major discussions in this book are
about the climate change it might induce.
So by combining (1) the science of global warming essentially
carried out by the mid-1960s, (2) the frightening upturn in the
global temperature data set at the end of the 1980s, (3) our
increased knowledge of how past climate has reacted to changes
in atmospheric carbon dioxide in the 1980s, (4) the emergence
of the global environmental awareness in the late 1980s, and
(5) the media’s savage interest in the confrontational nature of the
debate, we are led to the final recognition of the global warming
hypothesis. This has culminated in thousands of scientists
turning their attention to the problem to try to prove it right or
wrong. Landmarks since then have been the setting up of the
Intergovernmental Panel on Climate Change (IPCC) in 1988 by

the United Nations Environmental Panel and World Meteorological
Organization; the publication of key reports by the IPCC in 1990,
1996, and 2001; the formal signature of the United Nations
Framework Convention on Climate Change (UNFCCC) at the
Rio Earth Summit in 1992; the subsequent Conference of the
Parties (COP) at Kyoto in 1998, where the UNFCCC Protocols
were formally accepted, and then in Bonn in July 2001, where
the so-called ‘Kyoto’ Protocols were agreed by 186 countries.

The global warming hypothesis


Chapter 3
Your viewpoint
determines the future

Considering all the scientific evidence collected to support the
global warming hypothesis, why is there still a huge range of
opinions on what the future holds for us? An interesting way of
viewing this problem has been presented by Professor John Adams
at University College London. He suggests it is all down to how
each individual views risk, in particular how we view ‘nature’ as a
risk. Do we believe that nature is benign and able to take whatever
we throw at it or do we think of it as malevolent, having the power
to react harshly to our interference? As Douglas and Wildavsky
(1983) ask and answer in their book, ‘Can we know the risks we
face now and in the future? No, we cannot, but yes, we must act as
if we do.’ We all have to predict what the risks are around us, both
at the present time and in the future. This applies to anything from
the risk of crossing the road to the risk of climate change from
global warming. John Adams has developed ‘four myths of nature’
and ‘four myths of human nature’ and combined them to look at
the range of individual responses to risk and uncertainty (Adams,
1995). What I have done here is to alter these myths slightly so
they are more directly related to the issue of global warming.
It must be remembered that these are just another way of
appreciating how different people see the global warming


Adams (1995) suggest there are Four myths of nature which are
shown in Figure 8:
1. Nature benign. Nature, according to this myth, is predictable,
bountiful, robust, stable, and forgiving of any insults that
humankind might inflict upon it. However violently it might be
shaken, the ball always comes to rest in the bottom of the basin. If
nature is benign in the context of human activity then it does not
need to be managed and thus a non-interventionist approach can
be taken.
2. Nature ephemeral. Nature is fragile, precarious, and unforgiving.
It is in danger of catastrophic collapse thanks to human
interference. The objective of environmental management must
therefore be to protect nature from humans. This myth insists that
people must tread lightly on the Earth and that the guiding
management rule is one of precaution.
3. Nature perverse/tolerant. This is a combination of the first two
myths. Within limits, nature can be relied upon to behave

Your viewpoint determines the future

8. The four myths of nature

predictably. It is forgiving of modest shocks to the system but care
must be taken not to knock the ball out of the cup. Regulation is
required to prevent major excesses while leaving the system to
look after itself in minor matters. This is the ecologist’s equivalent
of a mixed-economy model. The management style is

Global Warming

4. Nature capricious. Nature is unpredictable. The appropriate
management style is laissez-faire, as there is no point to
management. The believer in this myth is an agnostic concerning
nature as the future may turn out to be good or bad, but in any event
it is beyond any human control.
Individuals base their views on many factors: on their own
belief system, their own personal agenda (either financial or
political), or whatever is expedient to believe at the time. However,
the basis to everyone’s views of the global warming hypothesis is
determined by how we each perceive the world. Cultural
geographers and sociologists have suggested a grid system to look
at individual beliefs. One axis on the horizontal from left to right is a
measure of how human nature can vary from an individualist to a
more collectivist point of view, while the vertical axis varies from the
top ‘Prescribed Inequality’, a measure of the amount of restrictions
one feels is imposed by a superior authority, assuming of course that
all social and economic transactions are characterized by inequality.
At the bottom, ‘Prescribing Equality’, there are no externally
prescribed constraints on choice and people negotiate the rules
as they go along. Combining these two axes produces four myths
of human nature which can then be combined with our views
of nature.
The four myths of human nature associated with this grid are
shown in Figure 9.
1. Individualists are enterprising ‘self-made’ people relatively free
from control by others, who strive to exert control over their

environment and the people in it. Their success is often measured
by their wealth and the number of followers they can command.
Victorian mill owners or self-made oil barons are good
representatives of this category.
2. Hierarchists, who inhabit a world with strong group boundaries
and binding prescriptions. Social relationships in this world are
hierarchical and everyone knows his or her place. Soldiers, civil
servants, and certain kinds of scientist are exemplars of this
3. Egalitarians have strong group loyalties but little respect for
externally imposed rules, other than those imposed by nature.
Group decisions are arrived at democratically and leaders rule by

Your viewpoint determines the future

9. Four myths of human nature

Global Warming

10. Four rationalities

force of personality and persuasion. Environmental pressure groups
are a classic example of this category.
4. Fatalists have minimal control over their own lives. They belong
to no groups responsible for decisions that rule their lives. They
are resigned to their fate and everyone else’s, and see no point in
trying to change it.
These two sets of myths can be related to each other to explain what
type of person is likely to believe which myth of nature (see Figure
10). What I have done in Figure 11 is to overlay some of the possible
climatic changes that could occur as a result of global warming,
changes which were discussed in Chapter 1. Now, for fun, you
should try putting the following people on the global warming belief
chart: yourself, President George Bush, a Greenpeace spokesperson,

11. Combined global warming scenarios with myths of human nature

Global Warming

and a cotton farmer in a less-developed country. Also, when you
have read Chapter 8 about the different groups involved in the
Kyoto Negotiations, it may be of interest to see where each group
lies on the global warming belief chart. By looking at global
warming in this way it shows that there are clear reasons why those
who do not believe in the threat of global warming may never
believe in it until it is too late, because they have their own view of
nature and thus perceive that there is a low potential risk of future
climate change. We must also remember that individuals can be
extremely fluid in their beliefs, particularly when it comes to risk
and uncertainty. People will, thus, shift in their opinion depending
on the evidence put forward. A classic example of this was in the
mid-1990s when journalists asked me if global warming was
occurring and whether I would be prepared to defend it; now, by
contrast, they ask how bad it will get. What I hope to do during the
rest of the book is try to shift your belief from the left-hand side of
the global warming belief chart to the right. Or, if you are already on
the right-hand side of the chart, show you why your instinctive view
of nature may well be correct.


Chapter 4
What is the evidence
for climate change?

Past climate change
Climate change in the geological past has been reconstructed using
a number of key archives, including marine and lake sediments, ice
cores, cave deposits, and tree rings. These various records reveal
that over the last 100 million years the Earth’s climate has been
cooling down, moving from the so-called ‘Greenhouse World’ of
the Cretaceous Period, when dinosaurs enjoyed warm and gentle
conditions, through to the cooler and more dynamic ‘Ice House
World’ of today (see Figure 12). It may seem odd that in geological
terms our planet is relatively cold, while this whole book is
concerned with our great fears of global warming. This is because
even today we have huge ice sheets on both Antarctica and
Greenland and nearly permanent sea ice in the Arctic Ocean.
So, compared to the time of the dinosaurs, when there were no
massive ice sheets, we live in chilly times.
This long-term, 100-million-year transition to cooler global climate
conditions was driven mainly by tectonic changes, such as the
opening of the Tasmanian–Antarctic gateway and the Drake
passage, which isolated Antarctica from the rest of the world,
the uplift of the Himalayas, and the closure of the Panama ocean
gateway. There is also geological evidence that levels of atmospheric
carbon dioxide have become significantly lower over the last
100 million years. These changes culminated in the glaciation of

12. The anatomy of past climatic changes

It may seem strange in a book about global warming to suggest that
we are currently in a geological ‘Ice House World’. This is, however,
an important point when we look at the consequences of the world
warming up, because, despite being in a relatively warm interglacial
period, both poles are still glaciated, which is a rare occurrence in
the geological history of our planet. Antarctica and Greenland are
covered by ice sheets, and the majority of the Arctic Ocean is
covered with sea ice. This means that there is a lot of ice that could
melt in a warmer world, and, as we will see, this is one of the biggest
unknowns that the future holds for our planet. The two glaciated
poles also make the temperature gradient or difference between
the poles and the Equator extremely large, from an average of
about + 30°C at the Equator down to −35°C or colder at the poles.
This temperature gradient is one of the main reasons that we have a
climate system, as excess heat from the tropics is exported both via
the oceans and the atmosphere to the poles, which causes our
weather. Geologically, we currently have one of the largest Equator–

The evidence for climate change

Antarctica about 35 million years ago and then the great northern
hemisphere ice ages, which began 2.5 million years ago. Since the
beginning of the great northern ice ages the global climate has
cycled from conditions that were similar or even slightly warmer
than today, to full ice ages, which caused ice sheets over 3 km thick
to form over much of North America and Europe. Between 2.5 and
0.9 million years ago these glacial-interglacial cycles occurred every
41,000 years and since 0.9 million years ago they have occurred
every 100,000 years. These great ice-age cycles are driven primarily
by changes in the Earth’s orbit with respect to the sun. In fact the
world has spent over 80% of the last 2.5 million years in conditions
colder than the present. Our present interglacial, the Holocene
Period, started about 10,000 years ago and is an example of the rare
warm conditions that occur between each ice age. The Holocene
began with the rapid and dramatic end of the last ice age; in less
than 4,000 years global temperatures increased by 6°C, relative sea
level rose by 120 m, atmospheric carbon dioxide increased by a
third, and atmospheric methane doubled.

Global Warming

pole temperature gradients, which leads to a very dynamic climate
system. So our ‘Ice House’ conditions cause our very energetic
weather system, which is characterized by hurricanes, tornadoes,
extra-tropical (temperate) winter storms, and monsoons. James
Lovelock in his book ‘The Ages of Gaia’ (New edition, 1995 p. 227)
suggests that interglacials, like the Holocene Period, are the fevered
state of our planet, which clearly over the last 2.5 million years
prefers a colder average global temperature. Lovelock sees global
warming as humanity just adding to the fever.
Climate, however, has not been constant during our interglacial,
i.e. the last 10,000 years. Palaeoclimate evidence suggests that the
early Holocene was warmer than the 20th century. Throughout
the Holocene there have been millennial-scale climate events, called
Dansgaard-Oeschger cycles, which involve a local cooling of 2°C.
These events have had a significant influence on classical
civilizations; for example, the cold arid event about 4,000 years ago
coincides with the collapse of many classical civilizations, such as
the Old Kingdom in Egypt (see discussion in Chapter 9). The last of
these millennial climate cycles was the Little Ice Age. This event is
really two cold periods; the first follows the Medieval Warm Period
which ended a thousand years ago, and is often referred to as the
Medieval Cold Period. The Medieval Cold Period played a role in
extinguishing Norse colonies on Greenland and caused famine and
mass migration in Europe. It started gradually before ad 1200
and ended at about ad 1650. The second cold period, more
classically referred to as the Little Ice Age, may have been the most
rapid and largest change in the North Atlantic region during the
late Holocene, as suggested by ice-core and deep-sea sediment
records. The reconstruction of temperature records for the last
thousand years includes the Little Ice Age and is essential data for
demonstrating that the last two centuries are very different from
the preceding eight (Figure 13). There are four main data sets which
have attempted to reconstruct temperatures for the northern
hemisphere over the last millennium: tree rings, corals, ice cores,
and/or the direct measurement of past temperatures from

boreholes. First, it should be noted that the different data sets
compare well with each other, which gives added confidence that we
are seeing real temperature variations in these reconstructions.
Second, the data show that the centuries before 1900 were much
colder. They also show that the Medieval Warm Period and the
Little Ice Age did occur, but that in much of the northern
hemisphere the climate changes seen are only small, with the
exception of northern Europe. Without this data the instrumental
temperature data set for the last 150 years would have no context.
As it is, it can now be clearly shown that temperatures, at least for
the northern hemisphere, have been warmer in the 20th century
than at any other time during the last thousand years.

Recent climate change
The three main indicators of global warming are temperature,
precipitation, and sea level. One of the key aims of scientists over
the last couple of decades has been to estimate how these have

The evidence for climate change

13. Northern Hemisphere temperature reconstruction for the last
thousand years

changed since the industrial revolution and to see if there is any
evidence for global warming being to blame. Below is the evidence
for each of these parameters.

Global Warming

As we have seen, temperatures for the northern hemisphere have
been reconstructed for the last thousand years, providing a context
to the 20th century. Temperatures for the last 150 years have been
estimated from a number of sources, both direct thermometerbased indicators and proxy-based. What is a proxy? As used here
and elsewhere, it is short for proxy variable. The term ‘proxy’ is
commonly used to describe a stand-in or substitute, as in ‘proxy
vote’ or ‘fighting by proxy’. In the same way, proxy variable in the
parlance of climatology means a measurable ‘descriptor’ that stands
in for a desired (but unobservable) ‘variable’, such as past ocean or
land temperature. So there is an assumption that you can use the
proxy variable to estimate a climatic variable that you cannot
measure directly. So, as we will see below, you can use the thickness
of tree rings as a way of estimating past land temperatures; in this
case, the tree-ring thickness is a proxy for temperature.
Thermometer-based indicators include sea-surface temperature
(SST), marine air temperatures (MAT), land surface-air
temperature, and temperatures in the free atmosphere, such as
those measured by sensors on balloons. Borehole temperature
measurements are defined as proxy-based because, despite the use
of direct measurements of temperatures, these have been altered
over time. Mathematical inversion procedures are required to
translate the modern temperature in the boreholes into changes of
ground temperature back through time. Other proxy-based
methods include infrared satellite measurements and tree-ring
width and thickness.
Thermometer-based measurements of air temperature have been
recorded at a number of sites in North America and Europe as far
back as 1760. The number of observation sites does not increase to

sufficient worldwide geographical coverage to permit a global land
average to be calculated until about the middle of the 19th century.
SST and marine air temperatures were systematically recorded by
ships from the mid-19th century, but even today the coverage of
the southern hemisphere is extremely poor. All these data sets
require various corrections to account for changing conditions and
measurement techniques. For example, for land data each station
has been examined to ensure that conditions have not varied
through time as a result of changes in the measurement site,
instruments used, instrument shelters, or the way monthly averages
were computed, or the growth of cities around the sites, which leads
to warmer temperatures caused by the urban heat island effect.

What is so interesting about the 130-year temperature data set are
the details, particularly as mentioned before the cooling observed in
the 1960s and 1970s. One of the key tests for climate models, used
to predict future climate changes, is whether they can reproduce the
changes seen since 1870. These models are discussed in more detail

The evidence for climate change

For SST and MAT there are a number of corrections that have to be
applied. First, up to 1941 most SST temperature measurements
were made in sea water hoisted on deck in a bucket. Since 1941
most measurements have been made at the ships’ engine water
intakes. Second, between 1856 and 1910 there was a shift from
wooden to canvas buckets, which changes the amount of cooling
caused by evaporation that occurs as the water is being hoisted on
deck. In addition, through this period there was a gradual shift from
sailing ships to steamships, which altered the height of the ship
decks and the speed of the ships, both of which can affect the
evaporative cooling of the buckets. The other key correction that
has to be made is for the global distribution of meteorological
stations through time. As shown in Figure 14, the number of stations
and their location varies greatly from 1870 to 1960. But by making
these corrections it is possible to produce a continuous record of
land-surface air and SST temperature for the last 130 years, which
shows a global warming of 0.65°C ±0.05°C over this period.

14. Global distribution of meteorological stations

in the next chapter but it should be noted that only by combining
natural forcing (such as solar 11-year cycles and stratospheric
aerosols from explosive volcanic eruptions), and anthropogenic
forcing (greenhouse gases and sulphur aerosols) can the
temperature record be simulated.

Satellite-based proxy records have been available for the last 20
years and have been the source of some key controversies in the
global warming debate. The advantage of satellite-mounted
microwave sensors is that they have a global coverage, unlike the
balloons which are predominately land-based and in the northern
hemisphere. There are, however, some major problems with the
microwave data set. First, the temperature record is based on eight
different satellites, and despite overlapping measurement times,
intercalibration between different instruments is problematic.
Second, there is a spurious warming trend after 1990 of 0.03–0.04
°C which is due to a drift in the orbital times, and a spurious cooling
trend of 0.12°C/decade due to the reduced altitude or height of the
satellites caused by friction with the atmosphere. Third, the height
within the atmosphere at which the microwave sensor measures
temperature is affected by the amount of ice crystals and raindrops
in the atmosphere. Hence, if the planet is warming up, moisture will
be found at great altitude, and the microwave sensor would in fact
measure temperature much higher in the atmosphere, i.e. in the
colder parts of the troposphere, thus giving a smaller temperature

The evidence for climate change

For the last 40 years balloon data has been available. In 1958 an
initial network of 540 stations was set up to release rawindsondes,
or balloons, which were regularly released to measure temperature,
relative humidity, and pressure through the atmosphere to a height
of about 20 km, where they burst. By the 1970s the network had
grown to 700–800 stations reporting twice daily. The balloon data
set shows a general surface and lower troposphere warming over
the last 30 years of about 0.1–0.2 °C/10 years, while weak cooling is
seen in the upper troposphere and strong cooling in the

Global Warming

increase than that which actually occurred. It is unsurprising that
reports on satellite recorded global temperature trends for the last
30 years have changed, as every new paper published contains yet
another correction that must be considered. For example, huge
controversy occurred when Christy et al. (1995) deduced a global
mean cooling trend of 0.05°C/decade for the period 1979–94, but
obtained a warming trend of 0.09°C/decade over this period by
~o and the climatic effect of the
removing the effects of El Nin
eruption of Mount Pinatubo. When the data set is corrected for
decreasing satellite altitude, the global mean cooling turns into a
warming of 0.07°C/decade. If the balloon, surface, and satellite
data are compared, there is some agreement and it shows that the
surface and lower troposphere have been warming up, while the
stratosphere has been cooling down. An excellent summary of the
corrections that have been made to each data set and why they were
applied can be found in Harvey (2000).
The Intergovernmental Panel on Climate Change (IPCC) has
collated all the published land-surface air and sea-surface
temperatures from 1861 to 1998, with all the corrections discussed
above. This data is shown relative to the average temperature
between 1961 and 1990 in Figure 13, and, as you can see, there has
been a sharp warming from the start of the 1980s onwards. The
mean global surface temperature has increased by about 0.3 to
0.6°C since the late 19th century. Including the evidence from
balloons and satellites, there seems to be a 0.2 to 0.3°C increase
over the last 40 years, which is the period with most reliable data.
Recent years have been among the warmest since 1860 – the period
for which instrumental records are available. This warming is
evident in both sea-surface and land-based surface air
temperatures. Indirect indicators, such as borehole temperatures
and glacier shrinkage, provide independent support for the
observed warming. It should also be noted that the warming has not
been globally uniform. The recent warming has been greatest
between 40°N and 70°N latitude, though some areas such as the
North Atlantic Ocean have cooled in the recent decades.


Relative sea level
The IPCC has also put together a key data set of sea level. In general
it shows that over the last 100 years, the global sea level has risen by
about 4 to 14 cm (Figure 16). But sea-level change is difficult to
measure, as relative sea-level changes have been derived mainly
from tide-gauge data. In the conventional tide-gauge system, the

The evidence for climate change

There are two global precipitation data sets: ‘Hulme’ and the
‘Global Historical Climate Network’ (GHCN). Unfortunately, unlike
temperature, rainfall and snow records are not as well documented
and the records have not been carried out for as long. It is also
known that precipitation over land tends to be underestimated by
up to 10–15% owing to the effects of airflow around the collecting
dish. The gradual realization and correction of this effect has
produced a spurious upward trend in global precipitation. After
correction, there is an overall increase of precipitation of 1% over
land, which is so small that it cannot be distinguished from zero,
i.e. no change. A detailed view suggests that, taking an average over
the Earth’s land surface, precipitation increased from the start of
the century up to about 1960, but has decreased since about 1980.
But yet again, as with main key data sets concerning global
warming, we have a huge gap, which is due to the lack of data on
precipitation over the oceans. However, what is observed are some
significant changes in where the precipitation has occurred (Figure
15). It seems that precipitation has increased over land at high
latitudes in the northern hemisphere, especially during the cold
season. One study also suggested that there was an increase in
the amount of rain falling during heavy rain events, especially
in the USA, the former Soviet Union, and China. Decreases in
precipitation occurred after the 1960s over the subtropics and the
tropics from Africa to Indonesia. These changes are consistent with
available data analyses of changes in stream flow, lake levels, and
soil surface. In terms of snowfall, Antarctic is a big winner with an
increase of 5–20% over the last two decades, while Greenland has
lost about 20% of its snow accumulation over the last 50 years.

Global Warming

15. Changes in precipitation over land a) 1955–1974 to 1975–1994 and
b) 1900 to 1994

sea level is measured relative to a land-based tide-gauge
benchmark. The major problem is that the land surface is much
more dynamic that one would expect, with a lot of vertical
movements, and these get incorporated into the measurements.
Vertical movements can occur as a result of normal geological
compaction of delta sediments, the withdrawal of groundwater
from coastal aquifers (both of which are discussed in more detail
in Chapter 6, Coastline section), uplift associated with colliding
tectonic plates (the most extreme of which is mountain building
such as the Himalayas), or ongoing postglacial rebound and
compensation elsewhere associated with the end of the last ice age.
The latter is caused by the rapid removal of weight when the giant

One of the biggest unknowns of global warming is whether the
massive ice sheets over Greenland and Antarctica will melt. A key
indicator of the expansion or contraction of these ice sheets is the
sea ice that surrounds them. The state of the cryosphere (or the
global ice) is extremely important, as shrinking of ice on land causes
the sea level to rise. Unfortunately, submarines have already
recorded a worrying thinning of the polar ice caps. Sea-ice draft is
the thickness of the part of the ice that is submerged under the sea.
Therefore, in order to understand the effects of global warming on
the cryosphere it is important to measure how much ice is melting
in the polar regions. Comparison of sea-ice draft data acquired on
submarine cruises between 1993 and 1997 with similar data
acquired between 1958 and 1976 indicates that the mean ice draft at
the end of the melt season has decreased by about 1.3 m in most of
the deep-water portions of the Arctic Ocean, from 3.1 m in 1958–76
to 1.8 m in the 1990s. In summary, ice draft in the 1990s is over a

The evidence for climate change

ice sheets melted, so that the land which has been weighed down
slowly rebounds back to its original position. An example of this is
Scotland, which is rising at a rate of 3 mm/year while England is
still sinking at a rate of 2 mm/year, despite the Scottish ice sheet
having melted 10,000 years ago. Again, using a number of
corrections, the global tide-gauge network suggests that the rise in
sea level since the beginning of the 20th century could be as much
as 18 cm (~1.8±0.1 mm/year). On this timescale, the warming and
the consequent thermal expansion of the oceans may account for
about 2–7 cm of the observed sea-level rise, while the observed
retreat of glaciers may account for about 2–5 cm. Other factors are
more difficult to quantify. The rate of observed sea-level rise
suggests that there may have been a net positive contribution from
the huge ice sheets of Greenland and Antarctica, but observations
of these ice sheets suggest that there may have been a net expansion
which would have contributed −0.05 mm/year to global sea level
over the last 100 years. The ice sheets remain a major source of
uncertainty in accounting for past changes in sea level because of
insufficient data about these ice sheets over the last 100 years.

16. Estimated sea-level rise 1910–1990

metre thinner than two to four decades earlier. The main draft has
decreased from over 3 metres to less than 2 metres and the volume
is down by some 40%. In addition, in 2000, for the first time in
recorded history, a hole large enough to be seen from space opened
in the sea ice above the North Pole. Unfortunately, because satellite
records are so short, we do not know if this is a frequent natural
occurrence or indicative of significant melting of Arctic sea ice.
Moreover, measurements of the size of Greenland suggest that it is
shrinking, particularly at its coastal margins.

Other evidence for global warming

There is evidence too that our weather patterns are changing. For

The evidence for climate change

Other evidence for global warming comes from permafrost regions
and weather patterns such as particular storm records. In the high
latitude and high altitude areas permafrost exists, where it is so cold
that the ground is frozen solid to a great depth. During the summer
months only the top metre or so of the permafrost gets warm
enough to melt, and this is called the active layer. Already in Alaska
there seems to have been a 3°C warming down to at least a metre
over the last 50 years, showing that the active layer has become
deeper. With the massive increases in atmospheric CO2 predicted
for the future, it is likely that there will be increases in the thickness
of the active layer of the permafrost or perhaps, in some areas, the
complete disappearance of so-called discontinuous permafrost over
the next century. This widespread loss of permafrost will produce a
huge range of problems in local areas, as it will trigger erosion or
subsidence, change hydrologic processes, and release into the
atmosphere even more CO2 and methane trapped as organic matter
in the frozen layers. Hence changes in permafrost will reduce the
stability of slopes and thus increase incidence of slides and
avalanches. A more dynamic cryosphere will increase the natural
hazards for people, structures, and communication links. Already
buildings, roads, pipelines, such as the oil pipelines in Alaska, and
communication links are being threatened.

Global Warming

example, in recent years massive storms and subsequent floods
have hit China, Italy, England, Korea, Bangladesh, Venezuela, and
Mozambique. In England in 2000, floods classified as ‘once in
30-year events’ occurred twice in the same month. Moreover, the
winter of 2000/1 was the wettest six months recorded in Britain
since records began in the 18th century, while in the summer of
2003 Britain recorded the first ever temperature of 100°F since
records began. In addition, on average, British birds nest 12±4 days
earlier than 30 years ago. Insect species – including bees and
termites – that need warm weather to survive are moving
northward, and some have already reached England by crossing
the Channel from France. Glaciers in Europe are in retreat,
particularly in the Alps and Iceland. Ice cover records from the
Tornio River in Finland, which has been recorded since 1693, show
that the spring thaw of the frozen river now occurs a month earlier.
There is also evidence that more storms are occurring in the
northern hemisphere. Wave height in the North Atlantic Ocean has
been monitored since the early 1950s, from lightships, Ocean
Weather Stations, and more recently satellites. Between the 1950s
and 1990s the average wave height increased from 2.5 to 3.5 m, an
increase of 40%. Storm intensity is the major control over wave
height, which provides evidence for an increase in storm activity
over the last 40 years. This is supported by German scientists who
suggested that storm-generated ocean waves pounding the coasts of
Europe produce long-wave vibrations which are picked up by the
sensitive equipment set up to record earthquakes. From this
evidence they calculated the number of storm days per month
during the winter. It seems that over the last 50 years these have
increased from seven to 14 days per month. This also fits with the
observed increase in winter extratropical cyclones, i.e. those
occurring in the mid-latitudes, which have increased markedly over
the last hundred years, with significant increases in both the Pacific
and Atlantic sectors since the early 1970s. There has, however, in
contrast, been a slight downturn in the number of hurricanes over
the last 50 years.

17. Mozambique floods of 2000

What do the sceptics say?
One of the best ways to summarize the evidence for global warming
and to persuade you, the reader, that there is evidence that
humanity has already altered global climate, is to review what the
sceptics say against the global warming hypothesis:

Global Warming

1. Ice-core data suggest atmospheric CO2 responds to global
temperature, therefore, atmospheric CO2 cannot cause global
temperature changes.
A detailed examination of the ice-core CO2 data at the end of the
last glacial period shows that the major stepwise increases occur at
the same time as warming in Antarctica. It is known that during the
last de-glaciation, gradual warming in Antarctica occurred before
step-like warming in the northern hemisphere (Figure 18). There
is, therefore, excellent evidence that atmospheric carbon dioxide
increases before overall global temperatures rise and the ice sheets
begin to melt. In fact, there is clear evidence that Antarctic
temperatures and atmospheric carbon dioxide levels are in step
(Figure 18), demonstrating the central role of carbon dioxide as a
climate amplifier. Moreover, time-series analysis of the last four
glacial-interglacial cycles by Professor Shackleton at Cambridge
University suggests atmospheric carbon dioxide response up to
5,000 years before variations in global ice sheets. This has
prompted many palaeoclimatologists to re-evaluate the role of
atmospheric carbon dioxide, placing it now as a primary driving
force of past climate instead of a secondary response and feedback.
2. Every data set showing global warming has been corrected or
tweaked to achieve this desired result.
For people who are not regularly involved in science this seems to be
the biggest problem with the whole ‘global warming has happened’
argument. As I have shown, all the data sets covering the last 150
years require some sort of adjustment. This, though, is part of the

18. Ice core records showing CO2 in phase with Antarctic warming.
A) 18O and D = temperature records, CO2 changing atmospheric
carbon dioxide levels, higher curve taking into account coral reef and
land vegetation changes since the last ice age. B) rate of change of
carbon dioxide most of which occurs in three large pulses

scientific process. For example, if great care had not been taken over
the spurious trends in the global precipitation data base we would
now assume that global precipitation was increasing. Moreover, as
science moves forward incrementally, it gains more and more
understanding and insight into the data sets it is constructing. This
constant questioning of all data and interpretations is the core
strength of science: each new correction or adjustment is due to a
greater understanding of the data and the climate system and thus
each new study adds to the confidence that we have in the results.
This is why the IPCC report refers to the ‘weight of the evidence’, as
our confidence in science increases if similar results are