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. 3
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assist considerably in helping to tie down the proportion that comes from fossil
fuel sources, such as leakage from mines and from natural gas pipelines.
The main process for the removal of methane from the atmosphere is through
chemical destruction. It reacts with hydroxyl (OH) radicals, which are present
in the atmosphere because of processes involving sunlight, oxygen, ozone and
water vapour. The average lifetime of methane in the atmosphere is determined
by the rate of this loss process. At about 12 years13 it is much shorter than the
lifetime of carbon dioxide.
Although most methane sources cannot be identi¬ed very precisely, the largest
sources apart from natural wetlands are closely associated with human activities.
51
OT H E R G R E E N H O U S E G A S E S




Rice paddy ¬elds have an adverse environmental impact because of the large quantities of methane gas
they generate. World methane production due to paddy ¬elds has been estimated to be in the range of
30 to 90 million tonnes per year.



It is interesting to note that the increase of atmospheric methane (Figure 3.6a) fol-
lows very closely the growth of human population since the Industrial Revolution.
However, even without the introduction of deliberate measures to control human-
related sources of methane because of the impact on climate change, it is not
likely that this simple relationship with human population will continue. The
IPCC Special Report on Emission Scenarios (SRES) presented in Chapter 6 include
a wide range of estimates of the growth of human-related methane emissions
during the twenty-¬rst century “ from approximately doubling over the century
to reductions of about 25%. In Chapter 10 (page 305) ways are suggested in which
52 THE GREENHOUSE GASES




Figure 3.6 Change in (a)
(a)
2000
methane and (b) nitrous oxide
concentration (mole fraction in
2000
ppb) over the last 10 000 years
(insets from 1750) determined
1500
from ice cores (symbols with
different colours from different
0.4
1500 1000
studies) and atmospheric




Radiative forcing (Wm“2)
samples (red lines). Radiative
Methane (ppb)




500
forcing since the pre-industrial
1900
1800 2000
era due to the increases is
Year
plotted on the right-hand axes.
0.2
1000




0


500




(b)
330

330



300
Radiative forcing (Wm“2)




270 0.1
300
Nitrous oxide (ppb)




240
1900
1800 2000
Year




0
270




10 000 5000 0
Time (before 2005)
53
OT H E R G R E E N H O U S E G A S E S




Table 3.2 Estimated sources and sinks of methane in millions of tonnes
per year. The ¬rst column of data shows the best estimate from each
source; the second column illustrates the uncertainty in the estimates by
giving a range of values

Best estimate Uncertainty

Sources
Natural
Wetlands 150 (90“240)
Termites 20 (10“50)
Ocean 15 (5“50)
Other (including hydrates) 15 (10“40)
Human-generated
Coal mining, natural gas, petroleum industry 100 (75“110)
Rice paddies 60 (30“90)
Enteric fermentation 90 (70“115)
Waste treatment 25 (15“70)
Land¬lls 40 (30“70)
Biomass burning 40 (20“60)
Sinks
Atmospheric removal 545 (450“550)
Removal by soils 30 (15“45)
Atmospheric increase 22 (35“40)

For some more recent estimates see Table 7.7 in Denman, K. L., Brasseur, G. et al.,
Chapter 7, in Solomon et al. (eds.) Climate Change 2007: The Physical Science Basis.
The ¬gure for atmospheric increase is an average for the 1990s; note that from 1999
to 2005 the increase was close to zero.


methane emissions could be reduced and methane concentrations in the atmos-
phere stabilised. Also see box on page 48“9 for possible destabilisation of methane
emissions from methane hydrates especially at high latitudes.


Nitrous oxide
Nitrous oxide, used as a common anaesthetic and known as laughing gas, is
another minor greenhouse gas. Its concentration in the atmosphere of about
0.3 ppm is rising at about 0.25% per year and is about 16% greater than in
54 THE GREENHOUSE GASES




pre-industrial times (Figure 3.6b). The largest emissions to the atmosphere are
associated with natural and agricultural ecosystems; those linked with human
activities are probably due to increasing fertiliser use. Biomass burning and the
chemical industry (for example, nylon production) also play some part. The sink
of nitrous oxide is photodissociation in the stratosphere and reaction with elec-
tronically excited oxygen atoms, leading to an atmospheric lifetime of about
120 years.


Chloro¬‚uorocarbons (CFCs) and ozone
The CFCs are man-made chemicals which, because they vaporise just below
room temperature and because they are non-toxic and non-¬‚ammable, appear
to be ideal for use in refrigerators, the manufacture of insulation and aerosol
spray cans. Since they are so chemically unreactive, once they are released into
the atmosphere they remain for a long time “ 100 or 200 years “ before being
destroyed. As their use increased rapidly through the 1980s their concentration
in the atmosphere has been building up so that they are now present (adding
together all the different CFCs) in about l ppb (part per thousand million “ or
billion “ by volume). This may not sound very much, but it is quite enough to
cause two serious environmental problems.
The ¬rst problem is that they destroy ozone.14 Ozone (O3), a molecule consist-
ing of three atoms of oxygen, is an extremely reactive gas present in small
quantities in the stratosphere (a region of the atmosphere between about
10 km and 50 km in altitude). Ozone molecules are formed through the action
of ultraviolet radiation from the Sun on molecules of oxygen. They are in turn
destroyed by a natural process as they absorb solar ultraviolet radiation at
slightly longer wavelengths “ radiation that would otherwise be harmful to
us and to other forms of life at the Earth™s surface. The amount of ozone in
the stratosphere is determined by the balance between these two processes,
one forming ozone and one destroying it. What happens when CFC molecules
move into the stratosphere is that some of the chlorine atoms they contain are
stripped off, also by the action of ultraviolet sunlight. These chlorine atoms
readily react with ozone, reducing it back to oxygen and adding to the rate of
destruction of ozone. This occurs in a catalytic cycle “ one chlorine atom can
destroy many molecules of ozone.
The problem of ozone destruction was brought to world attention in 1985
when Joe Farman, Brian Gardiner and Jonathan Shanklin at the British Antarctic
Survey discovered a region of the atmosphere over Antarctica where, during
the southern spring, about half the ozone overhead disappeared. The existence
of the ˜ozone hole™ was a great surprise to the scientists; it set off an inten-
sive investigation into its causes. The chemistry and dynamics of its formation
55
OT H E R G R E E N H O U S E G A S E S




Ozone depletion can be seen by comparing ozone levels in September 1980 and September 2008.
The dark blue and purple areas denote where the ozone layer is thinnest.


turned out to be complex. They have now been unravelled, at least as far as their
main features are concerned, leaving no doubt that chlorine atoms introduced
into the atmosphere by human activities are largely responsible. Not only is
there depletion of ozone in the spring over Antarctica (and to a lesser extent
over the Arctic) but also substantial reduction, of the order of 5%, of the total
column of ozone “ the amount above one square metre at a given point on the
Earth™s surface “ at mid latitudes in both hemispheres.
Because of these serious consequences of the use of CFCs, international action
has been taken. Many governments have signed the Montreal Protocol set up
in 1987 which, together with the Amendments agreed in London in 1991 and
in Copenhagen in 1992, required that manufacture of CFCs be phased out com-
pletely by the year 1996 in industrialised countries and by 2006 in developing
countries. Because of this action the concentration of CFCs in the atmosphere is
no longer increasing. However, since they possess a long life in the atmosphere,
little decrease will be seen for some time and substantial quantities will be
present well over 100 years from now.
So much for the problem of ozone destruction. The other problem with CFCs
and ozone, the one which concerns us here, is that they are both greenhouse
gases.15 They possess absorption bands in the region known as the longwave
atmospheric window (see Figure 2.5) where few other gases absorb. Because, as
we have seen, the CFCs destroy some ozone, the greenhouse effect of the CFCs is
partially compensated by the reduced greenhouse effect of atmospheric ozone.
56 THE GREENHOUSE GASES




First considering the CFCs on their own, a CFC molecule added to the atmos-
phere has a greenhouse effect 5000 to 10 000 times greater than an added mol-
ecule of carbon dioxide. Thus, despite their very small concentration compared,
for instance, with carbon dioxide, they have a signi¬cant greenhouse effect. It is
estimated that radiative forcing due to CFCs is about 0.3 W m’2 “ or about 12% of
the radiative forcing due to all greenhouse gases. This forcing will only decrease
slowly in the twenty-¬rst century.
Turning now to ozone, the effect from ozone depletion is complex because
the amount by which ozone greenhouse warming is reduced depends critically
on the height in the atmosphere at which it is being destroyed. Further, ozone
depletion is concentrated at high latitudes while the greenhouse effect of the
CFCs is uniformly spread over the globe. In tropical regions there is virtually no
ozone depletion so no change in the ozone greenhouse effect. At mid latitudes,
very approximately, the greenhouse effects of ozone reduction and of the CFCs
compensate for each other. In polar regions the reduction in the greenhouse
effect of ozone more than compensates for the greenhouse warming effect of
the CFCs.16
As CFCs are phased out, they are being replaced to some degree by other
halocarbons “ hydrochloro¬‚uorocarbons (HCFCs) and hydro¬‚uorocarbons
(HFCs). In Copenhagen in 1992, the international community decided that
HCFCs would also be phased out by the year 2030. While being less destructive
to ozone than the CFCs, they are still greenhouse gases. The HFCs contain no
chlorine or bromine, so they do not destroy ozone and are not covered by the
Montreal Protocol. Because of their shorter lifetime, typically tens rather than
hundreds of years, the concentration in the atmosphere of both the HCFCs and
the HFCs, and therefore their contribution to global warming for a given rate
of emission, will be less than for the CFCs. However, since their rate of manu-
facture could increase substantially their potential contribution to greenhouse
warming is being included alongside other greenhouse gases (see Chapter 10,
page 296).
Concern has also extended to some other related compounds which are
greenhouse gases, the per¬‚uorocarbons (e.g. CF4, C2F6) and sulphur hexa¬‚uoride
(SF6), which are produced in some industrial processes. Because they possess
very long atmospheric lifetimes, probably more than 1000 years, all emissions
of these gases accumulate in the atmosphere and will continue to in¬‚uence
climate for thousands of years. They are also therefore being included as poten-
tially important greenhouse gases.
Ozone is also present in the lower atmosphere or troposphere, where some
of it is transferred downwards from the stratosphere and where some is gener-
ated by chemical action, particularly as a result of the action of sunlight on the
57
PA R T I C L E S I N T H E ATM O S P H E R E




oxides of nitrogen. It is especially noticeable in polluted atmospheres near the
surface; if present in high enough concentration, it can become a health hazard.
In the northern hemisphere the limited observations available together with
model simulations of the chemical reactions leading to ozone formation suggest
that ozone concentrations in the troposphere have doubled since pre-industrial
times “ an increase which is estimated to have led to a global average radiative
forcing of about 0.35 W m’2 (Figure 3.11). Ozone is also generated at levels in
the upper troposphere as a result of the nitrogen oxides emitted from aircraft
exhausts; nitrogen oxides emitted from aircraft are more effective at producing
ozone in the upper troposphere than are equivalent emissions at the surface.
The radiative forcing in northern mid latitudes from aircraft due to this addi-
tional ozone17 is of similar magnitude to that from the carbon dioxide emitted
from the combustion of aviation fuel which currently is about 3% of current
global fossil fuel consumption.


Gases with an indirect greenhouse effect
I have so far described all the gases present in the atmosphere that have a direct
greenhouse effect. There are also gases which through their chemical action
on greenhouse gases, for instance on methane or on lower atmospheric ozone,
have an in¬‚uence on the overall size of greenhouse warming. Carbon monoxide
(CO) and the nitrogen oxides (NO and NO2) emitted, for instance, by motor vehi-
cles and aircraft are some of these. Carbon monoxide has no direct greenhouse
effect of its own but, as a result of chemical reactions, it forms carbon dioxide.
These reactions also affect the amount of the hydroxyl radical (OH) which in
turn affects the concentration of methane. Emissions of nitrogen oxides, for
instance, result in a small reduction in atmospheric methane which partially
compensates for the increase in ozone due to aircraft mentioned in the last
paragraph. Substantial research has been carried out on the chemical processes
in the atmosphere that lead to these indirect effects on greenhouse gases. It is
of course important to take them properly into account, but it is also important
to recognise that their combined effect is much less than that of the major
contributors to human-generated greenhouse warming, namely carbon dioxide
and methane.


Particles in the atmosphere
Small particles suspended in the atmosphere (often known as aerosol; see
Glossary) affect its energy balance because they both absorb radiation from the
Sun and scatter it back to space. We can easily see the effect of this on a bright
58 THE GREENHOUSE GASES




day in the summer with a light wind when downwind of an industrial area.
Although no cloud appears to be present, the Sun appears hazy. We call it ˜indus-
trial haze™. Under these conditions a signi¬cant proportion of the sunlight inci-
dent at the top of the atmosphere is being lost as it is scattered back and out of
the atmosphere by the millions of small particles (typically between 0.001 and
0.01 mm in diameter) in the haze. The effect of particles can also be seen often
when ¬‚ying over or near industrial or densely populated areas for instance in
Asia when although no cloud is present, it is too hazy to see the ground.18
Atmospheric particles come from a variety of sources. They arise partially
from natural causes; they are blown off the land surface, especially in desert
areas, they result from forest ¬res and they come from sea spray. From time to
time large quantities of particles are injected into the upper atmosphere from
volcanoes “ the Pinatubo volcano which erupted in 1991 provides a good exam-
ple (see Chapter 5). Some particles are also formed in the atmosphere itself,
for instance sulphate particles from the sulphur-containing gases emitted from
volcanoes. Other particles arise from human activities. Over the past ten years a
large number of observations especially from satellite-borne instruments have
provided much needed information about the aerosol distribution from both
natural and anthropogenic sources in both space and time (Figure 3.7a).
The most important of the aerosols from anthropogenic sources are sulphate
particles that are formed as a result of chemical action on sulphur dioxide, a
gas that is produced in large quantities by power stations and other industries
in which coal and oil (both of which contain sulphur in varying quantities) are
burnt. Because these particles remain in the atmosphere only for about ¬ve
days on average, their effect is mainly con¬ned to regions near the sources
of the particles, i.e. the major industrial regions of the northern hemisphere
(Figure 3.7b). Sulphate particles scatter sunlight and provide a negative forc-
ing, globally averaged estimated as ’0.4 ± 0.2 W m’2. Over limited regions of the
northern hemisphere the radiative effect of these particles is comparable in
size, although opposite in effect, to that of human-generated greenhouse gases
up to the present time. Figure 3.8 illustrates a model estimate of the substantial
effect on global atmospheric temperature of removing all sulphate aerosol in
the year 2000.
An important factor that will in¬‚uence the future concentrations of sulphate
particles is ˜acid rain™ pollution, caused mainly by the sulphur dioxide emis-
sions. This leads to the degradation of forests and ¬sh stocks in lakes especially
in regions downwind of major industrial areas. Serious efforts are therefore
under way, especially in Europe and North America, to curb these emissions
to a substantial degree. Although the amount of sulphur-rich coal being burnt
elsewhere in the world, for instance in Asia, is increasing rapidly, the damaging
(a)




0.1 0.2 0.3 0.4 0.5 0.6 0.7

Total aerosol optical depth

(b)




0 5 10 15 20 25 30
Mg [SO4] m “2

Figure 3.7 Distribution of atmospheric aerosols. (a) Total aerosol optical depth at a mid-visible wavelength
(for de¬nition see Glossary) due to natural plus anthropogenic aerosols determined from observations by the
satellite instrument MODIS, averaged from August to October 2001. Also indicated are the locations of aerosol
lidar network sites (red circles). (b) The amount of sulphate (SO4) aerosol in the atmosphere in mg[SO4] m’ 2
from human activities, ˜background™ non-explosive volcanoes and natural di-methyl sulphate (DMS) from
ocean plankton, averaged over the decade of the 1990s calculated by the Hadley Centre model HadGEM1.
60 THE GREENHOUSE GASES




effects of sulphur pollution are such that con-
2.0
Surface air temperature (°C)




trols on sulphur emissions are being extended
1.5
to these regions also. For the globe as a whole
1.0
therefore, sulphur emissions are likely to rise
much less rapidly than emissions of carbon
0.5

dioxide. In fact, they are likely to fall during
0.0
the twenty-¬rst century to below their 2000
“0.5
value (Figure 6.1) thus removing part of the
1950 2000 2050 2100
offset they are currently providing against the
Year
increase in radiative forcing from greenhouse
Figure 3.8 A model calculation of the effect on global
gases.
mean surface air temperature of removing all sulphate
The radiative forcing from particles can be
aerosols in the year 2000 (red line) compared with
positive or negative depending on the nature
maintaining the global burden of sulphate aerosols at
the 2000 level for the twenty-¬rst century (blue line). of the particles. For instance, soot particles
(also called black carbon) from fossil fuel burn-
ing absorb sunlight and possess a positive forcing globally averaged estimated
as 0.2 ± 0.15 W m’2. Other smaller anthropogenic contributions to aerosol radia-
tive forcing come from biomass burning (e.g. the burning of forests), organic
carbon particles from fossil fuel and nitrate and mineral dust particles. Because
of the interactions that occur between particles from different sources and with
clouds it is not adequate to add simply estimated radiative forcing from dif-
ferent particles to ¬nd the total forcing. That is why in Figure 3.11 estimates
are given of the total radiative forcing from aerosol particles together with the
associated uncertainty.
So far for aerosol we have been describing direct radiative forcing. There is
a further way by which particles in the atmosphere could in¬‚uence the cli-
mate; that is through their effect on cloud formation that is described as indirect
radiative forcing. The mechanism of indirect forcing that is best understood
arises from the in¬‚uence of the number of particles and their size on cloud
radiative properties (Figure 3.9). If particles are present in large numbers when
clouds are forming, the resulting cloud consists of a large number of smaller
drops “ smaller than would otherwise be the case “ similar to what happens
as polluted fogs form in cities. Such a cloud will be more highly re¬‚ecting to
sunlight than one consisting of larger particles, thus further increasing the
energy loss resulting from the presence of the particles. Further the droplet size
and number in¬‚uence the precipitation ef¬ciency, the lifetime of clouds and
hence the geographic extent of cloudiness. Figure 3.10 is an illustration of the
effect as it applies in the wakes of ships where clouds form possessing drop sizes
much smaller than those pertaining to other clouds in the vicinity. There is
now substantial observational evidence for these mechanisms but the processes
61
PA R T I C L E S I N T H E ATM O S P H E R E




Figure 3.9 Schematic illustrating More reflection higher albedo
the cloud albedo and lifetime
indirect effect on radiative
forcing. Larger numbers of
smaller particles in polluted
clouds lead to more re¬‚ection
of solar radiation from the cloud
Smaller cloud
top, less radiation at the surface, particles
less
less precipitation and a longer
precipitation
cloud lifetime.
Clean Polluted
Higher optical depth
less radiation at surface




Cloud droplet effective radius ( m)

4 8 12 16 20




Figure 3.10 Cloud droplet radii for ship track clouds and background water clouds in the same region
showing the smaller droplet sizes in the polluted ship track clouds. (Data from MODIS instrument on
NASA™s Aqua satellite.)


involved are not easy to model and will vary a great deal with the particular
situation. Substantial uncertainty therefore remains in estimates of their mag-
nitude as shown in Figure 3.1119. To re¬ne these estimates, more studies are
required especially through making careful measurements on suitable clouds.
62 THE GREENHOUSE GASES




RF values (W m “2)
RF terms Spatial scale LOSU


1.66 [1.49 to 1.83] Global High
CO2
Long-lived
N2O
greenhouse gases 0.48 [0.43 to 0.53]
0.16 [0.14 to 0.18] Global High
Halocarbons
CH4

“0.05 [“0.15 to 0.05] Continental
Med
Ozone Stratospheric Tropospheric to global
0.35 [0.25 to 0.65]
Anthropogenic




Stratospheric water
Global Low
0.07 [0.02 to 0.12]
vapour from CH4
“0.2 [“0.4 to 0.0] Local to Med
Land use
Surface albedo Black carbon continental “ Low
0.1 [0.0 to 0.2]
on snow
Continental Med
Direct effect “0.5 [“0.9 to “0.1]
to global “ Low
Total
aerosol Cloud albedo Continental Low
“0.7 [“1.8 to “0.3]
effect to global

Linear contrails 0.01 [0.003 to 0.03] Continental Low
Natural




Solar irradiance Global Low
0.12 [0.06 to 0.30]


Total net 1.6 [0.6 to 2.4]
anthropogenic

“2 “1 0 1 2
Radiation forcing (W m “2)

Figure 3.11 Global, annual mean radiative forcings (W m “2) due to a number of agents for the period from
pre-industrial (1750) to 2005. The size of the rectangular bar denotes a best estimate value; the horizontal
lines indicate estimates of the uncertainty (90% con¬dence) ranges. To each forcing an indication is given
of the geographical extent (spatial scale) and a ˜level of scienti¬c understanding™ (LOSU) index is accorded.
This latter represents a judgement about the reliability of the forcing estimate involving factors such as the
assumptions necessary to evaluate the forcing, the degree of knowledge of the mechanisms determining the
forcing and the uncertainties surrounding the quantitative estimate of the forcing.



The estimates for the radiative effects of particles as in Figure 3.11 can be
compared with the global average radiative forcing to date due to the increase
in greenhouse gases of about 2.6 W m’2. Comparing global average forcings,
however, is not the whole story. Although the effects of particles on the glo-
bal climate are well indicated by using globally averaged forcing estimates, for
their effects on regional climate, information about their regional distribution
(Figure 3.7) has also to be included (see Chapter 6).
A particular effect on cloudiness arises from aircraft ¬‚ying in the upper
troposphere which in¬‚uence high cloud cover through their emissions of water
63
E S T I M AT E S O F R A D I AT I V E F O RC I N G




vapour and of particles that can act as nuclei on which condensation can occur.
As we shall see in Chapter 5 (page 111) high cloud provides a blanketing effect
on the Earth™s surface similar to that of greenhouse gases and therefore leads to
positive radiative forcing. Extensive formation of contrails in the upper tropo-
sphere by aircraft frequently occurs; an estimate of radiative forcing from this
cause is included in Figure 3.11. Persistent contrails also tend to lead to increased
overall cloudiness in the region where the contrails have formed. This is called
aviation induced cloudiness and is dif¬cult to quantify. Because of these effects of
aircraft and also the effect of increased ozone (reduced by methane reduction)
mentioned on page 57, the overall greenhouse effect of aircraft has been esti-
mated as the equivalent of two or possibly up to four times the effect of their
carbon dioxide emissions.20


Global warming potentials
It is useful to be able to compare the radiative forcing generated by the differ-
ent greenhouse gases. Because of their different lifetimes, the future pro¬le of
radiative forcing due to releases of greenhouse gases varies from gas to gas. An
index called the global warming potential (GWP)21 has been de¬ned for green-
house gases that takes the ratio of the time-integrated radiative forcing from
the instantaneous release of 1 kg of a given gas to that from the release of 1 kg
of carbon dioxide. A time horizon has also to be speci¬ed for the period over
which the integration is carried out. The GWPs of the six greenhouse gases
included in the Kyoto Protocol are listed in Table 10.2. Applying the GWPs to the
emissions from a mixture of greenhouse gases enables the mixture to be con-
sidered in terms of an equivalent amount of carbon dioxide. However, because
the GWPs for different time horizons are very different, GWPs are of limited
application and must be used with care.


Estimates of radiative forcing
This chapter has summarised current scienti¬c knowledge about the sources
and sinks of the main greenhouse gases and aerosol particles, the natural bal-
ances that are maintained between the different components and the way in
which these balances are being disturbed by human-generated emissions.
This information has been employed together with information about the
absorption by the different gases of radiation in different parts of the spec-
trum (see Chapter 2) to calculate the effect of increases in gases and particles
on the amount of net solar radiation entering the atmosphere and of net ther-
mal radiation leaving. Estimates of the radiative forcing from 1750 to 2005
This view over the Chicago and Lake Michigan area was taken (30 November 2003) by a crewmember
on the International Space Station. Aircraft contrails are clearly visible.



for the different greenhouse gases and for tropospheric aerosols of different
origins are brought together in Figure 3.11, together with estimates of their
uncertainty.




SUMM ARY

From Figure 3.11 it will be seen that:
• The dominant forcing for climate change over the last two centuries has
been that from the increase of long-lived greenhouse gases, especially
carbon dioxide.
• Since the mid twentieth century, signi¬cant offset to the positive forcing
from greenhouse gases has arisen from negative forcing due to aerosols,
especially from sulphates.
65
QUESTIONS




• Other smaller forcings are due to changes in ozone (stratospheric and tropo-
spheric), stratospheric water vapour and land surface albedo (for de¬nition
see Glossary) (Figure 3.12) and persistent contrails from aircraft.
• Estimates of changes in solar irradiance are smaller than estimated in the
2001 IPCC report (see Chapter 6, page 166).
• Signi¬cant progress has been made in the understanding and estimating of
indirect aerosol forcing since the 2001 IPCC report “ although substantial
uncertainties remain.

For the future, different assumptions about future emissions of greenhouse
gases and aerosols are used to generate emission scenarios. From these sce-
narios estimates are made (for carbon dioxide, for instance, using a computer
model of the carbon cycle) of likely increases in greenhouse gas concentra-
tions in the future. Details of radiative forcings projected for the twenty-¬rst
century are presented in Chapter 6 (page 142). Chapters 5 and 6 will explain
how estimates of radiative forcing can be incorporated into computer climate
models so as to predict the climate change that is likely to occur because of
human activities. However, before considering predictions of future climate
change, it is helpful to gain perspective by looking at some of the climate
changes that have occurred in the past.



Q U E S TI O N S
QU
1 The lifetime of a carbon dioxide molecule in the atmosphere before it is
exchanged with the ocean is typically less than a year, while the time taken
for an increase in carbon dioxide concentration from fossil fuel burning to
diminish substantially is typically many years. Explain the reasons for this
difference.
2 Estimate how much carbon dioxide you emit each year through breathing.
3 Estimate the size of your share of carbon dioxide emissions from the burning
of fossil fuels.
4 A typical city in the developed world with a population of about one million
produces about half a million tonnes of municipal waste each year. Suppose
the waste is buried in a land¬ll site where the waste decays producing equal
quantities of carbon dioxide and methane. Making assumptions about the
likely carbon content of the waste and the proportion that eventually decays,
estimate the annual production of methane. If all the methane leaks away,
using the information in Note 12, compare the greenhouse effect of the
66 THE GREENHOUSE GASES




Surface albedo (0.86 m)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Figure 3.12 Global surface albedo for the period 1“16 January 2002, showing the proportion of incoming
solar radiation that is re¬‚ected from the Earth™s surface at a wavelength of 0.86 μm. (Data from MODIS
instrument on NASA™s Terra satellite; visualisation by Eric Moody, RS Information Systems Inc.)




carbon dioxide and methane produced from the land¬ll site with that of the
carbon dioxide produced if the waste were incinerated instead. Discuss how
far waste is a ˜renewable™ energy resource.
5 A new forest is planted containing 1 million trees which will mature in 40
years. Estimate the amount of carbon sequestered per year by the forest.
6 Find ¬gures for the amount of fuel used by a typical aircraft and the size of
¬‚eets of the world™s airlines and airforces and estimate the carbon dioxide
emitted globally each year by the world™s aircraft.
7 Search for information about the ozone hole and explain why it occurs
mainly in the Antarctic.
8 What are the main uses of CFCs? Suggest ways in which the emissions of
CFCs to the atmosphere could be reduced more rapidly.
9 Evidence is sometimes presented suggesting that the variations in global
average temperature over the last century or more can all be explained
as due to variations in the energy output of the Sun. There is therefore
67
N OT E S F O R C H A P T E R 3




nothing left to attribute to the increase in greenhouse gases. What is the
fallacy in this argument?
10 With the use of the formula given in the text, calculate the radiative forcing
due to carbon dioxide for atmospheric concentrations of 150, 280, 450,
560 and 1000 ppm.
11 Making approximate assumptions about particle size and scattering proper-
ties, estimate the optical depth equivalent to 25 mg[SO4] m’2 in Figure 3.7b.
Then estimate global average radiative forcing due to sulphate aerosol.




FURTHER READING AND REFERENCE
Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K. B., Tignor, M., Miller,
H. L. (eds.) 2007. Climate Change 2007: The Physical Science Basis. Contribution of
Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on
Climate Change. Cambridge: Cambridge University Press.
Technical Summary (summarises basic information about greenhouse gases, carbon
cycle and aerosols)
Chapter 2 Changes in atmospheric constituents and radiative forcing
Chapter 7 Couplings between changes in the climate system and biogeochemistry

World Resources Institute www.wri.org “ valuable for its catalogue of climate data
(e.g. greenhouse gas emissions).




N OTE S F O R C HA P TE R 3
1 It is convenient to de¬ne radiative forcing as 4 The process has been called the ˜plankton multiplier™:
the radiative imbalance at the top of the Woods, J., Barkmann, W. 1993. The plankton multi-
troposphere rather than at the top of the whole plier: positive feedback in the greenhouse. Journal of
atmosphere. The formal de¬ nition of radiative Plankton Research, 15, 1053“74.
forcing as in the IPCC reports is ˜the change in net 5 For more details see House, J. I. et al. 2003. Reconciling
(down minus up) irradiance (solar plus longwave apparent inconsistencies in estimates of terrestrial
in W m’2) at the tropopause after allowing for CO2 sources and sinks. Tellus, 55B, 345“63.
stratospheric temperatures to readjust to radiative 6 See Jones, C. D., Cox, P. M. 2001. Atmospheric science
equilibrium, but with surface and tropospheric letters. doi.1006/asle. 2001.0041; the respiration rate
varies approximately with a factor 2T“10/10.
temperatures and state held ¬ xed at the
unperturbed values™. 7 Cox, P. M., Betts, R. A., Collins, M., Harris, P.,
2 See Denman, K. L., Brasseur, G. et al. 2007. Chapter 7, Huntingford, C., Jones, C. D. 2004. Amazon dieback
Section 7.3.12, in Solomon et al. (eds.) Climate Change under climate-carbon cycle projections for the 21st
2007: The Physical Science Basis. century. Theoretical and Applied Climatology, 78, 137“56.
3 See Jansen, E., Overpeck, J. et al. 2007. Chapter 6 8 Some recent papers have pointed out the need for
(especially Box 6.2), in Solomon et al. (eds.) Climate urgent research into aspects of the climate/carbon-
Change 2007: The Physical Science Basis. cycle feedback, for instance the importance of
68 THE GREENHOUSE GASES



linking together all components of the ecosystem 25% arises because of its in¬‚uence on the overall
chemistry and dynamics including, for instance, the chemistry of the atmosphere. Increased methane
in¬‚uences of water and nitrogen. See Heimann, M., eventually results in small increases in water
Reichstein, M. 2008. Nature, 451, 289“92; Gruber, N., vapour in the upper atmosphere, in tropospheric
Galloway, J. N. 2008. Nature, 451, 293“6; and ozone and in carbon dioxide, all of which in turn
Friedlingstein, P. 2008. Nature, 451, 297“8. add to the greenhouse effect.
9 In the absence of climate feedbacks the Hadley 13 Taking into account the loss processes due to
model is similar to the other models with slightly reaction with OH in the troposphere, chemical
above average land carbon uptake and slightly reactions and soil loss lead to a lifetime of about
below average ocean uptake. ten years. However, the effective lifetime of meth-
10 The extensive and protracted forest ¬res in ane against a perturbation in concentration in the
Indonesia and neighbouring areas in 1997“8 have atmosphere (the number quoted here) is complex
been estimated to have resulted in the emission to because it depends on the methane concentration.
the atmosphere of 0.8 to 2.6 GtC; this may be one of This is because the concentration of the radical
the reasons for particularly high growth in atmos- OH (interaction with which is the main cause of
pheric carbon dioxide in 1998. methane destruction), due to chemical feedbacks,
11 In the 1990s the rate of increase substantially is itself dependent on the methane concentration
slowed. The reason for this is not known but one (more details in Prather et al. 2001. Chapter 4,
suggestion is that, because of the collapse of the in Houghton et al. (eds.), Climate Change 2001: The
Russian economy, the leakage from Siberian natural Scienti¬c Basis).
gas pipelines was much reduced. 14 For more detail, see Scienti¬c Assessment of Ozone
12 The ratio of the enhanced greenhouse effect of a Depletion: 2003. Geneva: World Meteorological
molecule of methane compared to a molecule of Organization.
carbon dioxide is known as its global warming 15 Prather et al., Chapter 4, in Houghton et al. (eds.),
potential (GWP); a de¬ nition of GWP is given later Climate Change 2001: The Scienti¬c Basis.
in the chapter. The ¬gure of about 8 given here 16 More detail on this and the radiative effects
for the GWP of methane is for a time horizon of 100 of minor gases and particles can be found in
years “ see Lelieveld, J., Crutzen, P. J. 1992. Nature, Ramaswamy et al. 2001, Chapter 6, in Houghton et
355, 339“41; see also Prather et al., Chapter 4, and al. (eds.), Climate Change 2001: The Scienti¬c Basis.
Ramaswamy et al., Chapter 6, in, Houghton, J. T., 17 More detail in Penner, J. E. et al. (eds.), 1999. Aviation
Ding, Y., Griggs, D. J., Noguer, M., van der Linden, P., and the Global Atmosphere. An IPCC Special Report.
Dai, X., Maskell, K., Johnson, C. A. (eds.) 2001. Cambridge: Cambridge University Press.
Climate Change 2001: The Scienti¬c Basis. Contribution 18 See for instance Ramanathan, V. et al. 2007,
of Working Group 1 to the Third Assessment Report of the Warming trends in Asia ampli¬ed by brown cloud
Intergovernmental Panel on Climate Change. Cambridge: solar absorption. Nature, 448, 575“8.
Cambridge University Press. The GWP is also often 19 For variations of radioactive forcing from different
expressed as the ratio of the effect for unit mass forcing agents over the period 1880“2004, see James
of each gas in which case the GWP for methane Hansen et al., Science 308, 1431.
(whose molecular mass is 0.36 of that of carbon 20 More detail in Penner et al. (eds.), Aviation and the
dioxide) becomes about 23 for the 100-year time Global Atmosphere.
horizon. About 75% of the contribution of methane 21 More detail on GWPs can be found in Ramaswamy
to the greenhouse effect is because of its direct et al., Chapter 6, in Houghton et al. (eds.), Climate
effect on the outgoing thermal radiation. The other Change 2001: The Scienti¬c Basis.
4
Climates of the past




Satellite image of the termini of retreating glaciers in the Himalayan Mountains of Bhutan.




T O OBTAIN some perspective against which to view future climate change, it is helpful to look
at some of the climate changes that have occurred in the past. This chapter will brie¬‚y consider
climatic records and climate changes in three periods: the last hundred years, then the last thousand
years and ¬nally the last million years. At the end of the chapter some interesting evidence for the
existence of relatively rapid climate change at various times during the past one or two hundred
thousand years will be presented.
70 C L I M AT E S O F T H E PA S T




The last hundred years
The 1980s and 1990s and the early years of the twenty-¬rst century have brought
unusually warm years for the globe as a whole as is illustrated in Figure 4.1,
which shows the global average temperature since 1850, the period for which the
instrumental record is available with good accuracy and coverage. An increase
over this period has taken place of 0.76 ± 0.19 °C (Figure 4.1a). The two warmest
years in the record are 1998 and 2005, 1998 ranking highest on one estimate
and 2005 highest on two other estimates. Also 12 of the 13 years 1995 to 2007
rank amongst the 13 warmest years in the whole record. A further striking sta-
tistic is that each of the ¬rst eight months of 1998 was very likely1 the warmest of
those months in the record up to that date. Although there is a distinct trend in
the record, the increase is by no means a uniform one. In fact, some periods of
cooling as well as warming have occurred and an obvious feature of the record
is the degree of variability from year to year and from decade to decade.
Note also that there has been little if any average increase in warming dur-
ing the years 2001“2006. Some have tried to argue that this shows the warm-
ing is over. However, as the ¬gure illustrates, seven years of record is too short
a period to establish a trend. Although the year 2007 was slightly cooler that
2006, the ¬rst seven years of the twenty-¬rst century were on average nearly
0.2 °C warmer than the last seven years of the twentieth century, even though
1998 was the warmest year so far. Further, studies of interannual variability
in the record demonstrate the strong in¬‚uence of variations in El Ni±o and
suggest that interannual variability may continue to offset anthropogenic
warming until around 2009.2
Shown also in Figure 4.1b are the patterns of recent warming at the surface
and averaged over the troposphere. Warming within the atmosphere is more
spatially uniform than the surface record which shows more warming over
land than over the ocean (see also Figure 4.2).
A sceptic may wonder how diagrams like those in Figure 4.1 can be prepared
and whether any reliance can be placed upon them. After all, temperature var-
ies from place to place, from season to season and from day to day by many tens
of degrees. But here we are not considering changes in local temperature but in
the average over the whole globe. A change of a few tenths of a degree in that
average is a large change.
First of all, just how is a change in global average temperature estimated from
a combination of records of changes in the near-surface temperature over land
and changes in the temperature of the sea surface? To estimate the changes
over land, weather stations are chosen where consistent observations have
been taken from the same location over a substantial proportion of the whole
71
THE L AST HUNDRED YEARS



0.6 14.6
(a)




Estimated actual global mean temperature (°C)
0.4 14.4
Difference (°C) from 1961“90



0.2 14.2



0.0 14.0



“0.2 13.8



“0.4 13.6



“0.6 13.4



“0.8 13.2
1860 1880 1900 1920 1940 1960 1980 2000
Year
Period Rate
°C per decade
Years
Annual mean
25 0.177±0.052
Smoothed series 50 0.128±0.026
100 0.074±0.018
5 “ 95% decadel error bars
150 0.045±0.012




Surface Troposphere
(b)




“0.75 “0.65 “0.55 “0.45 “0.35 “0.25 “0.15 “0.05 0 0.05 0.15 0.25 0.35 0.45 0.55 0.65 0.75
°C per decade

Figure 4.1 (a) Variations of the globally averaged Earth™s surface temperature (combined land surface air
temperature and sea surface temperature) for 1850“2006 relative to the 1961“90 mean. The black dots show
annual means and the right-hand axis shows the estimated actual average temperature. Linear trend ¬ts are
shown to the last 25, 50, 100 and 150 years indicating accelerated warming. (b) Patterns of linear global
temperature trends from 1979 to 2005 estimated at the surface (left), and for the troposphere (right) from the
surface to about 10 km altitude from satellite records. Grey areas indicate incomplete data.
72 C L I M AT E S O F T H E PA S T




Figure 4.2 Decadally smoothed
0.8
annual anomalies of global
HadSSt2 average sea surface temperature
Global anomaly (°C) relative to 1961“ 90




0.6
CRUTEM3 (blue), night marine air
HadMAT temperature (green) and land
0.4
surface air temperature (red)
relative to their 1961“90 means.
0.2


0.0


“0.2


“0.4


“0.6


“0.8
1860 1880 1900 1920 1940 1960 1980 2000
Year




Atmospheric temperature observed by satellites
Since 1979 meteorological satellites ¬‚own by the National Oceanic and Atmospheric Administration
(NOAA) of the United States have carried a microwave instrument, the Microwave Sounding Unit (MSU),
for the remote observation of the average temperature of the lower part of the atmosphere up to about
7 km in altitude.
Figure 4.3b shows the record of global average temperature deduced from the MSU and compares it
with data from sounding instruments carried on balloons for the same region of the atmosphere, showing
very good agreement for the period of overlap. Figure 4.3c shows the record of surface air temperature
for the same period. All three measurements show similar variability, the variations at the surface tracking
well with those in the lower troposphere. The plots also illustrate the dif¬culty of deriving accurate trends
from a short period of record where there is also substantial variability. Since 1979 the trend in the MSU
observations of 0.12 to 0.19 °C per decade shows good agreement with the trend in surface observations
of 0.16 to 0.18 °C per decade.
In the stratosphere the temperature trends are reversed (Figure 4.3a) ranging from a decrease of about
0.5 °C per decade in the lower stratosphere to 2.5 °C per decade in the upper stratosphere.
73
THE L AST HUNDRED YEARS




Lower stratosphere
(a)
1.5

1.0

0.5
°C




0.0
Satellite data
“0.5
Balloon instruments
“0.1

1960 1970 1980 1990 2000


Lower troposphere
(b)
Satellite data
0.5
Balloon instruments
°C




0.0


“0.5



1960 1970 1980 1990 2000


(c) Surface
Surface
Volcanic eruption
0.5
°C




0.0


“0.5



Agung El Chichón 1990 Pinatubo
1960 1970 1980 2000

Figure 4.3 Time series of analyses of observations of global average
temperature (°C) (relative to average for 1979“97) (a) for the lower
stratosphere (∼13 to 20 km) from balloon instruments (blue and red)
and since 1979 from satellite MSUs (purple and brown); (b) for the
lower troposphere (up to ∼7 km) from balloon instruments (blue and
red) and since 1979 from satellite MSUs (purple and brown); (c) for the
surface. All time series are monthly mean anomalies relative to 1979 to
1997 smoothed with a seven-month running mean ¬lter. Times of major
volcanic eruptions are indicated by vertical lines.
74 C L I M AT E S O F T H E PA S T




140-year period. Changes in sea surface temperature have been estimated by
processing over 60 million observations from ships “ mostly merchant ships “
over the same period. All the observations, from land stations and from ships,
are then located within a grid of squares, say 1° of latitude by 1° of longitude,
covering the Earth™s surface. Observations within each square are averaged; the
global average is obtained by averaging (after weighting them by area) over the
averages for each of the squares.
A number of research groups in different countries have made careful and
independent analyses of these observations. In somewhat different ways they
have made allowances for factors that could have introduced arti¬cial changes
in the records. For instance, the record at some land stations could have been
affected by changes in their surroundings as these have become more urban.
In the case of ships, the standard method of observation used to be to insert
a thermometer into a bucket of water taken from the sea. Small changes of
temperature have been shown to occur during this process; the size of the
changes varies between day and night and is also dependent on several other
factors including the material from which the bucket is made “ over the years
wooden, canvas and metal buckets have been variously employed. Nowadays, a
large proportion of the observations are made by measuring the temperature of
the water entering the engine cooling system. Careful analysis of the effects of
these details on observations both on land and from ships has enabled appropri-
ate corrections to be made to the record, and good agreement has been achieved
between analyses carried out at different centres.
Con¬dence that the observed variations are real is increased by noticing that
the trend and the shape of the changes are similar when different selections
of the total observations are made. For instance, the separate records from the
land and sea surface (Figures 4.1b and 4.2) and from the northern and southern
hemispheres are closely in accord. Further indirect indicators such as changes
in borehole temperatures and sub-surface ocean temperatures, decrease in
snow cover and glacier shrinkage provide independent support for the observed
warming (Table 4.1.)
During the last 30 years or so observations have been available from satellites
orbiting around the Earth. Their great advantage is that they automatically pro-
vide data with global coverage, which are often lacking in other data sets. The
length of the record from satellites, however, is less than 30 years, a compara-
tively short period in climate terms. At the time of the IPCC 2001 assessment
there were suggestions that the satellite measurements of lower atmospheric
temperature since 1979 showed a substantially smaller warming trend than the
surface observations. However, more careful analyses since 2001 of the satellite
75
THE L AST HUNDRED YEARS




observations now bring the two trends into agreement within their respective
uncertainties (see box).
The most obvious feature of the climate record illustrated in Figure 4.1 is that
of considerable variability, not just from year to year, but from decade to decade.
Some of this variability will have arisen through causes external to the atmos-
phere and the oceans, for instance as a result of volcanic eruptions such as those
of Krakatoa in 1883 or of Pinatubo in the Philippines in 1991 (the low global
average temperature in 1992 and 1993, compared with neighbouring years, is
almost certainly due to the Pinatubo volcano). But there is no need to invoke vol-
canoes or other external causes to explain all the variations in the record. Many
of them result from internal variations within the total climate system, for
instance between different parts of the ocean (see Chapter 5 for more details).
The warming during the twentieth century has not been uniform over the
globe. For instance, the recent warming has been greatest over northern hemi-
sphere continents at mid to high latitudes. There have also been areas of cool-
ing, for instance over some parts of the North Atlantic ocean associated with
changes in ocean circulation (see Chapter 5). Some of the regional patterns of
temperature change are related to different phases of atmosphere“ocean oscil-
lations, such as the El Ni±o Southern Oscillation (ENSO) and the North Atlantic
Oscillation (NAO). The positive phase of the NAO, with high pressure over the
sub-tropical Atlantic and southern Europe and mild winters over northwest
Europe, has tended to be dominant since the mid 1980s.
An interesting feature of the increasing temperature during the last few
decades has been that, in the daily cycle of temperature, minimum tempera-
tures over land have increased about twice as much as maximum temperatures.
A likely explanation for this, in addition to the effects of enhanced greenhouse
gases, is an increase in cloud cover which has been observed in many of the areas
with reduced temperature range. An increase in cloud tends to obstruct daytime
sunshine and tends also to reduce the escape of terrestrial radiation at night.
As might be expected the increases in temperature have led on average to
increases in precipitation, although precipitation shows even more variability
in both space and time than temperature. The increases have been particularly
noticeable in the northern hemisphere in mid to high latitudes, often appearing
particularly as increases in heavy rainfall events (see Table 4.1).
The broad features of these changes in temperature and precipitation are con-
sistent with what is expected because of the in¬‚uence of increased greenhouse
gases (see Chapter 5), although there is much variability in the record that arises
for reasons not associated with human activities. For instance, the particular
increase from 1910 to 1940 (Figure 4.1a) is too rapid to have been due to the rather
76 C L I M AT E S O F T H E PA S T




Table 4.1 Twentieth-century changes in the Earth™s atmosphere, climate and biophysical
system

Indicator Observed changes

Concentration indicators
Atmospheric concentration of CO2 280 ppm for the period 1000“1750 to 368 ppm in year 2000
(31 ± 4% increase) “ 380 ppm in 2006
Terrestrial biospheric CO2 exchange Cumulative source of about 30 GtC between the years 1800 and
2000; but during the 1990s a net sink of about 10 ± 6 GtC
Atmospheric concentration of CH4 700 ppb for the period 1000“1750 to 1750 ppb in year 2000
(151 ± 25% increase) “ 1775 ppb in 2005
Atmospheric concentration of N2O 270 ppb for the period 1000“1750 to 316 ppb in the year 2000
(17 ± 5% increase) “ 319 ppb in 2005
Increased by 35 ± 15% from the years 1750 to 2000, varies
Tropospheric concentration of O3
with region
Stratospheric concentration of O3 Decreased since 1970, varies with altitude and latitude
Atmospheric concentrations of HFCs, Increased globally over the last
PFCs and SF6 50 years

Weather indicators
Increased by 0.6 ± 0.2 °C over the twentieth century
Global mean surface temperature
“ 0.74 ± 0.18 over 100 years 1906“2005; land areas warmed
more than the oceans (very likely)
Northern hemisphere surface Increase over the twentieth century greater than during any
temperature other century in the last 1000 years; 1990s warmest decade of
the millennium (likely)
Diurnal surface temperature range Decreased over the years 1950 to 2000 over land; night-time
minimum temperatures increased at twice the rate of daytime
maximum temperatures (likely)
Hot days/heat index Increased (likely)
Cold/frost days Decreased for nearly all land areas during the twentieth century
(very likely)
Continental precipitation Increased by 5“10% over the twentieth century in the northern
hemisphere (very likely), although decreased in some regions
(e.g. north and west Africa and parts of the Mediterranean)
Heavy precipitation events Increased at mid and high northern latitudes (likely)
Drought Increased summer drying and associated incidence of drought in
a few areas (likely). Since 1970s, increase in total area affected
in many regions of the world (likely)
77
THE L AST HUNDRED YEARS




Table 4.1 (Cont.)

Tropical cyclones Since 1970s, trend towards longer lifetimes and greater storm
Intense extratropical storms intensity but no trend in frequency (likely) Since 1950s, net
increase in frequency/intensity and poleward shift in track
(likely)
Biological and physical indicators

Global mean sea level Increased at an average annual rate of 1“2 mm during the twentieth
century “ rising to about 3 mm from 1993“2003
Duration of ice cover of rivers and Decreased by about two weeks over the twentieth century in
lakes mid and high latitudes of the northern hemisphere (very likely)
Arctic sea-ice extent and thickness Thinned by 40% in recent decades in late summer to early
autumn (likely) and decreased in extent by 10“15% since the
1950s in spring and summer
Non-polar glaciers Widespread retreat during the twentieth century
Snow cover Decreased in area by 10% since global observations became
available from satellites in the 1960s (very likely)
Permafrost Thawed, warmed and degraded in parts of the polar, sub-polar
and mountainous regions
El Ni±o events Became more frequent, persistent and intense during the last 30
years compared to the previous 100 years
Growing season Lengthened by about one to four days per decade during the last
50 years in the northern hemisphere, especially at higher latitudes
Plant and animal ranges Shifted poleward and up in elevation for plants, insects, birds
and ¬sh
Breeding, ¬‚owering and migration Earlier plant ¬‚owering, earlier bird arrival, earlier dates of
breeding season and earlier emergence of insects in the northern
hemisphere
Coral reef bleaching Increased frequency, especially during El Ni±o events

Economic indicators
Weather-related economic losses Global in¬‚ation-adjusted losses rose by an order of magnitude
over the last 50 years. Part of the observed upward trend is
linked to socio-economic factors and part is linked to climatic
factors

Note: This table provides examples of key observed changes and is not an exhaustive list. It includes both changes
attributable to anthropogenic climate change and those that may be caused by natural variations or anthropogenic
climate change. Con¬dence levels (for explanation see Note 1) are reported where they are explicitly assessed by the
relevant Working Group of the IPCC.
78 C L I M AT E S O F T H E PA S T




small increase in greenhouse
Difference from 1961“ 90 (mm)




gases during that period. The
50
particular reasons for this
0
will be discussed in the next
chapter where comparisons
“50
of observed temperatures
“100
with simulations from cli-
mate models for the whole
“150
of the twentieth century
will be presented, not just as
1860 1900 1950 2000
they concern the global mean
Year
but also the regional pat-
Figure 4.4 Global average sea level from tide gauge (blue) and satellite
(red) data relative to the 1961“90 mean. terns of change. We conclude
therefore that, although the
expected signal is still emerging from the noise of natural variability, most of
the observed warming over the last 50 years is very likely to have been due to the
increase in greenhouse gas concentrations.
Signi¬cant cooling of the lower stratosphere (the region at altitudes between
about 10 and 30 km) has been observed over the last two decades (Figure 4.3a).
This is to be expected both because of the decrease in the concentration of ozone
(which absorbs solar radiation) and because of the increased carbon dioxide
concentration which leads to increased cooling at these levels (see Chapter 2).
Because warming of the troposphere and cooling of the stratosphere are occur-
ring because of the increase in greenhouse gases, an increase on average in the
height of the tropopause, the boundary between the troposphere and the strato-
sphere, is also expected. There is now observational evidence for this increase.
A further source of information regarding climate change comes from meas-
urements of change in sea level (Figure 4.4). Over the twentieth century sea
level rose by 17 ± 5 cm. The rate of rise increased to 3.1 ± 0.7 cm over the decade
1993 to 2003 of which 1.6 ± 0.5 cm is estimated to be from thermal expansion of
the ocean as its average temperature increased and 1.2 ± 0.4 cm from the melt-
ing of glaciers that have generally been retreating over the last century. The net
contribution from the Greenland and Antarctic ice caps is more uncertain but
is believed to be small.
In Chapter 1, we mentioned the increasing vulnerability of human popula-
tions to climate extremes, which has brought about more awareness of recent
extremes in the forms of ¬‚oods, droughts, tropical cyclones and windstorms.
It is therefore of great importance to know whether there is evidence of an
increase in the frequency or severity of these and other extreme events. The
available evidence regarding how these and other relevant parameters have
79
THE L AST THOUSAND YEARS




Nukuoro Atoll, Federated States of Micronesia is home to 900 people around the 6-km wide lagoon. It is
part of an island chain that stretches northeast of Papua New Guinea in the western Paci¬c. It was reported
in November 2005 that the islands have progressively become uninhabitable, with an estimate of their total
submersion by 2015. Storm surges and high tides continue to wash away homes, destroy vegetable gardens
and contaminate fresh water supplies. Photographed from the International Space Station.



changed during the twentieth century is summarised in Table 4.1 in terms of
different indicators: concentrations of greenhouse gases; temperature, hydro-
logical and storm-related indicators; and biological and physical indicators. To
what extent these changes are expected to continue or to intensify during the
twenty-¬rst century will be addressed in Chapter 6.


The last thousand years
The detailed systematic record of weather parameters such as temperature, rain-
fall, cloudiness and the like presented above, covering a good proportion of the
globe over the last 140 years, is not available for earlier periods. Further back, the
record is more sparse and doubt arises over the consistency of the instruments
80 C L I M AT E S O F T H E PA S T
Temperature anomaly (°C) relative to 1961“90




0.5




0.0




“0.5




“1.0


Overlap of reconstructed temperatures
800 1000 1200 1400 1600 1800 2000
Year


0 10 20 30 40 50 60 70 80 90 %

Figure 4.5 Northern hemisphere temperature anomalies (relative to the 1961“90 mean) during the last 1300
years from ten published overlapping reconstructions from proxy records “ e.g. tree rings, corals, ice cores and
historical records “ (shadings), and from 1860 from instrumental data (black line). For the percentage shading
scale, temperatures within ± 1 standard error (SE) of a reconstruction score 10% and regions within the
5“95% uncertainty range score 5%. The maximum of 100% is obtained only for temperatures that fall
within ± 1 SE of all ten reconstructions.



used for observation. Most thermometers in use 200 years ago were not well
calibrated or carefully exposed. However, many diarists and writers kept records
at different times; from a wide variety of sources weather and climate informa-

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