. 2
( 13)


been increasing over the past two hundred years and more substantially
over the past ¬fty years. To identify climate change related to this car-
bon dioxide increase, we need to look for trends in global warming over
similar lengths of time. They are long compared with both the memories
of a generation and the period for which accurate and detailed records
The problem of global warming 9

exist. Although, therefore, it can be ascertained that there was more
storminess, for instance, in the region of the north Atlantic during the
1980s and 1990s than in the previous three decades, it is dif¬cult to know
just how exceptional those decades were compared with other periods
in previous centuries. There is even more dif¬culty in tracking detailed
climate trends in many other parts of the world, owing to the lack of
adequate records; further, trends in the frequency of rare events are not
easy to detect.
The generally cold period worldwide during the 1960s and early
1970s caused speculation that the world was heading for an ice age. A
British television programme about climate change called ˜The ice age
cometh™ was prepared in the early 1970s and widely screened “ but the
cold trend soon came to an end. We must not be misled by our relatively
short memories.
What is important is continually to make careful comparisons be-
tween practical observations of the climate and its changes and what
scienti¬c knowledge leads us to expect. During the last few years, as the
occurrence of extreme events has made the public much more aware of
environmental issues,4 scientists in their turn have become more sure
about just what human activities are doing to the climate. Later chapters
will look in detail at the science of global warming and at the climate
changes that we can expect, as well as investigating how these changes
¬t in with the recent climate record. Here, however, is a brief outline of
our current scienti¬c understanding.

The problem of global warming
Human activities of all kinds whether in industry, in the ¬eld (e.g. de-
forestation) or concerned with transport or the home are resulting in
emissions of increasing quantities of gases, in particular the gas car-
bon dioxide, into the atmosphere. Every year these emissions currently
add to the carbon already present in atmospheric carbon dioxide a fur-
ther seven thousand million tonnes, much of which is likely to remain
there for a period of a hundred years or more. Because carbon dioxide
is a good absorber of heat radiation coming from the Earth™s surface,
increased carbon dioxide acts like a blanket over the surface, keeping
it warmer than it would otherwise be. With the increased temperature
the amount of water vapour in the atmosphere also increases, providing
more blanketing and causing it to be even warmer.
Being kept warmer may sound appealing to those of us who live in
cool climates. However, an increase in global temperature will lead to
global climate change. If the change were small and occurred slowly
enough we would almost certainly be able to adapt to it. However, with
10 Global warming and climate change

rapid expansion taking place in the world™s industry the change is un-
likely to be either small or slow. The estimate I present in later chap-
ters is that, in the absence of efforts to curb the rise in the emissions
of carbon dioxide, the global average temperature will rise by about a
third of a degree Celsius every ten years “ or about three degrees in a
This may not sound very much, especially when it is compared with
normal temperature variations from day to night or between one day and
the next. But it is not the temperature at one place but the temperature
averaged over the whole globe. The predicted rate of change of three
degrees a century is probably faster than the global average tempera-
ture has changed at any time over the past ten thousand years. And as
there is a difference in global average temperature of only about ¬ve or
six degrees between the coldest part of an ice age and the warm periods
in between ice ages (see Figure 4.4), we can see that a few degrees in this
global average can represent a big change in climate. It is to this change
and especially to the very rapid rate of change that many ecosystems and
human communities (especially those in developing countries) will ¬nd
it dif¬cult to adapt.
Not all the climate changes will in the end be adverse. While some
parts of the world experience more frequent or more severe droughts,
¬‚oods or signi¬cant sea level rise, in other places crop yields may increase
due to the fertilising effect of carbon dioxide. Other places, perhaps
for instance in the sub-arctic, may become more habitable. Even there,
though, the likely rate of change will cause problems: large damage to
buildings will occur in regions of melting permafrost, and trees in sub-
arctic forests like trees elsewhere will need time to adapt to new climatic
Scientists are con¬dent about the fact of global warming and climate
change due to human activities. However, substantial uncertainty remains
about just how large the warming will be and what will be the patterns
of change in different parts of the world. Although some indications can
be given, scientists cannot yet say in precise detail which regions will be
most affected. Intensive research is needed to improve the con¬dence in
scienti¬c predictions.

Adaptation and mitigation
An integrated view of anthropogenic climate change is presented in
Figure 1.5 where a complete cycle of cause and effect is shown. Begin
in the lower right-hand corner where economic activity, both large and
small scale, whether in developed or developing countries, results in
emissions of greenhouse gases (of which carbon dioxide is the most
Adaptation and mitigation 11

Climate Change - an integrated framework

Impacts on human
Climate Change
and natural systems
Temperature rise Food and water resources
Sea level rise Ecosystem and biodiversity
Precipitation change Human settlements
Droughts and floods Human health


Emissions and

development paths
Economic growth
Greenhouse gases Technology

important) and aerosols. Moving in a clockwise direction around the Figure 1.5 Climate
change “ an integrating
diagram, these emissions lead to changes in atmospheric concentrations
framework; see text for
of important constituents that alter the energy input and output of the
climate system and hence cause changes in the climate. These climate
changes impact both humans and natural ecosystems altering patterns of
resource availability and affecting human livelihood and health. These
impacts in their turn affect human development in all its aspects. An
anticlockwise arrow represents other effects of development on human
communities and natural systems, for instance changes in land use that
lead to deforestation and loss of biodiversity.
Figure 1.5 also shows how both causes and effects can be changed
through adaptation and mitigation. In general adaptation is aimed at
reducing the effects and mitigation is aimed at reducing the causes of
climate change, in particular the emissions of the gases that give rise to it.
12 Global warming and climate change

Uncertainty and response
Predictions of the future climate are surrounded with considerable un-
certainty that arises from our imperfect knowledge both of the science
of climate change and of the future scale of the human activities that
are its cause. Politicians and others making decisions are therefore faced
with the need to weigh all aspects of uncertainty against the desirabil-
ity and the cost of the various actions that can be taken in response
to the threat of climate change. Some mitigating action can be taken
easily at relatively little cost (or even at a net saving of cost), for in-
stance the development of programmes to conserve and save energy, and
many schemes for reducing deforestation and encouraging the planting
of trees. Other actions such as a large shift to energy sources that are
free from signi¬cant carbon dioxide emissions (for example, renewable
sources “ biomass, hydro, wind, or solar energy) both in the developed
and the developing countries of the world will take some time. Because
however of the long timescales that are involved in the development of
new energy infrastructure and in the response of the climate to emis-
sions of gases like carbon dioxide, there is an urgency to begin these
actions now. As we shall argue later (Chapter 9), to ˜wait and see™ is an
irresponsible response.
In the following chapters I shall ¬rst explain the science of global
warming, the evidence for it and the current state of the art regarding
climate prediction. I shall then go on to say what is known about the
likely impacts of climate change on human life “ on water and food
supplies for instance. The questions of why we should be concerned for
the environment and what action should be taken in the face of scien-
ti¬c uncertainty are followed by consideration of the technical possibil-
ities for large reductions in the emissions of carbon dioxide and how
these might affect our energy sources and usage, including means of
Finally I will address the issue of the ˜global village™. So far as the
environment is concerned, national boundaries are becoming less and
less important; pollution in one country can now affect the whole world.
Further, it is increasingly realised that problems of the environment are
linked to other global problems such as population growth, poverty, the
overuse of resources and global security. All these pose global challenges
that must be met by global solutions.

1 Look through recent copies of newspapers and magazines for articles which
mention climate change, global warming or the greenhouse effect. How
many of the statements made are accurate?
Notes 13

2 Make up a simple questionnaire about climate change, global warming and
the greenhouse effect to ¬nd out how much people know about these sub-
jects, their relevance and importance. Analyse results from responses to the
questionnaire in terms of the background of the respondents. Suggest ways
in which people could be better informed.

Notes for Chapter 1
1 See Table 8.3 in Vellinga, P. V., Mills, E., Bowers, L., Berz, G., Huq,
S., Kozak, L., Paultikof, J., Schanzenbacker, B., Shida, S., Soler, G., Benson,
C., Bidan, P., Bruce, J., Huyck, P., Lemcke, G., Peara, A., Radevsky, R.,
van Schoubroeck, C., Dlugolecki, A. 2001. Insurance and other ¬nancial
services. In McCarthy, J. J., Canziani, O., Leary, N. A., Dokken, D. J., White,
K. S. (eds.) 2001. Climate Change 2001: Impacts, Adaptation and Vulnera-
bility. Contribution of Working Group II to the Third Assessment Report of
the Intergovernmental Panel on Climate Change. Cambridge: Cambridge
University Press, Chapter 8.
2 Including windstorms, hurricanes or typhoons, ¬‚oods, tornadoes, hail-
storms, blizzards but not including droughts because their impact is not
immediate and occurs over an extended period.
3 A description of the variety of El Ni˜ o events and their impacts on different
communities worldwide over centuries of human history can be found in a
recent paperback by Ross Couiper-Johnston, El Ni˜ o: the Weather Pheno-
mena that Changed the World. 2000. London: Hodder and Stoughton.
4 A gripping account of some of the changes over the last decades can be
found in a recent book by Mark Lynas, High Tides: News from a Warming
World. 2004. London: Flamingo.
Chapter 2
The greenhouse effect

The basic principle of global warming can be understood by considering
the radiation energy from the Sun that warms the Earth™s surface and
the thermal radiation from the Earth and the atmosphere that is radiated
out to space. On average these two radiation streams must balance.
If the balance is disturbed (for instance by an increase in atmospheric
carbon dioxide) it can be restored by an increase in the Earth™s surface

How the Earth keeps warm
To explain the processes that warm the Earth and its atmosphere, I will
begin with a very simpli¬ed Earth. Suppose we could, all of a sud-
den, remove from the atmosphere all the clouds, the water vapour, the
carbon dioxide and all the other minor gases and the dust, leaving an
atmosphere of nitrogen and oxygen only. Everything else remains the
same. What, under these conditions, would happen to the atmospheric
The calculation is an easy one, involving a relatively simple radiation
balance. Radiant energy from the Sun falls on a surface of one square
metre in area outside the atmosphere and directly facing the Sun at a rate
of about 1370 watts “ about the power radiated by a reasonably sized
domestic electric ¬re. However, few parts of the Earth™s surface face the
Sun directly and in any case for half the time they are pointing away from
the Sun at night, so that the average energy falling on one square metre
of a level surface outside the atmosphere is only one-quarter of this1 or
about 343 watts. As this radiation passes through the atmosphere a small

How the Earth keeps warm 15

Figure 2.1 The radiation
balance of planet Earth.
The net incoming solar
radiation is balanced by
outgoing thermal radiation
from the Earth.

amount, about six per cent, is scattered back to space by atmospheric
molecules. About ten per cent on average is re¬‚ected back to space
from the land and ocean surface. The remaining eighty-four per cent, or
about 288 watts per square metre on average, remains actually to heat
the surface “ the power used by three good-sized incandescent electric
light bulbs.
To balance this incoming energy, the Earth itself must radiate on
average the same amount of energy back to space (Figure 2.1) in the form
of thermal radiation. All objects emit this kind of radiation; if they are
hot enough we can see the radiation they emit. The Sun at a temperature
of about 6000 —¦ C looks white; an electric ¬re at 800 —¦ C looks red. Cooler
objects emit radiation that cannot be seen by our eyes and which lies
at wavelengths beyond the red end of the spectrum “ infrared radiation
(sometimes called long-wave radiation to distinguish it from the short-
wave radiation from the Sun). On a clear, starry winter™s night we are
very aware of the cooling effect of this kind of radiation being emitted
by the Earth™s surface into space “ it often leads to the formation of frost.
The amount of thermal radiation emitted by the Earth™s surface de-
pends on its temperature “ the warmer it is, the more radiation is emitted.
The amount of radiation also depends on how absorbing the surface is;
the greater the absorption, the more the radiation. Most of the surfaces
on the Earth, including ice and snow, would appear ˜black™ if we could
see them at infrared wavelengths; that means that they absorb nearly all
the thermal radiation which falls on them instead of re¬‚ecting it. It can
be calculated2 that, to balance the energy coming in, the average temper-
ature of the Earth™s surface must be “6 —¦ C to radiate the right amount.3
This is much colder than is actually the case. In fact, an average of tem-
peratures measured near the surface all over the Earth “ over the oceans
as well as over the land “ averaging, too, over the whole year, comes to
about 15 —¦ C. Some factor not yet taken into account is needed to explain
this discrepancy.
16 The greenhouse effect

Table 2.1 The composition of the atmosphere, the
main constituents (nitrogen and oxygen) and the
greenhouse gases as in 2001

Mixing ratio or mole
fractiona expressed as
fraction* or parts per
Gas million (ppm)

Nitrogen (N2 ) 0.78*
Oxygen (O2 ) 0.21*
Water vapour (H2 O) Variable (0“0.02*)
Carbon dioxide (CO2 ) 370
Methane (CH4 ) 1.8
Nitrous oxide (N2 O) 0.3
Chloro¬‚uorocarbons 0.001
Ozone (O3 ) Variable (0“1000)

For de¬nition see Glossary.

The greenhouse effect
The gases nitrogen and oxygen that make up the bulk of the atmosphere
(Table 2.1 gives details of the atmosphere™s composition) neither ab-
sorb nor emit thermal radiation. It is the water vapour, carbon dioxide
and some other minor gases present in the atmosphere in much smaller
quantities (Table 2.1) that absorb some of the thermal radiation leaving
the surface, acting as a partial blanket for this radiation and causing the
difference of 21 —¦ C or so between the actual average surface temperature
on the Earth of about 15 —¦ C and the ¬gure of ’6 —¦ C which applies when
the atmosphere contains nitrogen and oxygen only.4 This blanketing
is known as the natural greenhouse effect and the gases are known as
greenhouse gases. It is called ˜natural™ because all the atmospheric gases
(apart from the chloro¬‚uorocarbons “ CFCs) were there long before hu-
man beings came on the scene. Later on I will mention the enhanced
greenhouse effect: the added effect caused by the gases present in the at-
mosphere due to human activities such as the burning of fossil fuels and
The basic science of the greenhouse effect has been known since
early in the nineteenth century (see box) when the similarity between the
radiative properties of the Earth™s atmosphere and of the glass in a green-
house (Figure 2.2) was ¬rst pointed out “ hence the name ˜greenhouse
effect™. In a greenhouse, visible radiation from the Sun passes almost
The greenhouse effect 17

Pioneers of the science of the greenhouse effect5
The warming effect of the greenhouse gases in the atmosphere was ¬rst
recognised in 1827 by the French scientist Jean-Baptiste Fourier, best
known for his contributions to mathematics. He also pointed out the
similarity between what happens in the atmosphere and in the glass
of a greenhouse, which led to the name ˜greenhouse effect™. The next
step was taken by a British scientist, John Tyndall, who, around 1860,
measured the absorption of infrared radiation by carbon dioxide and
water vapour; he also suggested that a cause of the Ice Ages might be
a decrease in the greenhouse effect of carbon dioxide. It was a Swedish
chemist, Svante Arrhenius, in 1896, who calculated the effect of an
increasing concentration of greenhouse gases; he estimated that doubling
the concentration of carbon dioxide would increase the global average
temperature by 5 —¦ C to 6 —¦ C, an estimate not too far from our present
understanding.6 Nearly ¬fty years later, around 1940, G. S. Callendar,
working in England, was the ¬rst to calculate the warming due to the
increasing carbon dioxide from the burning of fossil fuels.
The ¬rst expression of concern about the climate change which might
be brought about by increasing greenhouse gases was in 1957, when
Roger Revelle and Hans Suess of the Scripps Institute of Oceanography
in California published a paper which pointed out that in the build-up
of carbon dioxide in the atmosphere, human beings are carrying out a
large-scale geophysical experiment. In the same year, routine measure-
ments of carbon dioxide were started from the observatory on Mauna
Kea in Hawaii. The rapidly increasing use of fossil fuels since then, to-
gether with growing interest in the environment, has led to the topic of
global warming moving up the political agenda through the 1980s, and
eventually to the Climate Convention signed in 1992 “ of which more in
later chapters.

unimpeded through the glass and is absorbed by the plants and the soil
inside. The thermal radiation that is emitted by the plants and soil is,
however, absorbed by the glass that re-emits some of it back into the
greenhouse. The glass thus acts as a ˜radiation blanket™ helping to keep
the greenhouse warm.
However, the transfer of radiation is only one of the ways heat is
moved around in a greenhouse. A more important means of heat trans-
fer is due to convection, in which less dense warm air moves upwards
and more dense cold air moves downwards. A familiar example of this
process is the use of convective electric heaters in the home, which heat
a room by stimulating convection in it. The situation in the greenhouse
18 The greenhouse effect

Figure 2.2 A greenhouse has
a similar effect to the
atmosphere on the incoming
solar radiation and the
emitted thermal radiation.

is therefore more complicated than would be the case if radiation were
the only process of heat transfer.
Mixing and convection are also present in the atmosphere, although
on a much larger scale, and in order to achieve a proper understand-
ing of the greenhouse effect, convective heat transfer processes in the
atmosphere must be taken into account as well as radiative ones.
Within the atmosphere itself (at least in the lowest three-quarters or
so of the atmosphere up to a height of about 10 km which is called the
troposphere) convection is, in fact, the dominant process for transferring
heat. It acts as follows. The surface of the Earth is warmed by the sunlight
it absorbs. Air close to the surface is heated and rises because of its lower
density. As the air rises it expands and cools “ just as the air cools as it
comes out of the valve of a tyre. As some air masses rise, other air masses
descend, so the air is continually turning over as different movements
balance each other out “ a situation of convective equilibrium. Temper-
ature in the troposphere falls with height at a rate determined by these
convective processes; the fall with height (called the lapse-rate) turns
out on average to be about 6 —¦ C per kilometre of height (Figure 2.3).
A picture of the transfer of radiation in the atmosphere may be ob-
tained by looking at the thermal radiation emitted by the Earth and its
atmosphere as observed from instruments on satellites orbiting the Earth
(Figure 2.4). At some wavelengths in the infrared the atmosphere “ in
the absence of clouds “ is largely transparent, just as it is in the visible
part of the spectrum. If our eyes were sensitive at these wavelengths we
would be able to peer through the atmosphere to the Sun, stars and Moon
above, just as we can in the visible spectrum. At these wavelengths all
the radiation originating from the Earth™s surface leaves the atmosphere.
At other wavelengths radiation from the surface is strongly absorbed
by some of the gases present in the atmosphere, in particular by water
vapour and carbon dioxide.
The greenhouse effect 19

Figure 2.3 The distribution of temperature in a convective atmosphere (full
line). The broken line shows how the temperature increases when the amount of
carbon dioxide present in the atmosphere is increased (in the diagram the
difference between the lines is exaggerated “ for instance, for doubled carbon
dioxide in the absence of other effects the increase in temperature is about
1.2 —¦ C). Also shown for the two cases are the average levels from which thermal
radiation leaving the atmosphere originates (about 6 km for the unperturbed

Figure 2.4 Thermal radiation in the infrared region (the visible part of the
spectrum is between about 0.4 and 0.7 µm) emitted from the Earth™s surface
and atmosphere as observed over the Mediterranean Sea from a satellite
instrument orbiting above the atmosphere, showing parts of the spectrum
where different gases contribute to the radiation. Between the wavelengths of
about 8 and 14 µm, apart from the ozone band, the atmosphere, in the absence
of clouds, is substantially transparent; this is part of the spectrum called a
™window™ region. Superimposed on the spectrum are curves of radiation from a
black body at 7 —¦ C, ’13 —¦ C, ’33 —¦ C and ’53 —¦ C. The units of radiance are watts
per square metre per steradian per wavenumber.
20 The greenhouse effect

Figure 2.5 The blanketing
effect of greenhouse gases.

Objects that are good absorbers of radiation are also good emitters
of it. A black surface is both a good absorber and a good emitter, while
a highly re¬‚ecting surface absorbs rather little and emits rather little too
(which is why highly re¬‚ecting foil is used to cover the surface of a vac-
uum ¬‚ask and why it is placed above the insulation in the lofts of houses).
Absorbing gases in the atmosphere absorb some of the radiation
emitted by the Earth™s surface and in turn emit radiation out to space. The
amount of thermal radiation they emit is dependent on their temperature.
Radiation is emitted out to space by these gases from levels some-
where near the top of the atmosphere “ typically from between 5 and
10 km high (see Figure 2.3). Here, because of the convection processes
mentioned earlier, the temperature is much colder “ 30 to 50 —¦ C or so
colder “ than at the surface. Because the gases are cold, they emit cor-
respondingly less radiation. What these gases have to do, therefore, is
absorb some of the radiation emitted by the Earth™s surface but then to
emit much less radiation out to space. They, therefore, act as a radia-
tion blanket over the surface (note that the outer surface of a blanket is
colder than inside the blanket) and help to keep it warmer than it would
otherwise be7 (Figure 2.5).
There needs to be a balance between the radiation coming in and
the radiation leaving the top of the atmosphere “ as there was in the
very simple model with which this chapter started. Figure 2.6 shows
the various components of the radiation entering and leaving the top of
the atmosphere for the real atmosphere situation. On average, 240 watts
per square metre of solar radiation are absorbed by the atmosphere and
the surface; this is less than the 288 watts mentioned at the beginning of
the chapter, because now the effect of clouds is being taken into account.
Clouds re¬‚ect some of the incident radiation from the Sun back out to
space. However, they also absorb and emit thermal radiation and have
a blanketing effect similar to that of the greenhouse gases. These two
effects work in opposite senses: one (the re¬‚ection of solar radiation)
tends to cool the Earth™s surface and the other (the absorption of thermal
radiation) tends to warm it. Careful consideration of these two effects
shows that on average the net effect of clouds on the total budget of
radiation results in a slight cooling of the Earth™s surface.8
Mars and Venus 21

The numbers in Figure 2.6 demonstrate the required balance “ 240 Figure 2.6 Components
of the radiation (in watts
watts per square metre on average coming in and 240 watts per square
per square metre) which
metre on average going out. The temperature of the surface and hence of
on average enter and leave
the atmosphere above adjusts itself to ensure that this balance is main-
the Earth™s atmosphere and
tained. It is interesting to note that the greenhouse effect can only operate make up the radiation
if there are colder temperatures in the higher atmosphere. Without the budget for the
structure of decreasing temperature with height, therefore, there would atmosphere.
be no greenhouse effect on the Earth.

Mars and Venus
Similar greenhouse effects also occur on our nearest planetary neigh-
bours, Mars and Venus. Mars is smaller than the Earth and possesses, by
Earth™s standards, a very thin atmosphere. A barometer on the surface of
Mars would record an atmospheric pressure less than one per cent of that
on the Earth. Its atmosphere, which consists almost entirely of carbon
dioxide, contributes a small but signi¬cant greenhouse effect.
The planet Venus, which can often be seen fairly close to the Sun
in the morning or evening sky, has a very different atmosphere to Mars.
Venus is about the same size as the Earth. A barometer for use on Venus
would need to survive very hostile conditions and would need to be able to
measure a pressure about one hundred times as great as that on the Earth.
Within the Venus atmosphere, which consists very largely of carbon
dioxide, deep clouds consisting of droplets of almost pure sulphuric
acid completely cover the planet and prevent most of the sunlight from
reaching the surface. Some Russian space probes that have landed there
have recorded what would be dusk-like conditions on the Earth “ only one
22 The greenhouse effect

or two per cent of the sunlight present above the clouds penetrates that
far. One might suppose, because of the small amount of solar energy
available to keep the surface warm, that it would be rather cool; on
the contrary, measurements from the same Russian space probes ¬nd a
temperature there of about 525 —¦ C “ a dull red heat, in fact.
The reason for this very high temperature is the greenhouse effect.
Because of the very thick absorbing atmosphere of carbon dioxide, little
of the thermal radiation from the surface can get out. The atmosphere
acts as such an effective radiation blanket that, although there is not
much solar energy to warm the surface, the greenhouse effect amounts
to nearly 500 —¦ C.

The ˜runaway™ greenhouse effect
What occurs on Venus is an example of what has been called the ˜run-
away™ greenhouse effect. It can be explained by imagining the early
history of the Venus atmosphere, which was formed by the release of
gases from the interior of the planet. To start with it would contain a lot of
water vapour, a powerful greenhouse gas (Figure 2.7). The greenhouse
effect of the water vapour would cause the temperature at the surface to
rise. The increased temperature would lead to more evaporation of water
from the surface, giving more atmospheric water vapour, a larger green-
house effect and therefore a further increased surface temperature. The
process would continue until either the atmosphere became saturated
with water vapour or all the available water had evaporated.

Figure 2.7 Illustrating the evolution of the atmospheres of the Earth, Mars and
Venus. In this diagram, the surface temperatures of the three planets are plotted
against the vapour pressure of water in their atmospheres as they evolved. Also
on the diagram (dashed) are the phase lines for water, dividing the diagram into
regions where vapour, liquid water or ice are in equilibrium. For Mars and the
Earth the greenhouse effect is halted when water vapour is in equilibrium with
ice or liquid water. For Venus no such halting occurs and the diagram illustrates
the ™runaway™ greenhouse effect.
The enhanced greenhouse effect 23

A runaway sequence something like this seems to have occurred on
Venus. Why, we may ask, has it not happened on the Earth, a planet of
about the same size as Venus and, so far as is known, of a similar initial
chemical composition? The reason is that Venus is closer to the Sun than
the Earth; the amount of solar energy per square metre falling on Venus
is about twice that falling on the Earth. The surface of Venus, when there
was no atmosphere, would have started off at a temperature of just over
50 —¦ C (Figure 2.7). Throughout the sequence described above for Venus,
water on the surface would have been continuously boiling. Because of
the high temperature, the atmosphere would never have become saturated
with water vapour. The Earth, however, would have started at a colder
temperature; at each stage of the sequence it would have arrived at an
equilibrium between the surface and an atmosphere saturated with water
vapour. There is no possibility of such runaway greenhouse conditions
occurring on the Earth.

The enhanced greenhouse effect
After our excursion to Mars and Venus, let us return to Earth! The natural
greenhouse effect is due to the gases water vapour and carbon dioxide
present in the atmosphere in their natural abundances as now on Earth.
The amount of water vapour in our atmosphere depends mostly on the
temperature of the surface of the oceans; most of it originates through
evaporation from the ocean surface and is not in¬‚uenced directly by
human activity. Carbon dioxide is different. Its amount has changed
substantially “ by about thirty per cent so far “ since the Industrial
Revolution, due to human industry and also because of the removal of
forests (see Chapter 3). Future projections are that, in the absence of
controlling factors, the rate of increase in atmospheric carbon dioxide
will accelerate and that its atmospheric concentration will double from
its pre-industrial value within the next hundred years (Figure 6.2).
This increased amount of carbon dioxide is leading to global warm-
ing of the Earth™s surface because of its enhanced greenhouse effect.
Let us imagine, for instance, that the amount of carbon dioxide in
the atmosphere suddenly doubled, everything else remaining the same
(Figure 2.8). What would happen to the numbers in the radiation budget
presented earlier (Figure 2.6)? The solar radiation budget would not be
affected. The greater amount of carbon dioxide in the atmosphere means
that the thermal radiation emitted from it will originate on average from
a higher and colder level than before (Figure 2.3). The thermal radiation
budget will therefore be reduced, the amount of reduction being about
4 watts per square metre (a more precise value is 3.7).9
24 The greenhouse effect

Figure 2.8 Illustrating the enhanced greenhouse gas effect. Under natural
conditions (a) the net solar radiation coming in (S = 240 watts per square
metre) is balanced by thermal radiation (L) leaving the top of the atmosphere;
average surface temperature (Ts ) is 15 —¦ C. If the carbon dioxide concentration is
suddenly doubled (b), L is decreased by 4 watts per square metre. Balance is
restored if nothing else changes (c) apart from the temperature of the surface
and lower atmosphere, which rises by 1.2 —¦ C. If feedbacks are also taken into
account (d), the average temperature of the surface rises by about 2.5 —¦ C.

This causes a net imbalance in the overall budget of 4 watts per square
metre. More energy is coming in than going out. To restore the balance
the surface and lower atmosphere will warm up. If nothing changes apart
from the temperature “ in other words, the clouds, the water vapour, the
ice and snow cover and so on are all the same as before “ the temperature
change turns out to be about 1.2 —¦ C.
In reality, of course, many of these other factors will change, some
of them in ways that add to the warming (these are called positive
feedbacks), others in ways that might reduce the warming (negative
feedbacks). The situation is therefore much more complicated than this
simple calculation. These complications will be considered in more de-
tail in Chapter 5. Suf¬ce it to say here that the best estimate at the present
time of the increased average temperature of the Earth™s surface if car-
bon dioxide levels were to be doubled is about twice that of the simple
calculation: 2.5 —¦ C. As the last chapter explained, for the global average
temperature this is a large change. It is this global warming expected to
result from the enhanced greenhouse effect that is the cause of current
Having dealt with a doubling of the amount of carbon dioxide, it
is interesting to ask what would happen if all the carbon dioxide were
removed from the atmosphere. It is sometimes supposed that the outgoing
radiation would be changed by 4 watts per square metre in the other
direction and that the Earth would then cool by one or two degrees
Celsius. In fact, that would happen if the carbon dioxide amount were
Questions 25

to be halved. If it were to be removed altogether, the change in outgoing
radiation would be around 25 watts per square metre “ six times as big “
and the temperature change would be similarly increased. The reason
for this is that with the amount of carbon dioxide currently present in
the atmosphere there is maximum carbon dioxide absorption over much
of the region of the spectrum where it absorbs (Figure 2.4), so that a
big change in gas concentration leads to a relatively small change in
the amount of radiation it absorbs.10 This is like the situation in a pool
of water: when it is clear, a small amount of mud will make it appear
muddy, but when it is muddy, adding more mud only makes a small
An obvious question to ask is: has evidence of the enhanced green-
house effect been seen in the recent climatic record? Chapter 4 will look
at the record of temperature on the Earth during the last century or so,
during which the Earth has warmed on average by rather more than half
a degree Celsius. We shall see in Chapters 4 and 5 that there are good
reasons for attributing most of this warming to the enhanced greenhouse
effect, although because of the size of natural climate variability the
exact amount of that attribution remains subject to some uncertainty.
To summarise the argument so far:
r No one doubts the reality of the natural greenhouse effect, which keeps
us over 20 —¦ C warmer than we would otherwise be. The science of it
is well understood; it is similar science that applies to the enhanced
greenhouse effect.
r Substantial greenhouse effects occur on our nearest planetary neigh-
bours, Mars and Venus. Given the conditions that exist on those planets,
the sizes of their greenhouse effects can be calculated, and good agree-
ment has been found with those measurements which are available.
r Study of climates of the past gives some clues about the greenhouse
effect, as Chapter 4 will show.

First, however, the greenhouse gases themselves must be considered.
How does carbon dioxide get into the atmosphere, and what other gases
affect global warming?

1 Carry out the calculation described in Note 4 (refer also to Note 2) which
obtains an equilibrium average temperature of ’18 —¦ C for an Earth partially
covered with clouds such that thirty per cent of the incoming solar radiation
is re¬‚ected. If clouds are assumed to cover half the Earth and if the re¬‚ectivity
of the clouds increases by one per cent what change will this make in the
resulting equilibrium average temperature?
26 The greenhouse effect

2 It is sometimes argued that the greenhouse effect of carbon dioxide is neg-
ligible because its absorption band in the infrared is so close to saturation
that there is very little additional absorption of radiation emitted from the
surface. What are the fallacies in this argument?
3 Use the information in Figure 2.4 to estimate approximately the surface
temperature that would result if carbon dioxide were completely removed
from the atmosphere. What is required is that the total energy radiated by
the Earth plus atmosphere should remain the same, i.e. the area under the
radiance curve in Figure 2.4 should be unaltered. On this basis construct a
new curve with the carbon dioxide band absent.11
4 Using information from books or articles on climatology or meteorology
describe why the presence of water vapour in the atmosphere is of such
importance in determining the atmosphere™s circulation.
5 Estimates of regional warming due to increased greenhouse gases are gener-
ally larger over land areas than over ocean areas. What might be the reasons
for this?
6 (For students with a background in physics) What is meant by Local Thermo-
dynamic Equilibrium (LTE),12 a basic assumption underlying calculations
of radiative transfer in the lower atmosphere appropriate to discussions of
the greenhouse effect? Under what conditions does LTE apply?

Notes for Chapter 2
1 It is about one-quarter because the area of the Earth™s surface is four times
the area of the disc, which is the projection of the Earth facing the Sun; see
Figure 2.1.
2 The radiation by a black body is the Stefan“Boltzmann constant (5.67 —
10’8 J m’2 K’4 s’1 ) multiplied by the fourth power of the body™s abso-
lute temperature in Kelvin. The absolute temperature is the temperature in
degrees Celsius plus 273 (1 K = 1 —¦ C).
3 These calculations using a simple model of an atmosphere containing ni-
trogen and oxygen only have been carried out to illustrate the effect of the
other gases, especially water vapour and carbon dioxide. It is not, of course,
a model that can exist in reality. All the water vapour could not be removed
from the atmosphere above a water or ice surface. Further, with an average
surface temperature of ’6 —¦ C, in a real situation the surface would have
much more ice cover. The additional ice would re¬‚ect more solar energy out
to space leading to a further lowering of the surface temperature.
4 The above calculation is often carried out using a ¬gure of thirty per cent
for the average re¬‚ectivity of the Earth and atmosphere, rather than the
sixteen per cent assumed here; the calculation of surface temperature then
gives ’18 —¦ C for the average surface temperature rather than the ’6 —¦ C
found here. The higher ¬gure of thirty per cent for the Earth™s average
re¬‚ectivity is applicable when clouds are also included, in which case the
average temperature of ’18 —¦ C is not applicable to the Earth™s surface but
to some appropriate level in the atmosphere. Further, clouds not only re¬‚ect
Notes 27

solar radiation but also absorb thermal radiation, and so have a blanketing
effect similar to greenhouse gases. For the purposes of illustrating the effect
of greenhouse gases, therefore, it is more correct to omit the effect of clouds
from this initial calculation.
5 Further details can be found in Mudge, F. B. The development of greenhouse
theory of global climate change from Victorian times. 1997. Weather, 52,
pp. 13“16.
A range of 1.5 to 4.5 —¦ C is quoted in Chapter 6, page 120.
7 The formal theory of the greenhouse effect is presented in Houghton,
J. T. 2002. The Physics of Atmospheres, third edition, Chapter 2. Cambridge:
Cambridge University Press. See also Chapter 14 of that book.
8 More detail of the radiative effects of clouds is given in Chapter 5; see
Figures 5.14 and 5.15.
9 More detailed information about the enhanced greenhouse effect can be
found in Houghton, J. T. 2002. The Physics of Atmospheres, third edition,
Chapter 14. Cambridge: Cambridge University Press.
10 The dependence of the absorption on the concentration of gas is approxi-
mately logarithmic.
11 For some helpful diagrams and more information about the infrared spec-
trum of different greenhouse gases, see Harries, J. E. 1996. The greenhouse
Earth: a view from space. Quarterly Journal of the Royal Meteorological
Society, 122, pp. 799“818.
12 For information about LTE see, for instance, Houghton, J. T. 2002. The
Physics of Atmospheres, third edition. Cambridge: Cambridge University
Chapter 3
The greenhouse gases

The greenhouse gases are those gases in the atmosphere which, by
absorbing thermal radiation emitted by the Earth™s surface, have a blan-
keting effect upon it. The most important of the greenhouse gases is
water vapour, but its amount in the atmosphere is not changing directly
because of human activities. The important greenhouse gases that are
directly in¬‚uenced by human activities are carbon dioxide, methane, ni-
trous oxide, the chloro¬‚uorocarbons (CFCs) and ozone. This chapter will
describe what is known about the origin of these gases, how their con-
centration in the atmosphere is changing and how it is controlled. Also
considered will be particles in the atmosphere of anthropogenic origin
that can act to cool the surface.

Which are the most important greenhouse gases?
Figure 2.4 illustrated the regions of the infrared spectrum where the
greenhouse gases absorb. Their importance as greenhouse gases depends
both on their concentration in the atmosphere (Table 2.1) and on the
strength of their absorption of infrared radiation. Both these quantities
differ greatly for various gases.
Carbon dioxide is the most important of the greenhouse gases that
are increasing in atmospheric concentration because of human activities.
If, for the moment, we ignore the effects of the CFCs and of changes
in ozone, which vary considerably over the globe and which are there-
fore more dif¬cult to quantify, the increase in carbon dioxide (CO2 ) has
contributed about seventy per cent of the enhanced greenhouse effect

Carbon dioxide and the carbon cycle 29

to date, methane (CH4 ) about twenty-four per cent, and nitrous oxide
(N2 O) about six per cent (Figure 3.8).

Radiative forcing
In this chapter we shall use the concept of radiative forcing to compare
the relative greenhouse effects of different atmospheric constituents. It
is necessary therefore at the start to de¬ne radiative forcing.
In Chapter 2 we noted that, if the carbon dioxide in the atmosphere
were suddenly doubled, everything else remaining the same, a net radia-
tion imbalance near the top of the atmosphere of 3.7 W m’2 would result.
This radiation imbalance is an example of radiation forcing, which is de-
¬ned as the change in average net radiation at the top of the troposphere1
(the lower atmosphere; for de¬nition see Glossary) which occurs be-
cause of a change in the concentration of a greenhouse gas or because of
some other change in the overall climate system; for instance, a change
in the incoming solar radiation would constitute a radiative forcing. As
we saw in the discussion in Chapter 2, over time the climate responds to
restore the radiative balance between incoming and outgoing radiation.
A positive radiative forcing tends on average to warm the surface and a
negative radiative forcing tends on average to cool the surface.

Carbon dioxide and the carbon cycle
Carbon dioxide provides the dominant means through which carbon is
transferred in nature between a number of natural carbon reservoirs “ a
process known as the carbon cycle. We contribute to this cycle every time
we breathe. Using the oxygen we take in from the atmosphere, carbon
from our food is burnt and turned into carbon dioxide that we then exhale;
in this way we are provided with the energy we need to maintain our life.
Animals contribute to atmospheric carbon dioxide in the same way; so
do ¬res, rotting wood and decomposition of organic material in the soil
and elsewhere. To offset these processes of respiration whereby carbon is
turned into carbon dioxide, there are processes involving photosynthesis
in plants and trees which work the opposite way; in the presence of
light, they take in carbon dioxide, use the carbon for growth and return
the oxygen back to the atmosphere. Both respiration and photosynthesis
also occur in the ocean.
Figure 3.1 is a simple diagram of the way carbon cycles between
the various reservoirs “ the atmosphere, the oceans (including the ocean
biota), the soil and the land biota (biota is a word that covers all living
things “ plants, trees, animals and so on “ on land and in the ocean, which
30 The greenhouse gases

Figure 3.1 The global carbon cycle, showing the carbon stocks in reservoirs (in
Gt) and carbon ¬‚ows (in Gt year’1 ) relevant to the anthropogenic perturbation
as annual averages over the decade from 1989 to 1998. Net ocean uptake of the
anthropogenic perturbation equals the net air/sea input plus run-off minus
sediment. The units are thousand millions of tonnes or gigatonnes (Gt).

make up a whole known as the biosphere). The diagram shows that the
movements of carbon (in the form of carbon dioxide) into and out of
the atmosphere are quite large; about one-¬fth of the total amount in the
atmosphere is cycled in and out each year, part with the land biota and
part through physical and chemical processes across the ocean surface.
The land and ocean reservoirs are much larger than the amount in the
atmosphere; small changes in these larger reservoirs could therefore have
a large effect on the atmospheric concentration; the release of just two
per cent of the carbon stored in the oceans would double the amount of
atmospheric carbon dioxide.
It is important to realise that on the timescales with which we are
concerned anthropogenic carbon emitted into the atmosphere as car-
bon dioxide is not destroyed but redistributed among the various carbon
reservoirs. Carbon dioxide is therefore different from other greenhouse
gases that are destroyed by chemical action in the atmosphere. The car-
bon reservoirs exchange carbon between themselves on a wide range of
Carbon dioxide and the carbon cycle 31

timescales determined by their respective turnover times “ which range
from less than a year to decades (for exchange with the top layers of
the ocean and the land biosphere) to millennia (for exchange with the
deep ocean or long-lived soil pools). These timescales are generally
much longer than the average time a particular carbon dioxide molecule
spends in the atmosphere, which is only about four years. The large range
of turnover times means that the time taken for a perturbation in the at-
mospheric carbon dioxide concentration to relax back to an equilibrium
cannot be described by a single time constant. Although a lifetime of
about a hundred years is often quoted for atmospheric carbon dioxide so
as to provide some guide, use of a single lifetime can be very misleading.
Before human activities became a signi¬cant disturbance, and over
periods short compared with geological timescales, the exchanges be-
tween the reservoirs were remarkably constant. For several thousand
years before the beginning of industrialisation around 1750, a steady
balance was maintained, such that the mixing ratio (or mole fraction, for
de¬nition see Glossary) of carbon dioxide in the atmosphere as measured
from ice cores (see Chapter 4) kept within about ten parts per million of
a mean value of about 280 parts per million (ppm) “ see Figure 3.2(a).
The Industrial Revolution disturbed this balance and since its be-
ginning in about 1700 approximately 600 thousand million tonnes (or
gigatonnes, Gt) of carbon have been emitted into the atmosphere from
fossil fuel burning. This has resulted in a concentration of carbon diox-
ide in the atmosphere that has increased by about thirty per cent, from
280 ppm around 1700 to a value of over 370 ppm at the present day
(Figure 3.2(b)). Accurate measurements, which have been made since
1959 from an observatory near the summit of Mauna Loa in Hawaii,
show that carbon dioxide is currently increasing on average each year
by about 1.5 ppm, although there are large variations from year to year
(Figure 3.2(c)). This increase spread through the atmosphere adds about
3.3 Gt to the atmospheric carbon reservoir each year.
It is easy to establish how much coal, oil and gas is being burnt world-
wide each year. Most of it is to provide energy for human needs: for heat-
ing and domestic appliances, for industry and for transport (considered
in detail in Chapter 11). The burning of these fossil fuels has increased
rapidly since the Industrial Revolution (Figure 3.3 and Table 3.1); cur-
rently the annual total is between 6 and 7 Gt of carbon, nearly all of
which enters the atmosphere as carbon dioxide. Another contribution to
atmospheric carbon dioxide due to human activities comes from land-
use change, in particular from the burning and decay of forests balanced
in part by aforestation or forest regrowth. This contribution is not easy
to quantify but some estimates are given in Figure 3.3 and Table 3.1.
For the 1980s (see Table 3.1), annual anthropogenic emissions from
32 The greenhouse gases

Figure 3.2 Atmospheric 380
carbon dioxide concentration, (a) Taylor Dome
(a) over the last 10 000 years

CO2 concentration (ppm)
from the Taylor Dome
Antarctic ice core; (b) from
Antarctic ice cores for the past
millennium; recent
atmospheric measurements
from the Mauna Loa
observatory in Hawaii are also
shown; (c) monthly changes
in atmospheric carbon dioxide 180
concentration, ¬ltered so as to 12500 10000 7500 5000 2500 0
remove seasonal cycle; vertical Age (yr BP)
arrows denote El Ni˜ o events;
note the small rate of growth 380
from 1991 to 1994 which 360
may be connected with events
CO2 concentration (ppm)

such as the Pinatubo volcanic 320
eruption in 1991 or the 300
unusual extended El Ni˜ o of
n 280
1991“4 (denoted by
Mauna Loa
horizontal line).
240 Law Dome
Adelie Land
South Pole
800 1000 1200 1400 1600 1800 2000

monthly atmospheric increase (filtered)
fossil fuel emissions
ppm (yr)



1960 1970 1980 1990 2000

fossil fuel burning, cement manufacture and land-use change amounted
to about 7.1 Gt; over three-quarters of these resulted from fossil fuel
burning. Since the annual net increase in the atmosphere was about
3.3 Gt, about forty-¬ve per cent of the 7.1 Gt of new carbon remained to
increase the atmospheric concentration. The other ¬fty-¬ve per cent was
taken up between the other two reservoirs: the oceans and the land biota.
Carbon dioxide and the carbon cycle 33

Table 3.1 Components of annual average global carbon budget for
1980s and 1990s “ in Gt of carbon per year a (positive values are ¬‚uxes
to the atmosphere, negative values represent uptake from the

1980s 1990s

5.4 ± 0.3 6.4 ± 0.4
Emissions (fossil fuel, cement)
3.3 ± 0.1 3.2 ± 0.1
Atmospheric increase
’1.9 ± 0.6 ’1.7 ± 0.5
Ocean“atmosphere ¬‚ux
Land“atmosphere ¬‚ux— ’0.2 ± 0.7 ’1.4 ± 0.7

partitioned as follows
Land-use change 1.7 (0.6 to 2.5) 1.4 to 3.0
’1.9 (’3.8 to 0.3) ’4.8 to ’1.6
Residual terrestrial sink

The entries in the ¬rst four rows are from Table 3.3 in Prentice et al. 2001 (see
Note 2). Note that the ranges quoted represent sixty-seven per cent certainty.
The entries in the ˜partitioning of land-atmosphere ¬‚ux™ are from House et al.
2003 (see Note 2).

Figure 3.3 (a) Fossil carbon emissions (based on statistics of fossil fuel and
cement production) and estimates of global reservoir changes: atmosphere
(deduced from direct observations and ice core measurements), ocean
(calculated with the Geophysical Fluid Dynamics Laboratory (GFDL), University
of Princeton, ocean carbon model) and net terrestrial biosphere (calculated as
remaining imbalance) from 1840 to 1990. The calculation implies that the
terrestrial biosphere was a net source to the atmosphere prior to 1940 (negative
values) and has been a net sink since about 1960. (b) Estimates of contributions
to the carbon balance of the terrestrial biosphere. The curve showing the
terrestrial reservoir changes is taken from (a). Emissions from land-use changes
(including tropical deforestation) are plotted negatively because they represent a
loss of biospheric carbon. These estimates are subject to large uncertainties (see
uncertainty estimates in Table 3.1).
34 The greenhouse gases

Figure 3.5 shows that these fractions may change substantially in the
About ninety-¬ve per cent of fossil fuel burning occurs in the
northern hemisphere, so there is more carbon dioxide there than in the
southern hemisphere. The difference is currently about two parts per
million and, over the years, has grown in parallel with fossil fuel emis-
sions, thus adding further compelling evidence that the atmospheric
increase in carbon dioxide levels results from these emissions.
We turn now to what happens in the oceans. We know that carbon
dioxide dissolves in water; carbonated drinks make use of that fact. Car-
bon dioxide is continually being exchanged with the air above the ocean
across the whole ocean surface (about 90 Gt per year is so exchanged “
Figure 3.1), particularly as waves break. An equilibrium is established
between the concentration of carbon dioxide dissolved in the surface
waters and the concentration in the air above the surface. The chemical
laws governing this equilibrium are such that if the atmospheric concen-
tration changes by ten per cent the concentration in solution in the water
changes by only one-tenth of this: one per cent.
This change will occur quite rapidly in the upper waters of the ocean,
the top hundred metres or so, so enabling part of the anthropogenic (i.e.
human generated) carbon dioxide added to the atmosphere (most of the
ocean™s share of the ¬fty-¬ve per cent mentioned above) to be taken up
quite rapidly. Absorption in the lower levels in the ocean takes longer;
mixing of surface water with water at lower levels takes up to several
hundred years or for the deep ocean over a thousand years. This process
whereby carbon dioxide is gradually drawn from the atmosphere in the
ocean™s lower levels is sometimes known as the solubility pump.
So the oceans do not provide as immediate a sink for increased
atmospheric carbon dioxide as might be suggested by the size of the
exchanges with the large ocean reservoir. For short-term changes only
the surface layers of water play a large part in the carbon cycle.
Biological activity in the oceans also plays an important role. It may
not be immediately apparent, but the oceans are literally teeming with
life. Although the total mass of living matter within the oceans is not
large, it has a high rate of turnover. Living material in the oceans is
produced at some thirty to forty per cent of the rate of production on
land. Most of this production is of plant and animal plankton that go
through a rapid series of life cycles. As they die and decay some of the
carbon they contain is carried downwards into lower levels of the ocean
adding to the carbon content of those levels. Some is carried to the very
deep water or to the ocean bottom where, so far as the carbon cycle is
concerned, it is out of circulation for hundreds or thousands of years.
Carbon dioxide and the carbon cycle 35

The biological pump in the oceans3

global temperature began to increase when the last
In temperate and high latitudes there is a peak each
Ice Age receded nearly 20 000 years ago and as the
spring in ocean biological activity. During the win-
carbon dioxide in the atmosphere began to increase
ter, water rich in nutrients is transferred from deep
(Figure 4.4), the methyl sulphonic acid concen-
water to levels near the surface. As sunlight in-
tration decreased. An interesting link is thereby
creases in the spring an explosive growth of the
provided between the carbon dioxide in the atmo-
plankton population occurs, known as the ˜spring
sphere and marine biological activity. During the
bloom™. Pictures of the colour of the ocean taken
cold periods of the Ice Ages, enhanced biological
from satellites orbiting the Earth can demonstrate
activity in the ocean could have been responsible
dramatically where this is happening.
for maintaining the atmospheric carbon dioxide
Plankton are small plants (phytoplankton) and
at a lower level of concentration “ the ˜biological
animals (zooplankton) that live in the surface wa-
pump™ was having an effect.
ters of the ocean; they range in size between about
There is some evidence from the paleo record
one-thousandth of a millimetre across and the size
of the biological activity in the ocean being stimu-
of typical insects on land. Herbivorous zooplank-
lated by the presence of iron-containing dust blown
ton graze on phytoplankton; carnivorous zooplank-
over the oceans from the land surface. This has led
ton eat herbivorous zooplankton. Plant and an-
to some proposals in recent years to enhance the
imal debris from these living systems sinks in
˜biological pump™ through arti¬cially introducing
the ocean. While sinking, some decomposes and
iron over suitable parts of the ocean. While an in-
returns to the water as nutrients, some (perhaps
teresting idea, it seems from careful studies that
about one per cent) reaches the deep ocean or
even a very large-scale operation would not have a
the ocean ¬‚oor, where it is lost to the carbon cy-
large practical effect.
cle for hundreds, thousands or even millions of
The question then remains as to why the Ice
years. The net effect of the ˜biological pump™ is
Ages should be periods of greater marine biolog-
to move carbon from the surface waters to lower
ical activity than the warm periods in between. A
levels in the ocean. As the amount of carbon in
British oceanographer, Professor John Woods, has
the surface waters is reduced, more carbon diox-
suggested that the key may lie in what happens in
ide from the atmosphere can be drawn down in or-
the winter as nutrients are fed into the upper ocean
der to restore the surface equilibrium. It is thought
ready for the spring bloom. When there is less at-
that the ˜biological pump™ has remained substan-
mospheric carbon dioxide, the cooling by radiation
tially constant in its operation during the last cen-
from the surface of the ocean increases. Since con-
tury unaffected by the increase in carbon dioxide
vection in the upper layers of the ocean is driven
by cooling at the surface, the increased cooling
Evidence of the importance of the ˜biological
results in a greater depth of the mixed layer near
pump™ comes from the paleoclimate record from
the top of the ocean where all the biological ac-
ice cores (see Chapter 4). One of the constituents
tivity occurs. This is an example of a positive bi-
from the atmosphere trapped in bubbles in the ice
ological feedback; a greater depth of layer means
is the gas methyl sulphonic acid, which originates
more plankton growth. Woods calls it the ˜plankton
from decaying ocean plankton; its concentration is
therefore an indicator of plankton activity. As the
36 The greenhouse gases

This process, whose contribution to the carbon cycle is known as the
biological pump (see box), was important in determining the changes
of carbon dioxide concentration in both the atmosphere and the ocean
during the ice ages (see Chapter 4).
Computer models “ which calculate solutions for the mathemati-
cal equations describing a given physical situation, in order to predict
its behaviour (see Chapter 5) “ have been set up to describe in detail
the exchanges of carbon between the atmosphere and different parts of
the ocean. To test the validity of these models, they have also been ap-
plied to the dispersal in the ocean of the carbon isotope 14 C that entered
the ocean after the nuclear tests of the 1950s; the models simulate this
dispersal quite well. From the model results, it is estimated that about
2 Gt (±0.8 Gt) of the carbon dioxide added to the atmosphere each year
ends up in the oceans (Table 3.1 and Figure 3.3). Observations of the
relative distribution of the other isotopes of carbon in the atmosphere
and in the oceans also con¬rm this estimate (see Box below).
Further information regarding the broad partitioning of added
atmospheric carbon dioxide between the atmosphere, the oceans and
the land biota as presented in Table 3.1 comes from comparing the
trends in atmospheric carbon dioxide concentration with the trends in
very accurate measurements of the atmospheric oxygen/nitrogen ratio.5
This possibility arises because the relation between the exchanges of
carbon dioxide and oxygen with the atmosphere over land is different to
that over the ocean. On land, living organisms through photosynthesis
take in carbon dioxide from the atmosphere and build up carbohydrates,
returning the oxygen to the atmosphere. In the process of respiration
they also take in oxygen from the atmosphere and convert it to
carbon dioxide. In the ocean, by contrast, carbon dioxide taken from
the atmosphere is dissolved, both the carbon and the oxygen in the
molecules being removed. How such measurements can be interpreted
for the period 1990“4 is shown in Figure 3.4. These data are consistent
with budget for the 1990s shown in Table 3.1.
The global land“atmosphere ¬‚ux in Table 3.1 represents the balance
of a net ¬‚ux due to land-use changes which has generally been posi-
tive or a source of carbon to the atmosphere (Figure 3.3) and a residual
component that is, by inference, a negative ¬‚ux or carbon sink. The esti-
mates of land-use changes (Table 3.1) are dominated by deforestation in
tropical regions although some uptake of carbon has occurred through
forest regrowth in temperate regions of the northern hemisphere and
other changes in land management. The main processes that contribute
to the residual carbon sink are believed to be the carbon dioxide ˜fer-
tilisation™ effect (increased carbon dioxide in the atmosphere leads to
increased growth in some plants “ see box in Chapter 7 on page 166),
Carbon dioxide and the carbon cycle 37

What we can learn from carbon isotopes
Isotopes are chemically identical forms of the same element but with dif-
ferent atomic weights. Three isotopes of carbon are important in studies
of the carbon cycle: the most abundant isotope 12 C which makes up
98.9 per cent of ordinary carbon, 13 C present at about 1.1 per cent and
the radioactive isotope 14 C which is present only in very small quantities.
About 10 kg of 14 C is produced in the atmosphere each year by the action
of particle radiation from the Sun; half of this will decay into nitrogen
over a period of 5730 years (the ˜half-life™ of 14 C).
When carbon in carbon dioxide is taken up by plants and other
living things, less 13 C is taken up in proportion than 12 C. Fossil fuel
such as coal and oil was originally living matter so also contains less
C (by about eighteen parts per thousand) than the carbon dioxide in
ordinary air in the atmosphere today. Adding carbon to the atmosphere
from burning forests, decaying vegetation or fossil fuel will therefore
tend to reduce the proportion of 13 C.
Because fossil fuel has been stored in the Earth for much longer
than 5730 years (the half-life of 14 C), it contains no 14 C at all. Therefore,
carbon from fossil fuel added to the atmosphere reduces the proportion
of 14 C the atmosphere contains.
By studying the ratio of the different isotopes of carbon in the at-
mosphere, in the oceans, in gas trapped in ice cores and in tree rings, it
is possible to ¬nd out where the additional carbon dioxide in the atmo-
sphere has come from and also what amount has been transferred to the
ocean. For instance, it has been possible to estimate for different times
how much carbon dioxide has entered the atmosphere from the burning
or decay of forests and other vegetation and how much from fossil fuels.
Similar isotopic measurements on the carbon in atmospheric
methane provide information about how much methane from fossil fuel
sources has entered the atmosphere at different times.

the effects of increased use of nitrogen fertilisers and of some changes in
climate. The magnitudes of these contributions (Table 3.1 and Figure 3.3)
are dif¬cult to estimate directly and are subject to much more uncertainty
than their total, which can be inferred from the requirement to balance
the overall carbon cycle budget.
A clue to the uptake of carbon by the land biosphere is provided from
observations of the atmospheric concentration of carbon dioxide which,
each year, show a regular cycle; the seasonal variation, for instance, at
the observatory site at Mauna Loa in Hawaii approaches about 10 ppm.
Carbon dioxide is removed from the atmosphere during the growing sea-
son and is returned as the vegetation dies away in the winter. Since there
38 The greenhouse gases



O2 concentration, difference from standard (ppm)



’35 fossil
bur ning







atmospheric land ocean
uptake uptake
345 350 355 360 365 370 375 380 385


. 2
( 13)