The greenhouse gases

Industrial activity: a source of carbon dioxide and other gaseous and particulate pollution.

THE GREENHOUSE gases are those gases in the atmosphere which, by absorbing thermal radiation emitted by the Earth's surface, have a blanketing 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 influenced by human activities are carbon dioxide, methane, nitrous oxide, the chlorofluorocarbons (CFCs) and ozone. This chapter will describe what is known about the origin of these gases, how their concentration in the atmosphere is changing and how it is controlled. Also considered will be particles in the atmosphere of anthropogenic origin, some of which can act to cool the surface.

Which are the most important greenhouse gases?

Figure 2.5 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 therefore more difficult to quantify, the increase in carbon dioxide (CO2) has contributed about 72% of the enhanced greenhouse effect to date, methane (CH4) about 21% and nitrous oxide (N2O) about 7% (Figure 3.11).

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 first to define 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 radiation 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 defined as the change in average net radiation at the top of the troposphere1 (the lower atmosphere; for definition see Glossary) which occurs because 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 fires, rotting wood and decomposition of organic material in the soil and elsewhere. To offset these

Atmosphere = 760 Accumulation 3.3 ± 0.2

Fossil fuels and cement production 6.3 ± 0.6

Net terrestrial uptake 0.7 ± 1.0

Global net primary productivity, respiration and fire = 60

s Bin I Vegetation = 500 Soils and detritus =2000 =2500

Net terrestrial uptake 0.7 ± 1.0

s Bin I Vegetation = 500 Soils and detritus =2000 =2500

Fossil organic carbon and carbonate minerals

Air/sea exchange = 90

Ocean = 39 000

Sedimentation = 0.2

Figure 3.1 The global carbon cycle, showing the approximate carbon stocks in reservoirs (in Gt) and carbon flows (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). (More detail in Fig. 7.3 in Chapter 7 of IPCC AR4 WGI 2007.)

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 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-fifth 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 2% 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 carbon 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 carbon reservoirs exchange carbon between themselves on a wide range of 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 atmospheric carbon dioxide concentration to relax back to an equilibrium cannot be described by a single time constant. About 50% of an increase in atmospheric carbon dioxide will be removed within 30 years, a further 30% within a few centuries and the remaining 20% may remain in the atmosphere for many thousands of years.2 Although a lifetime of about 100 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 significant disturbance, and over periods short compared with geological timescales, the exchanges between the reservoirs were remarkably constant. For thousands of years before the beginning of industrialisation around 1750, a steady balance was maintained, such that the mixing ratio (or mole fraction; for definition see Glossary) of carbon dioxide in the atmosphere as measured from ice cores (see Chapter 4) kept within about 20 parts per million (ppm) of a mean value of about 280 ppm (Figure 3.2a).

The Industrial Revolution disturbed this balance and since its beginning over 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 dioxide in the atmosphere that has increased by about 36%, from 280 ppm around 1700 to a value of over 380 ppm at the present day (Figure 3.2a), a greater concentration than for at least 650 000 years. Accurate measurements, which have been made since 1959 from an observatory near the summit of Mauna Loa in Hawaii, show that from 1995 to 2005 carbon dioxide increased

Didcot power station, near Oxford, UK.

on average each year by about 1.9 ppm (an increase from the average for the 1990s of about 1.5 ppm, although there are large variations from year to year (Figure 3.2b)). This increase spread through the atmosphere adds about 3.8 Gt to the atmospheric carbon reservoir each year.

It is easy to establish how much coal, oil and gas is being burnt worldwide each year. Most of it is to provide energy for human needs: for heating 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 (Table 3.1). Over the 1990s emissions rose about 0.7% per year; from 1999 to 2005 annual emissions rose systematically from 6.5 to 7.8 Gt of carbon (an annual increase averaging about 3%), 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 tropical deforestation balanced in part by afforestation or forest regrowth. This contribution is not easy to quantify but some estimates are given in Table 3.1. For the 1990s (see Table 3.1), annual anthropogenic emissions from fossil fuel burning, cement manufacture (about 3% of the total) and land-use change amounted to about 8.0 Gt; over three-quarters of these resulted from fossil fuel burning. Since the annual net increase in the atmosphere was about 3.2 Gt, about 40% of the 8 Gt of new carbon remained to increase the atmospheric concentration

Figure 3.2 Atmospheric carbon dioxide concentration. (a) Over the last 10 000 years (inset since 1750) from various ice cores (symbols with different colours for different studies) and atmospheric samples (red lines). Corresponding radiative forcings shown on right-hand axis. (b) Annual changes in global mean and their flve-year means from two different measurement networks (red and black stepped lines). The flve-year means smooth out short-term perturbations associated with strong El Niño Southern Oscillation (ENSO) events in 1972, 1982, 1987 and 1997. The upper dark green line shows the annual increases that would occur if all fossil fuel emissions stayed in the atmosphere and there were no other emissions.

250-

250-

10 000

5000

Time (before 2005)

10 000

5000

Time (before 2005)

r"

1970

1980 Year

1960

1990

2000

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

1980s

1990s

2000-2005

Emissions (fossil fuel, cement)

5.4 ± 0.3

6.4 ± 0.4

7.2 ± 0.3

Atmospheric increase

3.3 ± 0.1

3.2 ± 0.1

4.1 ± 0.1

Ocean-atmosphere flux

-1.8 ± 0.8

-2.2 ± 0.4

-2.2 ± 0.5

Land-atmosphere flux*

-0.3 ± 0.9

-1.0 ± 0.6

-0.9 ± 0.6

*partitioned as follows

Land-use change

1.4 (0.6 to 2.3)

1.6 (0.5 to 2.7)

not available

Residual terrestrial sink

-1.7 (-3.4 to 0.2)

-2.6 (-4.3 to -0.9)

not available

(Figure 3.2b). The other 60% was taken up between the other two reservoirs: the oceans and the land biota. Figure 3.5 shows that, as global average temperatures increase the fractions taken up by both land and ocean are likely to reduce.

About 95% 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 2 ppm (Figure 3.3) and, over the years, has grown in parallel with fossil fuel emissions, thus adding further compelling evidence that the atmospheric increase in carbon dioxide levels results from these emissions.

We now turn to what happens in the oceans. We know that carbon dioxide dissolves in water; carbonated drinks make use of that fact. Carbon 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 concentration changes by 10% the concentration in solution in the water changes by only one-tenth of this: 1%.

This change will occur quite rapidly in the upper waters of the ocean, the top 100 m or so, thus enabling part of the anthropogenic (i.e. human-generated) carbon dioxide added to the atmosphere (most of the ocean's share of the 60% 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 into the ocean's lower levels is sometimes known as the solubility pump.

Figure 3.3 Carbon dioxide concentrations (monthly averages) observed from Mauna Loa, Hawaii, 19 °N, green and from Baring Head, New Zealand, 41°S, red. Also shown are measurements of deviations in the O2/N2 ratio from an arbitrary reference multiplied by 106 from samples from Alert, Canada, 82° N, blue and from Cape Grim, Australia, 41° S, dark blue (after Manning and Keeling).

320-

o cp cp

380-

320-

Figure 3.3 Carbon dioxide concentrations (monthly averages) observed from Mauna Loa, Hawaii, 19 °N, green and from Baring Head, New Zealand, 41°S, red. Also shown are measurements of deviations in the O2/N2 ratio from an arbitrary reference multiplied by 106 from samples from Alert, Canada, 82° N, blue and from Cape Grim, Australia, 41° S, dark blue (after Manning and Keeling).

380-

1970 1975 1980 1985

1990

1995 2000 2005

Year

I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I

1970 1975 1980 1985

1990

1995 2000 2005

Year

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. Further, it is likely that a warmer regime will be associated on average with weaker overturning in the ocean and therefore with reduced carbon dioxide uptake.

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 30-40% of the rate of production on land. Most of this production is of plant and animal plankton which 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. 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 mathematical 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 applied to the dispersal in the ocean of the carbon isotope 14C that entered the ocean after the nuclear tests of the 1950s;

A large aquamarine-coloured plankton bloom is shown stretching across the length of Ireland in the North Atlantic Ocean in this image, captured on 6 June 2006 by Envisat's MERIS satellite, a dedicated ocean colour sensor able to identify phytoplankton concentrations.

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 (see Table 3.1). Observations of the relative distribution of the other isotopes of carbon in the atmosphere and in the oceans also confirm this estimate (see box).

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