The Present Increase In C02

Since the late 1700s, the carbon dioxide level in the atmosphere has been increasing (Figure 7.10). The record of air bubbles trapped in polar ice shows the 18th century beginning stages of this rise, which has gently accelerated over time into a steep increase of about 1 % a year. The change in C02 levels is also recorded in the stomatal densities of the leaves of trees preserved in herbaria; leaves collected around 1750 have a lower number of stomatal pores per unit number of epidermal cells in the skin of the leaf. This is just as one would expect from experiments that involve manipulating C02 concentrations (Chapter 8), where plants adjust the density of stomata on their leaves to take best advantage of the circumstances.

It is thought that the beginning of the increase in C02 was mostly due to deforestation in eastern North America, as settlers cleared the land for farming. Over time, more ancient sources of plant carbon from fossil fuels such as coal and oil became more important, as the industrial revolution took hold. Nevertheless, around 25-30% of the increase in atmospheric C02 that occurs each year is still due to deforestation, mostly in the tropics. Presently, the vast tract of forest in the Amazon Basin is the largest single source of C02 from deforestation, with South-Fast Asia following second.

However, not all forests arc losing carbon. Forests areas in several parts of the world have clearly now switched from being a "source'' of C02, to what is known as a "sink". A sink, in the language of carbon cycle science, is something that is taking up carbon and storing it. For example, in the last 150 years, forests in the eastern USA have become a carbon sink (Figure 7.11 *). They made a big comeback, starting in the late 1800s as farms were abandoned as uneconomic in competition with the fertile plains lands farther west. 1'he eastern USA is once again a mainly forested land, and its forests are still relatively young and the trees still growing, so they are storing up carbon rapidly. In China, replanting of previously deforested uplands since the late 1970s has led to a large carbon sink as the trees mature. It is likely that the large-scale movement of population to the cities, and a shift from wood-burning to coal-burning, has also helped forests to recover. An analogous process of forest recovery has occurred in eastern Europe, where a slump in agriculture and movement to the towns has left much land to return to forest. An important thing to bear in mind when thinking about forests as carbon sinks is that no forest can continue soaking up carbon forever. The size of the trees, the amount of fallen woody debris and the amount of organic carbon in soils underneath, will eventually reach a sort of max-

* See also color section.

2600 2400 2200 2000

C* 1800

a 1600

a 1400

if 800 600 400 200

United States Canada Latin America Europe

North Africa and Middle East Tropical Africa Former Soviet Union China

Tropical Asia Pacific Developed Region Total flax

Year

Figure 7.11. Annual net flux of carbon to the atmosphere from land use change: 1850 2000. The changing history of forests has led to some regions (such as in the USA) shifting from a net source to a sink of carbon. Other regions (such as Amazonia) have now taken over in becoming a major source of carbon. When a region goes into the negative on this graph, it is a net sink of carbon. If it goes above zero on the vertical axis, it is a net source of carbon. Source: Houghton and Hackler (CDIAC).

imum steady state. Carbon may be continually fixed by photosynthesis, and released by respiration and decay, but this is just turnover without change in the size of the carbon reservoir in the forest. Individual trees may continue to die and be replaced by new ones, but overall on the scale of the whole landscape there will be no net increase in the amount of carbon contained in the forest ecosystem. Thus, any forest carbon sink will eventually start to saturate and stop taking up carbon. However, starting from newly planted or recovering forest this steady state will only be reached after several hundred years.

Overall, then, it is a complex picture: carbon release from the tropics and carbon uptake in the temperate zone arc having competing influences on the increasing C02 content of the atmosphere. The loss of carbon from the tropics is large enough to win out over temperate forest uptake, raising the atmosphere's C02 content significantly. However, this is considerably smaller than the contribution from fossil fuel derived C03 increase. The two sources combined—deforestation carbon plus fossil fuel carbon currently give an increase in atmosphere C02 of about 1.5 ppm/yr (Figure 7.12).

7.5.2 Seasonal and year-to-year wiggles in CO2 level

As well as affecting the long-term balance of C02, the world's vegetation also affects the C02 level in the atmosphere in smaller, more subtle ways that vary with the seasons, and from one year to another. On the seasonal timescale, there is a "wiggle" in the CO2 level (Figure 7.12), with C02 slightly higher in the winter and early spring than in summer and autumn. In the northern hemisphere, this seasonal variation is greatest in the far north, and it flattens out as one approaches the equator (Figure 7.13). Going south from the equator the seasonal wiggle reappears, but

E 335

345 340 335 330 325

Year

Figure 7.13. The seasonal cycle in C02 concentration varies with latitude. It is strongest in the far north and flattens out towards the equator. It reappears in a much weaker form in the high latitudes of the southern hemisphere. Acronyms initials refer to the names of the CO: monitoring stations (e.g., MLO = Mauna Loa observatory).

reversed in terms of months of the year and corresponding now to the southern hemisphere summer and winter. It is striking, however, that the seasonal wiggle is much smaller in the southern hemisphere.

What causes the seasonal wiggle in C02? It is the result of the way that plants change their activities with the seasons. In the spring and summer, plants in the mid and high latitudes are busy photosynthesizing—taking up carbon dioxide into sugars which is stored as starch, or built into the tough cell walls of leaves and wood. Plants in the higher latitudes take a few weeks to unfurl their leaves and start photosynthesizing rapidly, so there is a delay during May and June as leaves mature. During this stage, decaying leaf litter on the soil surface pushes out a lot of C02, so the atmospheric C02 level in the northern hemisphere reaches a peak in late spring. Then, by late June and July photosynthesis begins in earnest, and so much uptake over such huge areas is enough to produce a noticeable decrease in C02 level in the atmosphere across the whole hemisphere. When autumn comes, the leaves stop photosynthesizing and are dropped to the ground; trees stand leafless and herbaceous vegetation dies back. As winter cold sets in, the uptake of C02 from the atmosphere has essentially stopped over vast swathes of the continents. But in mid-latitudes with milder winters where the ground is not totally frozen, leaves now begin to decay on the forest tloor, releasing CÜ2 which joins the steady trickle of C02 up from the dead organic matter in soils, and the C02 level goes up a tad.

Why then is there much less of a seasonal wiggle in the southern hemisphere after all, don't the plants there also experience seasons? This weaker seasonal cycle occurs because there is much less land south of the equator, and much of the land that does occur is arid and sparsely vegetated, so there is less seasonal activity of plants taking up or releasing carbon.

Taken from one year to the next, the CÜ2 level always goes up due to the amount added by humans burning fossil fuels and clearing tropical forests. But, in some years the increase is greater than in others: as much as a two-fold difference. Human activity does not vary enough from one year to the next to explain such variations, so they must have something to do with natural processes. Partly, these year-to-year differences are due to purely physical changes in the temperature and circulation of the oceans. For example, in an HI Niño year in the Pacific, upwelling of deep waters off the coast of Peru slowrs, and less C02 than usual escapes to the atmosphere. This tends to make the atmospheric C02 increase a little less than usual in that year because uptake in other parts of the oceans is no longer partly balanced by this upwelling C02 source. I lowever, detailed study of the way carbon isotopes in CO: vary from one year to the next shows that changes in the oceans arc not the main cause of year-to-year differences in the rate of increase in CÜ2. Year-to-year differences in C02 are matched by differences in the abundance of the "biological" isotope of C02—carbon-12—that plants are particularly good at taking up. Whenever changes in the photosynthesis or decay of plants (or other organic matter) cause a change in CO: levels, there is a corresponding change in the carbon-12 composition of the atmosphere (see Box 7.1 on carbon isotopes). The fact that inter-annual changes in C02 are paralleled by fairly big changes in carbon-12 suggests that something plants arc doing is a large part of the reason C02 varies on this timescale.

The present increase in C()2 179

Box 7.1 Plants and carbon isotopes

Isotopes are different forms of atoms of the same element, differing in the number of neutron particles in the nucleus of the atom. There are two main stable isotopes of carbon, and one radioactive isotope. By far the most abundant is the lightest form, carbon-12. A small percentage of any sample of carbon consists of the heavier carbon-13. Also, a very small proportion of carbon atoms are the radioactive form carbon-14, which is continuously made at the top of the atmosphere when nitrogen gas is hit by cosmic rays from outer space.

The radioactive carbon-14 form gets into plants when they take up radioactive UC02 from the atmosphere. As well as entering all the living plant materials it is passed down the food chain so everything in the ecosystem picks up some of this radioactive carbon. 14C disappears by radioactive decay at a very precise rate, so the level of radioactivity emanating from carbon in a sample of plant or animal material gives an accurate estimate of how old it is. This technique, radiocarbon-dating, is immensely valuable in finding out the age of samples in archeology, and also in dating ecological changes in the past (Chapter 1) from buried fragments of wood, leaves or soil organic matter. However, by about 50,000 years after the material was first fixed by plants taking in C02 from the atmosphere, all the UC has gone, so the technique cannot be used back beyond this age.

The two stable carbon isotopes can also reveal a lot about both past and present day ecosystem processes. When plants photosynthesize, the enzyme (called rubisco) in their cells that takes in the C02 tends to go preferentially for the lighter ,2C isotope. So, any living material is slightly enriched in 12C, and depleted in I3C (by about 22 parts in a thousand). This difference carries over into the animals that eat the plants, and into organic matter buried in soils and rocks.

If we study the isotope composition of the C02 in the earth s atmosphere over the past couple of hundred years, one thing we can tell is that it is getting "lighter" that is, that more L C is entering the atmosphere (Figure 7.14). We can tell that this carbon is coming from a source that was once living, because it is a source rich in L C. This source could be either present-day forest and soils, or fossil carbon like oil and coal. The fact that the UC content of the atmosphere is also going down rapidly means, even though its rate of production at the top of the atmosphere has stayed constant, it is being diluted by a large portion of very old carbon. This old carbon must be coming from fossil fuels. The combined picture from both the stable and radioactive carbon Isotopes tallies with general expectations from observing forest clearance and the rate of use of fossil fuels: that most of the C02 rise is from fossil fuels with a smaller part from tropical forest clearance.

Carbon isotopes can also reveal broad trends in the global carbon cycle going back many millions of years. In the distant history of the earth, after about 450 million years ago, there was a big decline in the amount of I:C in ocean carbonate minerals. Such minerals reflect the composition of the C02 "left behind" in the atmosphere after some of it has been extracted by plants. What the decline in 12C

1 10

Late Miocene global carbon shfft

Late Miocene global carbon shfft

• North American horse teeth □ Pakistan mammal teeth -O Pakistan paleosois Ocean carbonate shift

• North American horse teeth □ Pakistan mammal teeth -O Pakistan paleosois Ocean carbonate shift

613CPOB (parts per thousand)

Figure 7.15. A carbon isotope shift around 7 million years ago indicates that C4 plants suddenly became much more common. Source: Cerling et al. (1993).

is less depleted in l?C, by only around 8 atoms in a thousand as opposed to 22 atoms in C3 plants. It is easy then to trace whether a soil has had C4 or C3 plants growing in it by measuring the composition of soil carbon derived from the plants that grew there. Around 7 million years ago there was a sudden decrease in :C in many soils in many parts of the world (Figure 7.15), revealing a widespread shift from C3 plants to C4 plants. It is thought that this switchover reflects a drying of the earth's climate, plus a decrease in CO? levels which would also favor C4 plants.

7.6 THE SIGNAL IN THE ATMOSPHERE

Although its trend is always upwards, the actual amount by which the global CO: concentration increases tends to vary from one year to another. Years in which tropical forest regions are slightly hotter than usual tend to have a greater CO: increase. This suggests that in these warmer than usual years the tropical forests lose carbon through some sort of temperature-dependent process, perhaps increased respiration by the leaves, or increased rotting of dead wood and other litter in the

Sop to Aug: 1959 to 2003; Surface Air Temperature Seasonal correlator with Jan to Dec C02 (C02 lags by 4 moo?«)

Ar to Awj 1S6C to ?05J &ri»ce P»eoptrtar Rats Season* corraUbor Jjnt> D»c C02

Figure 7.16. (a) Tin's map shows the strength of correlation between temperature and global CO2 increment each year and that C02 increment in a given year is correlated with mean temperature in the tropics. When temperatures in South-Hast Asia and Amazonia are higher, there tends to be a big increase in global CO> in that year (NCEP, NCAR re-analysis, NO A A/ ORES Climate Diagnostics Center), (b) A map showing the correlation between the amount of rainfall and the size of the global C02 increment around the world. The relationship to rainfall in forest regions of the tropics is much more scattered and weaker overall, suggesting that heat rather than lack of rainfall may be more important in producing a burst of carbon from the tropics is some years. This might be due to some combination of faster decay, poor photosynthesis and growth of trees under heal stress, or more rapid evaporation stressing trees and preventing photosynthesis. Source: Author, in collaboration with Gianluca Piovesan.

Sop to Aug: 1959 to 2003; Surface Air Temperature Seasonal correlator with Jan to Dec C02 (C02 lags by 4 moo?«)

Ar to Awj 1S6C to ?05J &ri»ce P»eoptrtar Rats Season* corraUbor Jjnt> D»c C02

Figure 7.16. (a) Tin's map shows the strength of correlation between temperature and global CO2 increment each year and that C02 increment in a given year is correlated with mean temperature in the tropics. When temperatures in South-Hast Asia and Amazonia are higher, there tends to be a big increase in global CO> in that year (NCEP, NCAR re-analysis, NO A A/ ORES Climate Diagnostics Center), (b) A map showing the correlation between the amount of rainfall and the size of the global C02 increment around the world. The relationship to rainfall in forest regions of the tropics is much more scattered and weaker overall, suggesting that heat rather than lack of rainfall may be more important in producing a burst of carbon from the tropics is some years. This might be due to some combination of faster decay, poor photosynthesis and growth of trees under heal stress, or more rapid evaporation stressing trees and preventing photosynthesis. Source: Author, in collaboration with Gianluca Piovesan.

forests. It seems that forests in South-East Asia and in the Amazon Basin particularly dominate this temperature response (Figure 7.16a,b*).

Could it be the effect of drought on the rainforest which is actually causing the big rise in global C02 in a hot year in the tropics? After all, the sunny, cloudless skies associated with high temperatures may also tend lo be associated with lack of rain. However, when we look at the data in detail, drought in the tropical forests does not seem to have nearly as strong an effect as temperature itself; the statistical relationship of global C02 increment with rainfall is much weaker whereas if drought were so important we would cxpcct it to be stronger. Nevertheless, there are exceptions—the big droughts that are associated with some El Niño events do seem to have at least some effect on carbon release in South-East Asia, where fires set by farmers often spread into tropical forests and burn across huge areas, sending smoke and haze across the region. For example, such extensive fires occurred across Indonesia and parts of Malaysia during droughts in 1982/1983, and also in 2003/2004, that they shut dow n airports hundreds of miles from the sources of the smoke.

El Niño events arc generally strongly correlated with a large global C02 increment. However, cxccpt for the really extreme ones associated with strong droughts and fires, it looks like El Niño operates more through bringing about high temperatures that affect the carbon balance of the forests (El Niños are generally associated with warmer conditions in the main tropical rainforest regions), rather than causing drought.

The mid-latitude forests of the USA, Scandinavia and northeastern Asia also seem to play a role in affecting variability in CG2 increase each year, but their effect is weaker. It is also opposite to the trend in the tropics: in a warm year the mid-latitude forests tend to take up more carbon. The trend is also rather complex; a particularly cool year seems to shut down the decay of leaves and wood on the floor of the boreal forests of Siberia and Canada, and because there is so little decay, much less C02 is released from this forest litter to the atmosphere. I bis more than cancels out the smaller C02 uptake due to reduced photosynthesis in the tree leaves in a cooler year. A year like this occurred in 1991/1992 after the big volcano Pinatubo exploded in the Philippines and altered climate around the northern hemisphere with the cloud of sulfuric acid that it pushed into the stratosphere. The northern forest zones were cooled, and with less decay the C02 increment in the world's atmosphere during that year was unusually small.

In the tropics there is also a weaker and rather mysterious two-year delay between a blip in temperature and a blip in their contribution to the global CÜ2 increment. Compared with the "immediate" (same year) effect of temperature on C02 release by the tropics, the two-year lagged effect is the opposite: it takes up rather than releases more C'02 in response to a warmer year. It is thought that this lagged response has something to do with the effect of increased temperature on recycling of nitrogen in forest ecosystems. In a warmer year more decay occurs, enabling nitrogen bound up in dead leaves and other material on the forest floor to be released as nitrates and ammonia that can then be used by the trees again. This produces a burst of growth of new leaves and wood about two years later when the trees have adjusted to the increased supply of nitrogen; and the addition of those new leaves takes up C02 from the atmosphere as they begin to photosynthesize.

As one would expect if plant and fossil fuel carbon is being burned to give C02, the oxygen concentration in the atmosphere is slowly declining. The amount of the decline shows a seasonal wiggle that is the opposite of the CQ2 wiggle: in the summer

The missing sink is not quite as mysterious as it might sound; no-one believes this carbon is leaving for outer space, or being sucked deep into the earth's interior, so it must be going somewhere within cither the oceans or land ecosystems. Probably, it is a result of some simple, basic errors in calculation of uptake by familiar processes in the oceans or forests. However, it might be the result of other poorly understood mechanisms, such as a direct C02 fertilization effect making existing forests grow faster than before and accumulate carbon (see Chapter 8). It is important to try to understand the missing sink in order to predict how it might behave in future. For instance, perhaps before long it will begin to saturate and stop taking up CO:, or even go into reverse and start releasing carbon? For this reason there have been many studies in recent years which aim to narrow the uncertainties. These include eddy flux studies (next section).

An important clue to the missing sink comes from detailed measurement of average C02 concentrations around the world. The C02 concentration over the mid-latitudes of the northern hemisphere (Europe, Russia, China, North America and the northern Atlantic) is lower than it should be, given the calculated rate at which C02 is supposed to be getting taken up into ocean waters and temperate forests. This "dip" in CO: over the northern mid-latitudes suggests that this is where the missing carbon in the calculations is going. It looks like there is an especially-strong absorption of C02 going on over the eastern part of the USA and Canada. Even though these regions are actually big sources of greenhouse gas due to fuel-burning, the amount of CO? coming off them should be substantially larger than it actually is, showing that some is going missing. Most scientists who work on this subject think that the extra C02 is going into maturing forests in the temperate regions, and that they are just taking it up faster than anyone had expected. It could be though that part of the unaccounted-for carbon is being absorbed in the North Atlantic, and that previous studies of C02 uptake in that region underestimated how-fast it is dissolving in the ocean water. It might also be that the estimates of the release of carbon from tropical forest clearance are a bit too high, tending to widen a "gap" (compared with C02 accumulation in the atmosphere) that really was not so large in the first place.

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