The Present Increase In C02

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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 CO2 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 CO2 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 CO2 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 CO2 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 CO2 from deforestation, with South-East Asia following second.

However, not all forests are losing carbon. Forests areas in several parts of the world have clearly now switched from being a "source" of CO2, 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. The 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. Recent estimates suggest that most of the net uptake into forests is occurring in the southern half of China, while forests in the northeast of China continue to be more heavily exploited. 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 maximum 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 (Figure 7.12). 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.

* See also color section.

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O 12 00

1850 1860 1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

United States Canada

-Latin America


North Africa and Middle East Tropica] Africa

-Former Soviet Union


- Tropical Asia

Pacific Developed Region Total flux

1850 1860 1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000


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. 0ther 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).

Figure 7.12. When an area of land is allowed to return to forest, there is an initial accumulation of carbon which levels off after a certain amount of time and reaches a balance between the growth and death of trees.






Figure 7.12. When an area of land is allowed to return to forest, there is an initial accumulation of carbon which levels off after a certain amount of time and reaches a balance between the growth and death of trees.

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Figure 7.13. The record of atmospheric C02 increase since the 1950s, measured directly at the Mauna Loa Observatory in Hawaii (monthly average C02 concentration).

Overall, then, it is a complex picture: carbon release from the tropics and carbon uptake in the temperate zone are 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 C02 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 7.13).

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