The carbon cycle, described by David Schimel in Chapter 11, is the flow of carbon in various chemical forms through the Earth's atmosphere, oceans, biosphere and lithosphere. It involves chemical (geological) processes such as the formation of carbonate rock, physical processes such as the dissolution of atmospheric carbon dioxide in surface waters, mechanical processes such as the transport of dissolved carbon dioxide to the deep ocean-and the biological processes of photosynthesis and respiration. Compared with the other processes, the biological cycle is fast: although the amounts of carbon stored in rocks and the deep ocean are much greater than those in biomass or the atmosphere (see Fig. 1 1. l), the amount of carbon taken up annually by photosynthesis and released back to the atmosphere by respiration is 1,000 times greater than the annual flow of carbon through the geological cycle.
Over geological time, the flow of carbon through the carbon cycle has remained essentially responsive to climatic conditions, as they in turn have been determined by changes in the Earth’s orbit, volcanic action and continental drift. The concentration of C02 in the atmosphere has always changed in response to these natural changes in Earth’s climate. But now the boot is on the other foot. Anthropogenic (man-made) COZ emissions, coming mainly from fossil fuel burning, cement production and changes in land use, now constitute some 4% of total CO2 emissions to the atmosphere. The balance of the carbon cycle is a fine one and some of the steps in it are slower than others. In particular, the rate at which we are adding COz to the atmosphere is faster than natural uptake processes can adapt to take it out again. The concentration of C02 in the atmosphere is rising by about 0.4% per year as a result mainly of fossil fuel burning and land use change, and global warming is upon us. The UN’s Intergovernmental Panel on Climate Change (IPCC) estimates that greenhouse gas emissions have already led to an increase in global mean surface temperature of about 0.6 C, and that this is likely to increase by a further 1.4-5.8 C over the next 100 years (Climate Change, 2001).
Photosynthesis removes CO:! from the atmosphere and stores it as fixed carbon for the lifetime of the photosynthetic organism. The subsequent decay of dead photosynthetic biomass returns the temporarily fixed carbon to the atmosphere as C02 (except in the case of marine photosynthetic organisms, which may be exported to the deep ocean). The mass of carbon fixed annually by photosynthesis is termed primary production. Gross primary production (GPP), also known as assimilation, is the gross amount of carbon fixed per year. Perhaps surprisingly, GPP in the oceans (-103 GtC per year) is nearly as great as that on land (-12OGtC per year).* This is because phytoplankton, despite containing only about 5 GtC, or 0.2% of total photosynthetic biomass, turn over much faster than land plant^.^ Falkowski, Geider and Raven discuss the evolution and role of aquatic photosynthesis in Chapter 6.
Photosynthetic organisms consume some of their fixed carbon by autotrophic respiration, and net primary production (NPP) is what remains after this has occurred. On land, NPP is -60 GtC per year, and in the oceans it is -45 GtC per year. NPP is thus about half GPP, both on land and in the oceans. The remaining photosynthetic biomass sooner or later dies and returns its carbon as CO2 to the atmosphere through decay, fire or conversion to compounds resistant to decomposition, so the total amount of photosynthetically fixed carbon varies only slowly over time.
Figure 1.2 shows the distribution of land stocks of carbon between different vegetation types. The longer-lived the photosynthetic organism, the more carbon it tends to amass in its lifetime. OF the -2500 G C stored in terrestrial biomass. nearly all is contained in long-lived higher land plants-a term that includes trees-and their associated soils. Tropical. temperate and boreal trees contain nearly half the total, in the forests that cover about one-quarter of the land area of Earth. The forest soils of lemperate forests store much more carbon than do tropical moist forests, but tropical forests grow much faster because the fall and regrowth of leaves occur continuously throughout the year and the forest is always active.
Planting and growing more 1ree.s could make a useful, though one-off, contribution to arresting the increase of atmospheric C02 levels by fixing more carbon in wood and soils. Richard Tipper and Rebecca Cam discuss these possibilities, and how they might be encouraged and validated, in Chapter 12. It is also possible, though currently judged unlikely in the long tern, that the feedback from increased levels of C 0 2 in the atmosphere could ‘fertilise’ photosynthesis, i.e. increase its net rate; David Schimel discusses this in Chapter 11.
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