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Terrestrial ecosystems are important components in the biogeochemical cycles that create many of the sources and sinks of carbon dioxide, methane, and nitrous oxide and thereby influence global responses to human-induced emissions of greenhouse gases (GHGs). The dynamics of terrestrial ecosystems depend on interactions between a variety of biogeochemical cycles, particularly the carbon cycle, the nutrient cycles, and the circulation of water-all of which may be modified indirectly by climate changes and by direct human actions (e.g., land-use/cover change).
Analyses of air bubbles in ice cores from Greenland and the Antarctic have given us a reasonably clear idea about variations in atmospheric CO2 concentration since the end of the last glacial maximum. It was then about 200 ppmv; it rose gradually to about 250 ppmv 8,000 years ago and subsequently by 25 ppmv during the following 7,000 years. During the past millennium until the beginning of the industrial revolution, CO2 varied between 275 and 285 ppmv. There seems to have been an increase of about 10 ppmv around 1300 AD, followed by a 10 ppmv decrease around 1600 AD (i.e., during the Little Ice Age) (Barnola et al., 1995; Etheridge et al., 1996; Inderm�hle et al., 1999). All of these changes took place gradually, and the rate of change in the atmospheric reservoir probably seldom exceeded a few Gt C per decade (Ciais, 1999).
The CO2 concentration has risen from the range noted above to a concentration of 366 ppmv in 1998 (Keeling and Whorf, 1999). The decadal rate of change over the past century has been persistent and more rapid than during any other period in the last millennium. This rate of change can be explained by the cumulative effects of emissions from fossil fuel combustion and land clearing and the response of the oceans and biosphere to this anthropogenic perturbation.
From 1850 to 1998, 270 � 30 Gt C were emitted from fossil fuel burning and cement production (Marland et al., 1999); 176 � 10 Gt C accumulated in the atmosphere (Etheridge et al., 1996; Keeling and Whorf, 1999). The cumulative ocean uptake during this time has been estimated (with the aid of ocean carbon cycle models) to be 120 � 50 Gt C (Kheshgi et al., 1999; Joos et al., 1999). This estimate of ocean uptake is more uncertain than estimates of total emissions from fossil fuel burning and the accumulation in the atmosphere (Siegenthaler and Joos, 1992; Enting et al., 1994). Nevertheless, balancing the carbon budget for this period yields a global net terrestrial source of about 26 � 60 Gt C. In other words, it is likely that the terrestrial system has been a source during this period.
It is relevant to compare the magnitude of this global net terrestrial source with direct estimates of emissions during this time resulting from the expansion of cropland, deforestation, and other land-use changes (see Section 1.4.1). The area covered by cropland in temperate regions (particularly in North America and the former Soviet Union) reached a maximum by the middle of the 20th century (Ramankutty and Foley, 1998). The rate of increase of croplands in tropical regions (mainly Latin America), however, surpassed that of temperate regions around 1960 (Houghton, 1994, 1999).
During the period 1850-1998, net cumulative global CO2 emissions from land-use change are estimated to have been 136 � 55 Gt C (assuming that the relative uncertainty of land-use change emissions is the same as the estimate for the 1980s). Of these emissions, about 87 percent were from forest areas and about 13 percent from cultivation of mid-latitude grasslands (Houghton, 1999; Houghton et al., 1999, 2000). A residual global terrestrial sink of 110 � 80 Gt C is therefore required to reconcile the difference between the net terrestrial source estimated by balancing the carbon budget (26 � 60 Gt C) and the larger terrestrial source estimated by accounting for the effects land-use change on carbon stocks (136 � 55 Gt C). Thus, this residual terrestrial carbon sink-popularly referred to as the "missing carbon sink"-was comparable in size to the net ocean uptake over this period.
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