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1.2. Biogeochemical Cycles of Greenhouse Gases
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).
1.2.1. The Global Carbon Cycle
1.2.1.1. Natural and Human-Induced Changes in the Past
Carbon Cycle
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; Indermühle 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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