Current warming and likely future impacts - Climate Change: Biological and Human Aspects - page 262

century and are thought to possibly be related to global warming. However, countering
this two separate analyses suggest that El Ni no-related droughts were not the cause
of these tropical rainforest changes, although the researchers advise further checks,
so this matter has yet to be resolved. Third, rainfall changes (as distinct from drought,
which sees a decline in rainfall, river flow and ground water) might be the cause, but
this too was discounted on checking meteorological records.
The final possible reason was that the changes might be driven by accelerated
forest productivity. The researchers believed that this was the most likely reason,
due to rising atmospheric carbon dioxide, although increased cloudiness and higher
airborne nutrient transfer from increased forest fires (both climate-related factors)
could be the cause. Either way, greenhouse changes seem to be the spur.
In recent decades the rate of warming in Amazonia has been about 0.25 ◦ C per
decade. Under mid-range greenhouse gas emission scenarios, temperatures are pro-
jected to rise 3.3 ◦ C (range 1.8-5.1 ◦ C) this century, slightly more in the interior in the
dry season, or by up to 8 ◦ C if significant forest dieback affects regional biophysical
properties. At the end of the last glacial, Amazonia warmed by only approximately
0.1 ◦ C per century. Changes in precipitation, particularly in the dry season, are prob-
ably the most critical determinant of the climatic fate of the Amazon. There has
been a drying trend in northern Amazonia since the mid-1970s, and no consistent
multi-decadal trend in the south, but some global climate models project significant
Amazonian drying over the 21st century. The zones of highest drought risk are the
south east and east, which are also the areas of most active deforestation. In contrast,
the north-western Amazon is least likely to experience major drought (Malhi et al.,
2007).
These changes could have both local and global implications. Currently, undis-
turbed tropical rainforests appear to function as carbon sinks and so they help slow
global warming. A 1998 long-term monitoring study of plots in mature humid trop-
ical forests concentrated in South America revealed that biomass gain by tree growth
exceeded losses from tree death in 38 of 50 Neotropical sites. These more humid
parts of the Amazon forest plots have gained 0.71 0.34 t of carbon (t C) ha − 1 year − 1

above ground (i.e. excluding soil carbon) (Phillips et al., 1998). A similar 2009 study
in Africa (which included the results of previous studies), where a third of the Earth's
tropical forests are found, for the period 1968-2007 gave an average above-ground
carbon accumulation rate of 0.63 t C ha − 1 year − 1 . The data suggest that undisturbed
Neotropical forests may (currently) be a significant carbon sink, reducing the rate of
increase in atmospheric carbon dioxide. Scaling up to tropical rainforests of the whole
African continent suggests total above-ground accumulation of 0.34 gigatonnes of
carbon (GtC) year − 1 . Including this with the aforementioned and other studies up
to 2009 suggest that globally tropical forests above ground might be accumulating
carbon at a rate of 1.31 GtC year − 1 within the range 0.79-1.56 GtC year − 1 in recent
decades (Lewis et al., 2009). This compares with an estimated global land sink (all
terrestrial biomes) of very approximately 2.2 GtC year − 1 between 1980 and 2000
(1.4 GtC year − 1 for 1990-9; IPCC, 2001b). The tentative suggestion is that at the
moment tropical rainforests are increasing as a carbon sink.
Yet ecological changes such as seen in the rainforest study by Laurance et al.
(2004), if pervasive, could modify this sink effect. Increases in carbon storage by

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