Tropical deforestation and the global carbon budget (original) (raw)
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A commentary on: Tropical deforestation and atmospheric carbon dioxide
Climatic Change, 1991
's contribution confirms what many had suspected: that the rate of CO 2 emission to the atmosphere from tropical deforestation is substantially larger than what it was in 1980, the year on which previous analyses of the role of tropical deforestation in the global carbon cycle have been based. Houghton estimates a likely 1989 emission of 1.5-3.0 x 1012 kg C, compared to a 1980 emission of 1.0-2.0 x 1012 kg C using the same methodology and assumptions. This increase is a direct consequence of a dramatic increase in rates of deforestation for a variety of social, political, and economic reasons. The most serious consequence of this deforestation in my opinion is not its effect on climate or atmospheric carbon dioxide, but the massive species extinctions-a biological holocaust-which it implies. Absorption of atmospheric CO 2 by the oceans will remove about 85% of the emitted CO 2 within a few hundred years, dissolution of marine carbonate sediments will remove another 10% or so within a few thousand years, and silicate weathering will take care of the rest within about 100 000 years, which is a very short period of time from an evolutionary perspective. Species extinction, in contrast, is irreversible. This having been said, there are a number of important implications of Houghton's contribution with regard to the immediate issues of atmospheric pollution, understanding the carbon cycle at decadal to century time scales, and policy responses. The first of these is addressed by Keller et al. (this issue): an increase in CO2 emissions due to burning of forests implies an increase in emissions of CO, NOx, and other trace gases. Andrea et al. (1988) estimated that 5-20% of the carbon in combusted material in burning tropical forests is released as CO rather than as CO2, and about half of the carbon released in 1989 by deforestation appears to have been a result of burning in that year. The emitted CO is ultimately oxidized to CO2 (thanks to the presence of OH in the atmosphere), but not before a number of far-reaching chemical effects, including production of ozone (a greenhouse gas) and indirect extension of the atmospheric lifetime of CH 4 and the CFCs (further greenhouse gases). One of the persistent problems facing Earth System scientists is the elementary task of balancing the carbon cycle, that is, accounting for the difference between anthropogenic emissions and the observed rate of increase of atmospheric CO 2 in terms of plausible sinks of CO 2. The rate of atmospheric CO 2 increase during the period January 1986-January 1990 was about 3.5 x 1012 kg C yr-1. Fossil-fuel emissions averaged about 5.5 x 1012 kg C yr-1, which, combined with Houghton's likely deforestation emission of 1.5-3.0 x 1012 kg C yr-1, gives a total anthropogenic emission of 7.0-8.5 x 1012 kg C yr-1 and requires sinks of 3.5-5.0 x 1012 kg C yr-1. A recent study by Tans et al. (1990) indicates that the oceans were taking up
Philosophical Transactions of the Royal Society B: Biological Sciences, 2004
The remaining carbon stocks in wet tropical forests are currently at risk because of anthropogenic deforestation, but also because of the possibility of release driven by climate change. To identify the relative roles of CO 2 increase, changing temperature and rainfall, and deforestation in the future, and the magnitude of their impact on atmospheric CO 2 concentrations, we have applied a dynamic global vegetation model, using multiple scenarios of tropical deforestation (extrapolated from two estimates of current rates) and multiple scenarios of changing climate (derived from four independent offline general circulation model simulations). Results show that deforestation will probably produce large losses of carbon, despite the uncertainty about the deforestation rates. Some climate models produce additional large fluxes due to increased drought stress caused by rising temperature and decreasing rainfall. One climate model, however, produces an additional carbon sink. Taken together, our estimates of additional carbon emissions during the twenty-first century, for all climate and deforestation scenarios, range from 101 to 367 Gt C, resulting in CO 2 concentration increases above background values between 29 and 129 p.p.m. An evaluation of the method indicates that better estimates of tropical carbon sources and sinks require improved assessments of current and future deforestation, and more consistent precipitation scenarios from climate models. Notwithstanding the uncertainties, continued tropical deforestation will most certainly play a very large role in the build-up of future greenhouse gas concentrations.
Tropical forests and atmospheric carbon dioxide: current conditions and future scenarios
Avoiding Dangerous …, 2006
Tropical forests affect atmospheric carbon dioxide concentrations, and hence modulate the rate of climate change -by being a source of carbon, from land-use change (deforestation), and as a sink or source of carbon in remaining intact forest. These fluxes are among the least understood and most uncertain major fluxes within the global carbon cycle. We synthesise recent research on the tropical forest biome carbon balance, suggesting that intact forests presently function as a carbon sink of approx. 1.2 Pg C a Ϫ1 , and that deforestation emissions at the higher end of the reported 1-3 Pg C a Ϫ1 spectrum are likely. Scenarios suggest that the source from deforestation will remain high, whereas the sink in intact forest is unlikely to continue, and remaining tropical forests may become a major carbon source via one or more of (i) changing photosynthesis/respiration rates, (ii) functional/ biodiversity changes within intact forest, or widespread forest collapse via (iii) drought, or (iv) fire. Each scenario risks possible positive feedbacks with the climate system suggesting that current estimates of the possible rate, magnitude and effects of global climate change over the coming decades may be conservative.
Determination of tropical deforestation rates and related carbon losses from 1990 to 2010
Global Change Biology, 2014
We estimate changes in forest cover (deforestation and forest regrowth) in the tropics for the two last decades (1990-2000 and 2000-2010) based on a sample of 4000 units of 10 910 km size. Forest cover is interpreted from satellite imagery at 30 9 30 m resolution. Forest cover changes are then combined with pan-tropical biomass maps to estimate carbon losses. We show that there was a gross loss of tropical forests of 8.0 million ha yr À1 in the 1990s and 7.6 million ha yr À1 in the 2000s (0.49% annual rate), with no statistically significant difference. Humid forests account for 64% of the total forest cover in 2010 and 54% of the net forest loss during second study decade. Losses of forest cover and Other Wooded Land (OWL) cover result in estimates of carbon losses which are similar for 1990s and 2000s at 887 MtC yr À1 (range: 646-1238) and 880 MtC yr À1 (range: 602-1237) respectively, with humid regions contributing two-thirds. The estimates of forest area changes have small statistical standard errors due to large sample size. We also reduce uncertainties of previous estimates of carbon losses and removals. Our estimates of forest area change are significantly lower as compared to national survey data. We reconcile recent low estimates of carbon emissions from tropical deforestation for early 2000s and show that carbon loss rates did not change between the two last decades. Carbon losses from deforestation represent circa 10% of Carbon emissions from fossil fuel combustion and cement production during the last decade (2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010). Our estimates of annual removals of carbon from forest regrowth at 115 MtC yr À1 (range: 61-168) and 97 MtC yr À1 (53-141) for the 1990s and 2000s respectively are five to fifteen times lower than earlier published estimates.
Perturbations in the carbon budget of the tropics
The carbon budget of the tropics has been perturbed as a result of human influences. Here, we attempt to construct a 'bottom-up' analysis of the biological components of the budget as they are affected by human activities. There are major uncertainties in the extent and carbon content of different vegetation types, the rates of land-use change and forest degradation, but recent developments in satellite remote sensing have gone far towards reducing these uncertainties. Stocks of carbon as biomass in tropical forests and woodlands add up to 271 AE 16 Pg with an even greater quantity of carbon as soil organic matter. Carbon loss from deforestation, degradation, harvesting and peat fires is estimated as 2.01 AE 1.1 Pg annum À1 ; while carbon gain from forest and woodland growth is 1.85 AE 0.09 Pg annum À1 . We conclude that tropical lands are on average a small carbon source to the atmosphere, a result that is consistent with the 'top-down' result from measurements in the atmosphere. If they were to be conserved, they would be a substantial carbon sink. Release of carbon as carbon dioxide from fossil fuel burning in the tropics is 0.74 Pg annum À1 or 0.57 MgC person À1 annum À1 , much lower than the corresponding figures from developed regions of the world.
Tropical deforestation and greenhouse gas emissions
2004
A recent (2002) analysis concluded that rates of tropical deforestation and atmospheric carbon emissions during the 1990-1997 interval were lower than previously suggested. We challenged this assertion with respect to tropical carbon emissions, but our conclusions were disputed by the authors of the original study. Here we provide further evidence to support our conclusion that the effect of tropical deforestation on greenhousegas emissions and global warming is substantial. At least for Brazilian Amazonia, the net impact of tropical deforestation on global warming may be more than double that estimated in the recent study.