Tropical Land-Use Change Emissions—Smaller, but Still Very Significant

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Before the United Nations Climate Change Conference (COP15) held in December 2009 in Copenhagen, the Brazilian media picked up the issue of REDD (Reduction of Emissions from Deforestation and Forest Degradation). A variety of somewhat conflicting statements came from all quarters: the scientific community, government authorities, environmental NGOs and other interest groups. As expected, they spanned a wide range of views on the issue.

Broadly speaking, tropical deforestation has been declining. Thus, a fundamental question has been put forward: are land-use change emissions of GHGs quantitatively significant enough to warrant a special mechanism under the UNFCCC? Some critics of REDD maintain that emissions from tropical land-use change are not as large as has been assumed, and that it is not as important as emissions from other sectors such as fossil fuel combustion.

Even before I get into the details, let me emphasize that tropical deforestation and REDD are still just as significant as before, and as important, for instance, as the share from transportation emissions. As I will describe in this post, the latest calculations using new data that has become available after the last IPCC report (2007)—following the same methodology as the IPCC—shows that the share of tropical land-use change in overall CO2 emissions has fallen. However, looking at the big picture, tropical deforestation is still a massive issue to tackle in the battle against climate change and attention should not be diverted from REDD. 

After the 2007 IPCC report, it became standard to place the share of total CO2 emissions coming from tropical land-use change at 20 percent. The IPPC Summary for Policy Makers (SPM)¹ (see below) refers to estimates of land-use change emissions for the 1990s as an average value of 1.6 GtC/year (billion tons of carbon per year), but with a very large uncertainty range of 0.5 to 2.7 GtC/year. The average fossil fuel emissions for the 1990s were estimated at 6.4 GtC/year. Therefore, land use change emissions were calculated as 1.6/(6.4 + 1.6) = 0.2. That is how the 20 percent figure was arrived at.

But, as new data shows, things have changed in this decade. Fossil fuel emissions increased at the fast pace of 3.5 percent a year, in part as a result of the large growth of two emerging economies—China and India—with only a small blip in 2008 and 2009 due to the global economic downturn. In the opposite direction, tropical land-use change emissions stabilized and even declined somewhat. According to updated estimates of the Global Carbon Project (www.carbonproject.org)²  the average tropical land-use change emissions were 1.5 GtC/year in the period 2000-2006, roughly corresponding to 15¨to 16 percent of global emissions of CO2. That emission rate has further declined over the next two years, 2007 and 2008, reaching a lower value of 1.2 GtC/year on average in 2008. That is attributed to reductions in deforestation rates both in Brazil and Indonesia and also to fewer peatland fires in the latter.

Currently, the total fossil fuel emissions are about 8.5 GtC/year (preliminary data for 2008). Therefore, in the CO2 emission budget for 2008, the global total CO2 emissions tally at 9.7 GtC/year (8.5 + 1.2).  Therefore, the tropical land use change fraction is (1.2/(8.5 + 1.2)) = 12.4 percent, a value considerably smaller than the 20 percent of the 1990s. The decrease of the tropical land-use change fraction of close to 8 percentage points is due to both increase of fossil fuel emissions (accounting for about 4 percent) and reductions of tropical land-use emissions (accounting for another 4 percent).

As I said earlier, even at 1.2 GtC/year, these emissions are still very significant and comparable to the CO2 emissions of transportation, which stand at approximately 14 percent. More importantly, what matters is the absolute value of emissions since this is what is causing global warming and tropical land-use change emissions are large by any measure.

Further, the figure has to be seen as a lower limit for the total GHG emissions from tropical land-use change, since there are significant emissions of other GHG (CH4 and N2O) associated with vegetation fires. A recent paper in Science³ by Bowman et al seems to indicate that emissions directly associated to fires can account for a much larger fraction of GHG emissions (up to 17 percent according to their estimates). Therefore, all things considered, one might say that land-use change emissions today account for less than 20 percent for CO2 alone, but makes up for a larger percent for all GHGs.

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¹The IPCC SPM WGI 2007 states: "The primary source of the increased atmospheric concentration of carbon dioxide since the pre-industrial period results from fossil fuel use, with land-use change providing another significant but smaller contribution. Annual fossil carbon dioxide emissions increased from an average of 6.4 [6.0 to 6.8] GtC (23.5 [22.0 to 25.0] GtCO2) per year in the 1990s to 7.2 [6.9 to 7.5] GtC (26.4 [25.3 to 27.5] GtCO2) per year in 2000–2005 (2004 and 2005 data are interim estimates). Carbon dioxide emissions associated with land-use change are estimated to be 1.6 [0.5 to 2.7] GtC (5.9 [1.8 to 9.9] GtCO2) per year over the 1990s, although these estimates have a large uncertainty. {7.3}."

²LeQuéré at al., 2009. Nature Geosciences, 2. doi: 10.1038/ngeo689

³Bowman et al., 2009: Fire in the Earth System. Science, 324:481-484. doi: 10.1126/science.1163886.