Additional info from Skeptical Science
The Intergovernmental Panel on Climate Change (IPCC) statement on Amazonian forests can be found in Section 13.4.1 of the IPCC Fourth Assessment Report:
"Up to 40% of the Amazonian forests could react drastically to even a slight reduction in precipitation; this means that the tropical vegetation, hydrology and climate system in South America could change very rapidly to another steady state, not necessarily producing gradual changes between the current and the future situation (Rowell and Moore, 2000)."
The reference is Global Review of Forest Fires, a non-peer-reviewed report by the World Wildlife Fund (WWF). The WWF report makes the following statement:
"Up to 40% of the Brazilian forest is extremely sensitive to small reductions in the amount of rainfall. In the 1998 dry season, some 270,000 sq. km of forest became vulnerable to fire, due to completely depleted plant-available water stored in the upper five meters of soil. A further 360,000 sq. km of forest had only 250 mm of plant-available soil water left. [Nepstad et al. 1999]"
The WWF correctly states that 630,000 km2 of forests were severely drought stressed in 1998 — this figure comes from Nepstad et al. 1999. However, the 40% figure comes from several other papers by the same author that the WWF failed to cite. A 1994 paper estimated that around half of the Amazonian forests lost large portions of their available soil moisture during drought. In 2004, new rainfall data showed that half of the forest area of the Amazon Basin had either fallen below, or was very close to, the critical level of soil moisture below which trees begin to die. The results from these papers are consistent with the original statement that "up to 40% of the Brazilian forest is extremely sensitive to small reductions in the amount of rainfall."
Subsequent research has provided additional confirmation of the Amazonian forest's vulnerability to drought. Field measurements of the soil moisture critical threshold found that tree mortality rates increase dramatically during drought. Another study measured the effect of the intense 2005 drought on Amazonian biomass. The drought caused massive tree mortality leading to a fall in biomass. This turned the region from a large carbon sink to a carbon producer. The paper concluded that "such events appear capable of strongly altering the regional carbon balance and thereby accelerating climate change."
An investigation into the peer-reviewed scientific literature shows the information presented by the IPCC on Amazonian forests is correct. The error is that the WWF erroneously omitted the citations supporting the "up to 40% of the Brazilian forest is extremely sensitive …" statement. The lesson here is that the IPCC could have avoided this glitch if they'd quoted directly from the original peer-reviewed papers. Critics of the IPCC, if their goal is a clearer understanding of the science, would also do well to follow this advice.
Adapted from © John Cook and Skeptical Science