Earlier this week in the Telegraph, the intrepid James Delingpole debuted "Amazongate." Like Himalayagate, this is a case in which the IPCC relied on a World Wildlife Fund (WWF) report, rather than a peer-reviewed scientific study, to make a scary claim about global warming.
The IPCC (Working Group II, Ch. 13, p. 596) says that, "Up to 40% of the Amazonian forests could react drastically to even a slight reduction in precipitation" due to global warming. The IPCC's reference for this claim is Rowell and Moore (2000), which turns out to be an IUCN/WWF report, Global Review of Forest Fires.
The IUCN/WWF report does cite a peer-reviewed study to support the 40% estimate: Nepsted et al. 1999. Large - scale Impoverishment of Amazonian Forests by Logging and Fire, Nature, Vol. 398, p. 505. The study is available here.
But the Neptsed study doesn't quite say what WWF suggests it does. The study says: "Because of the severe drought of 1997 and 1998, we calculate that approximately 270,000 km2 of Amazonian forest had completely depleted plant-available water stored in the upper five metres of soil by the end of the 1998 dry season. In addition, 360,000 km2 of forest had less than 250mm of plant-available soil water left by this time . . ."
The IUCN/WWF report reproduces that statement almost verbatim:
"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 metres of soil. A further 360,000 sq. km of forest had only 250 mm of plant-available soil water left."
However, IUCN/WWF's paraphase adds something -- the "up to 40%" estimate. That figure does not appear in Nepsted et al.
Since the Amazon rain forest is estimated at 8.2 million km2 (http://rainforests.mongabay.com/amazon/), it would seem that about 8% (630,000 km2) of the region was threatened by the drought in 1998, not 40%.
Note also that the terms "climate change" and "global warming" do not appear in the text of the Nepsted study (although footnote 14 references a paper titled "Amazonian deforestation and regional climate change").
Apparently, the IPCC recycled two claims in the IUCN/WWF report that the report's supposed source -- Nepsted et al. (1999) -- did not make: namely, 40% of the rain forest is risk, and this is due to global climate change.
There may be other reasons to conclude that climate change endangers 40% of the rain forest, but they are not to be found in Nepsted et al. (1999) -- the source for the IPCC's source.