THE WOODS HOLE RESEARCH CENTER

 149 Woods Hole Road ⋅ Falmouth, MA 02540-1644 USA 

 Telephone 508.540.9900 ⋅ Fax 508.540-9700 ⋅ www.whrc.org

Senior Scientist Daniel Nepstad endorses the correctness of the IPCC’s (AR4) 

statement on Amazon forest susceptibility to rainfall reduction

February 2010

The Intergovernmental Panel on Climate Change has been recently criticized in media coverage (e.g. Sunday Times) for 

presenting inaccurate information on the susceptibility of the forests of the Amazon Basin to rainfall reduction in its fourth 

assessment. The statement that has drawn the criticism reads as follows: 

"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)." (IPCC 

2007, Magrin et al. 2007) 

The Rowell and Moore review report that is cited as the basis of this IPCC statement cites an article that we published in 

the journal Nature in 1999 as the source for 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 metres of soil. A further 360,000 sq. km of forest had only 250 mm of plant-available soil water 

left.[Nepstad et al. 1999]" (Rowell and Moore 2000) 

The IPCC statement on the Amazon is correct, but the citations listed in the Rowell and Moore report were incomplete. 

(The authors of this report interviewed several researchers, including the author of this note, and had originally cited the 

IPAM website where the statement was made that 30 to 40% of the forests of the Amazon were susceptible to small 

changes in rainfall). Our 1999 article (Nepstad et al. 1999) estimated that 630,000 km2 of forests were severely drought 

stressed in 1998, as Rowell and Moore correctly state, but this forest area is only 15% of the total area of forest in the 

Brazilian Amazon. In another article published in Nature, in 1994, we used less conservative assumptions to estimate that 

approximately half of the forests of the Amazon depleted large portions of their available soil moisture during seasonal or 

episodic drought (Nepstad et al. 1994). After the Rowell and Moore report was released in 2000, and prior to the 

publication of the IPCC AR4, new evidence of the full extent of severe drought in the Amazon was available. In 2004, we 

estimated 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 in 1998. This estimate incorporated new rainfall data and results from an

experimental reduction of rainfall in an Amazon forest that we had conducted with funding from the US National Science 

Foundation (Nepstad et al. 2004). Field evidence of the soil moisture critical threshold is presented in Nepstad et al. 2007.

In sum, the IPCC statement on the Amazon was correct. The report that is cited in support of the IPCC statement (Rowell 

and Moore 2000) omitted some citations in support of the 40% value statement. 

Daniel Nepstad, PhD

Senior Scientist, Woods Hole Research Center

Coordinator of Research, Instituto de Pesquisa Ambiental da Amazonia

Email Dan Nepstad
 THE WOODS HOLE RESEARCH CENTER

 149 Woods Hole Road ⋅ Falmouth, MA 02540-1644 USA 

 Telephone 508.540.9900 ⋅ Fax 508.540-9700 ⋅ www.whrc.org

Literature citations:

IPCC (2007), Summary for Policymakers, in Climate Change 2007: Impacts, Adaptation and Vulnerability. Contribution of 

Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, edited by M. L. 

Parry, O. F. Canziani, J. P. Palutikof, P. J. van der Linden and C. E. Hanson, pp. 7-22, Cambridge University Press, 

Cambridge, UK.

Magrin, G., C. G. García, D. C. Choque, A. R. M. J.C. Giménez, G. J. Nagy, C. Nobre, and A. Villamizar (2007), Latin 

America, in Climate Change 2007: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Fourth 

Assessment Report of the Intergovernmental Panel on Climate Change, edited by M. L. Parry, O. F. Canziani, J. P. 

Palutikof, P. J. van der Linden and C. E. Hanson, pp. 581-615, Cambridge University Press, Cambridge, UK., section 13.4.1

Nepstad, D. C., et al. (1999), Large-scale impoverishment of Amazonian forests by logging and fire, Nature, 398, 505-

508.

Nepstad, D., P. Lefebvre, U. Lopes da Silva, J. Tomasella, P. Schlesinger, L. Solorzano, P. Moutinho, D. Ray, and J. 

Guerreira Benito (2004), Amazon drought and its implications for forest flammability and tree growth: a basin-wide 

analysis, Global Change Biology, 10, 704-717.

Nepstad, D., I. Tohver, I., D. Ray, P. Moutinho, G. Cardinot. 2007. Mortality of large trees and lianas following 

experimental drought in an Amazon forest. Ecology88(9): 2259-2269

Nepstad, D., C. Reis de Carvalho, E. Davidson, P. Jipp, P. Lefebvre, G. Hees Negreiros, E. Silva, T. Stone, S. Trumbore, S. 

Vieira. 1994 The role of deep roots in the hydrologic and carbon cycles of Amazonian forests and pastures. Nature: 372: 

666-669

Rowell, A., and P. F. Moore (1999), Global Review of Forest Fires, WWF and IUCN. 64 pp http://data.iucn.org/dbtwwpd/edocs/2000-047.pdf