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Dry outlook for the Amazon rain forest

One of the more irritating aspects, if you will, of global change is that air pollution has so far prevented the planet from warming more rapidly than it actually did. Clean air is of course a good thing. But reducing pollution might expose an as of yet ‘masked’ portion of global warming.

This could have a dramatic affect on the Amazon rainforest. A team led by Peter Cox of the University of Exeter, UK, reports in a paper in this week’s Nature that reductions in aerosol pollution will tremendously increase the risk of severe drought in the Amazon region. Here is an editor’s summary of the paper.

Although it accounts for nearly a quarter of the world’s fresh water, drought is not unknown in Amazonia.

In the dry season, from July to October, rainfall in the region is linked to sea surface temperatures (SST) in the tropical Atlantic. In years with a pronounced temperature gradient – warming of the tropical Atlantic north of the equator relative to the south – the normal’ position of high and low atmospheric pressure systems can shift, delaying or suppressing the onset of the South American monsoon.

The effect has been observed in 2005, when large parts of the Amazon region were hit by the worst drought in decades. See a Nature news story by Mike Hopkin here (subscription required) and a New York Times story here about the devastating event.

Cox thinks that the 2005 drought was a harbinger of things to come. Their “simulations for the 21st century show a strong tendency for the SST conditions associated with the 2005 drought to become much more common, owing to continuing reductions in reflective aerosol pollution in the Northern Hemisphere.”

Droughts like in 2005 will happen every two years by 2025, and in nine out of ten years by 2060, the model suggests.

How robust is this dire prediction? The Amazonian climate, for reasons not quite understood, is notoriously difficult to simulate. But the Hadley Centre’s climate model which was used for this study has previously reproduced features of the regional climate with greater accuracy than other models.

In Mike Hopkin’s words, “the ultimate fear is that the Amazon forest – often touted as an invaluable piece of armour against climate change – could become part of the problem rather than a key element of the solution. Droughts make it more likely that it will become a net source of greenhouse gases to the atmosphere, rather than mopping them up.”

Quirin Schiermeier

You can vote or comment on the importance of the new paper in the Journal Club of Nature Reports Climate Change.


  1. Report this comment

    Bishop Hill said:

    Where do you see this Alexander? They say they can hindcast, but there is no testing of the forecast that I can see.

  2. Report this comment

    Alexander Ač said:

    Dear Bishop,

    true, they are reporting good hindcast – and based on this, one can have higher confidence in the forcast, no?

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    sangram ganguly said:

    Please refer to a new paper that came out in Geophysical Research Letters contradicting the reports that came out in Science about “Amazon forests greening up in 2005 drought”.

    Samanta, A., S. Ganguly, H. Hashimoto, S. Devadiga, E. Vermote, Y. Knyazikhin, R. R. Nemani, and R. B. Myneni (2010), Amazon forests did not green-up during the 2005 drought,Geophys. Res. Lett.,doi:10.1029/2009GL042154, in press.

    [PDF] (accepted 26 January 2010)

    Optical remote sensing data can be severely affected from aerosol contamination and analysis based on corrupted data do not capture the trends or changes in greenness variability as expected.

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