Published online 29 October 2010 | Nature | doi:10.1038/news.2010.571

News: Explainer

Drought strikes the Amazon rainforest again

Climate change may explain why history is repeating itself in Brazil.

Black River (Rio Negro, in Portuguese) is seen empty due to drought in Manaus, in the Brazilian northern state of Amazonas, on October 21, 2010. Fourty of the 62 municipalities belong Amazonas state declared state of emergency because of the lack of waterA severe drought is affecting the Amazon for the second time in five years.E. QUEIROZ/AP/Press Association Images

Five years ago, vast areas of the Amazon were hammered by a historic drought, which destroyed trees, impacted the livelihoods of fishermen and others who are dependent on the river and presented scientists with what was seen as a rare opportunity to investigate the world's largest rainforest in extreme distress. Drought has now struck again, reinforcing fears that the invisible hand of climate change may be involved. Nature takes a closer look.

How does the current drought compare with the one in 2005?

So far it seems the drought is similar in size, although some features vary. Luiz Aragao, a remote-sensing expert at the University of Exeter, UK, who has reviewed Brazilian data from ground stations and satellites, says that the drought appears to be broader in scope but slightly less intense than 2005. The current drought has affected a large area covering the northwest, central and southwest Amazon, including parts of Columbia, Peru and northern Bolivia. Fewer clouds and less rain also translate into higher temperatures, and Aragao says that the maximum temperatures in September are 1 °C higher than 2005, and 2–3 °C higher than average. Water levels in the primary tributary Rio Negro — or Black River — are at historic lows.

Although deforestation in Brazil has decreased, there have been reports of increased fire activity. Is that related to the drought?

Yes. Forest fires are generally associated with deforestation, but drought amplifies the impact of fires that are set in order to clear land.

Preliminary data for 2010 indicate that deforestation has fallen by some 85% compared with its recent peak in 2004. When the final data are released in the coming weeks, Brazil might be able to claim that it has met its commitment to reduce deforestation by 80% (compared with a rolling average) a decade early. For perspective, the Union of Concerned Scientists, based in Cambridge, Massachusetts, estimates the corresponding reduction in greenhouse-gas emissions at 1 gigatonne, which is roughly comparable to what the United States and the European Union have each committed to over the next decade.

Despite that remarkable news, Aragao says that fire activity has increased to about 80% of the 2005 levels. Natural fires are a rare occurrence in the Amazon, but during a drought, fires that are set to clear smaller plots are more likely to escape into the forest.

Is climate change to blame for the drought?

It is hard to pinpoint a culprit, but both the 2005 and the 2010 droughts align well with longer-term projections by some climate modellers for a drying out of the Amazon due to climate change. Much of the Amazon normally experiences a dry season that begins in around July or August and continues to September or October. Periodic droughts are also associated with El Niño — a periodic warming of surface waters in the eastern Pacific Ocean — but there are some indications that the 2005 and 2010 droughts could be associated with warmer surface temperatures in the Atlantic Ocean north of the equator, which effectively pull the trade winds — and all of the moisture they carry — to the north. Scientists described the 2005 drought as a once-in-a-century phenomenon. But clearly it wasn't.

What kind of impact did the 2005 drought have?

Five years on, remote-sensing scientists are still debating that question with surprising intensity. A controversial 2007 study in Science1 found that the Amazon 'greened up' in 2005, suggesting that the rainforest was more resilient than previously believed. But other researchers have looked at the same data and failed to detect any such greening (see 'Amazon drought raises research doubts')2. Some go as far as to say that the satellite data aren't good enough to answer the question at all.

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The remote-sensing data discussion continues in the scientific literature, but ground data provides clear evidence of biomass loss and an increase in tree deaths. A study published in Science last year suggests that the drought reduced cumulative carbon storage in affected areas by some 1.6 gigatonnes3. From this perspective, fewer clouds and more sunlight could spur an initial increase in photosynthesis resulting in more plant growth, but the reduced soil moisture caused by prolonged drought has a bigger effect in the end. A recent analysis in the Proceedings of the National Academy of Sciences4 builds on this by suggesting that drought conditions could spur leaf loss and new leaf budding while simultaneously leading to a rise in tree deaths.

Could the 2010 drought help to answer those questions?

It will certainly give scientists another opportunity to pore over the various ground and satellite data, and this time they will have an even better idea what to look for. It's not clear how the debate over satellite data will play out. As the recent PNAS paper indicates, much work remains to be done to explain why satellites see green — if in fact they do — and assess the broader impacts of drought on carbon storage. Having a second set of data to analyse certainly won't hurt. 

  • References

    1. Saleska, S. R., Didan, K., Huete, A. R. & da Rocha, H. R. Science 318, 612 (2007). | Article
    2. Samanta, A. et al. Geophys. Res. Lett. 37, L05401 (2010). | Article
    3. Phillips, O. L. et al. Science 323, 1344-1347 (2009). | Article | ChemPort |
    4. Brando, P. M. et al. Proc. Natl Acad. Sci. USA advance online publication doi:10.1073/pnas.0908741107 (2010).
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