May 08 2008
Can lower sulphate emissions in America affect rainfall in Amazonia?
New Scientist had an article recently titled How cleaning up America dried up the Amazon by Mason Inman. It reported on a Nature paper by Cox et al., Increasing risk of Amazonian drought due to decreasing aerosol pollution. This paper shows that positive changes can have unintended consequences long distances from their source.
The gist is that the sulfates generated in the United States reflect incoming solar radiation. But the amount of these aerosols has been decreasing since the 1970s, when anti-pollution laws started restricting the amount of aerosols in the atmosphere. Since there are now less sulfates in the atmosphere, more radiation is being absorbed by the ocean near the United States. This change in where the radiation is absorbed may have changed the storm tracks in the North Atlantic, bringing them farther North. This has led to less rainfall in the Amazon rainforest.
Abstract:
The Amazon rainforest plays a crucial role in the climate system, helping to drive atmospheric circulations in the tropics by absorbing energy and recycling about half of the rainfall that falls on it. This region (Amazonia) is also estimated to contain about one-tenth of the total carbon stored in land ecosystems, and to account for one-tenth of global, net primary productivity. The resilience of the forest to the combined pressures of deforestation and global warming is therefore of great concern, especially as some general circulation models (GCMs) predict a severe drying of Amazonia in the twenty-first century. Here we analyse these climate projections with reference to the 2005 drought in western Amazonia, which was associated with unusually warm North Atlantic sea surface temperatures (SSTs). We show that reduction of dry-season (July–October) rainfall in western Amazonia correlates well with an index of the north–south SST gradient across the equatorial Atlantic (the ‘Atlantic N–S gradient’). Our climate model is unusual among current GCMs in that it is able to reproduce this relationship and also the observed twentieth-century multidecadal variability in the Atlantic N–S gradient, provided that the effects of aerosols are included in the model. Simulations for the twenty-first century using the same model 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.
The paper is interesting because it looks at a direct effect of aerosols on climate. The study used model simulations to see how the introduction of sulfate aerosols influenced the dynamics of the atmosphere. The model was run twice, once with aerosol effects and once without. Only when the sulfate aerosols were included did the model accurately produce rainfall rates in the Amazon.
The authors attribute the change in precipitation to the change in the Atlantic North-South Gradient (ANSG) in sea surface temperatures. Their model results show that when the ANSG is high, there is less rainfall during the summer months, and when the ANSG is low there is more rainfall.
The model shows that the North Atlantic to warm faster than the South Atlantic. This would lead to higher values of ANSG, and lower expected precipitation rates. Add to that the fact that the sulfate concentrations in the North Atlantic will likely continue to decline, and the expected decrease in the precipitation rate in the Amazon would fall rapidly.
There was a large drought in the Amazon in 2005. Cox et al. show that when aerosols are included in the models that such drought events will not be the extremes, but will rather be the norm.
We estimated the probability of a ‘2005-like’ year occurring in the HadCM3LC run with aerosols, based on the fraction of years in a centred 20-yr window that exceed the ANSG index for 2005. The model suggests that 2005 was an approximately 1-in-20-yr event, but will become a 1-in-2-yr event by 2025 and a 9-in-10-yr event by 2060.
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2 Responses to “Can lower sulphate emissions in America affect rainfall in Amazonia?”
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The lack of sulphates would explain higher solar radiation reaching the NH since the 1990s.
Would that also not explain any warming?
The UK Met is one of the easiest places to find sunshine records.
Most of the last few years have parts of the Uk with 150% or more of the usual sunshine hours.
Can’t that affect climate?
Atmoz,
Thanks for starting a thread on this. If this study assumes aerosols are decreasing in the Western NH, why I have seen summary results on the web from two different climate modelers that assume aerosols are getting worse? Are climate modelers free to choose whatever aerosol load they want to put in their models? Since the SH had very little aerosols in it in the first place, is this why there has been very little heating in the SH for the last 30 years (via satellite data)? No one over at rc.org cares to answer my questions as if they don’t matter. They probably don’t have the answers anyway and would rather just blindly follow their models even though they don’t match reality.