I’m sure everyone is aware of the recent eruption of the Chaitén Volcano in southern Chile. There have been some pretty amazing photographs, satellite images, and videos. Besides the obvious negative effects to the local people, what other possible climate effects will this eruption have? We can use previous volcanic eruptions to learn what aspect of a volcanic eruption influences the global temperatures.

Solid, Liquid, or Gas?

Before the effects of a volcanic eruption can be determined, first we need to know what comes out of a volcano. The most obvious substance is the liquid hot magma (molten rock). In explosive volcanoes this is ejected into the atmosphere as tephra, which I think most of us would simply call “ash”.
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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.
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There’s an interesting new paper in press about how volcanic aerosols can influence cloud radiative properties. It has been highlighted as an Image of the Day at NASA Earth Observatory.

Gasso, S. (2008), Satellite observations of the impact of weak volcanic activity on marine clouds, J. Geophys. Res., doi:10.1029/2007JD009106, in press.
pre-print (when published)
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The roles of aerosols and clouds as radiative forcing is the least well known aspect of the climate change problem. According the the IPCC 2007 Summary for Policymakers, aerosols represent a radiative forcing of approximately -1.2 W/m2, combining the direct effect of aerosols and the cloud albedo effect (also known as the first indirect effect or the Twomey effect). The estimate error in this value is +/- 1.2 W/m2. Other radiative forcings, such as CO2, have a much higher level of scientific understanding. The result is that the total net anthropogenic forcing is estimated at 1.6 W/m2 +/- 0.9 W/m2. The large uncertainty in the net forcing is almost all the result of unknown, or not-well understood, effects from clouds and aerosols.

Kim and Ramanathan (2008) use multiple satellite observing systems, along with ground-based measurements to compare the radiative forcing from aerosols and clouds with model results.
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