Warming of surface waters in the tropical North Atlantic Ocean has been linked to increasing hurricane activity (Nature News, subscription), but climatologists differ on what’s driving the temperature rise. Now a study published in Science (subscription) suggests that a large chunk of the warming trend is due neither to anthropogenic climate change nor to a natural cycle in ocean circulation – the two main contenders previously. The new theory: it’s dust in the wind.

Not just dust, actually: sulfates spewed by volcanoes seem to be an even bigger player. Both dust and sulfates are light-reflecting aerosol particles, so they cool the ocean by preventing sunbeams from reaching it. Amato Evans of the University of Wisconsin-Madison and his colleagues used a simple model to estimate the degree to which fluctuating amounts of these aerosols – measured by satellite since 1982 – have affected sea surface temperature in the tropical North Atlantic. They find that their model, driven by changes in aerosols alone, reproduces no less than 69% of the warming trend from 1982-2007.

How can the cooling influence of aerosols account for warming seas? Two big volcanic eruptions, along with fierce African dust storms, cooled the region in the 1980s and early 1990s, paving the way for a big rise in sea temperatures later on as the dust subsided and the volcanoes lay dormant.

I’ve covered the research in an online story for Nature News. With its new angle on the divisive topic of tropical Atlantic warming, the paper could well stir up controversy. “I, for one, remain skeptical”, says Tim Barnett of Scripps Institution of Oceanography, who has published landmark papers on anthropogenic ocean warming. Barnett thinks what’s needed is not a trend analysis, but a study of how aerosols affect the total heat flowing through this ocean basin.

What the paper doesn’t imply, says oceanographer Greg Foltz of the University of Washington, is that the previously proposed causes for warming are moot. He wrote in an email:

This does not mean that the AMO [Atlantic meridional overturning circulation] is unimportant or that global warming is unimportant (greenhouse gas emissions will most likely lead to significant warming of the tropical North Atlantic during the next few decades), but that there is a lot of natural variability (for example, volcanic eruptions and changes in dust coming off of Africa) that may obscure or enhance the global warming signal on time scales of 0-20 years.

Anna Barnett

Image: Saharan dust storm blows into the tropical Atlantic / NASA