Climate Feedback

Settling of dust warms tropical Atlantic

africandust.jpgWarming 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


  1. Report this comment

    russ said:

    The reported loss of Africa dust and its warming influence on the Atlantic Ocean has a far more dire and immediate consequence. The same series of satellite data used by these climate modelers has shown a more than 26% decline in net primary productivity of the Atlantic, read forced extinction of green plants, the phytoplankton. This massive loss of ocean plankton has staggering effects up and down the food chain destabilizing the Atlantic ecosystem.

    One of its most damaging forcing effects is in the role in destruction of Atlantic fish stocks. While it is fashionable and politically correct to point fingers for fisheries declines only at the bad fishers the loss of ocean pasture capacity is worse. Further those fish were not mere farm animals awaiting slaughter they are or were the worms of the surface ocean whose toil of life churned and recycled ocean nutrients greatly enriching those ocean pastures. As we starve and remove the worms, fish and other marine life, of the seas we destroy the ecosystem.

    The same data series that shows the decline in African dust and ties it as the primary cause of Atlantic Ocean warming also shows that dust decline is due to greening of the grassy regions of N. Africa which are made greener by the massive dose of CO2 we’ve emitted over the past century. That dose of CO2, a deadly airborne carbon bomb, will remain for centuries even if were to never emit another molecule of CO2, which of course isn’t going to stop. So as high CO2 nurtures green plants on land making better ground cover and reducing dust the CO2 also acidifies the ocean and worst missing dust denies the ocean vital mineral micronutrients. The more than one thousand billion tonnes of fossil CO2 already in the air is more than sufficient to seal the doom of ocean plants and ecosystems. The ONLY hope is to replenish and restore those ocean plants and the ocean pastures.

    Only ocean plants can compete with the acid forming reaction H2O+CO2+H2CO3 carbonic acid. And it matters NOT if we reduce todays or tomorrows CO2 emissions as the deadliest problem is yesterdays CO2. We simply must immediately begin to replenish the micronutrients to the oceans and restore ocean plant life. By restoring ocean plant life to the level seen some 26 years ago when the satellite data started coming in we might expect to see those restored ocean plants taking the same amount, 4-5 billion tonnes of CO2 and converting it to ocean life instead of its destiny today acid death.

    During those same 26 years scientists from around the world in both public and private sectors have spent hundreds of millions of dollars researching the prospect and promise of ocean restoration by means of replenishing the vital mineral micronutrients our fossil fuel age has denied the oceans. The means and ability to safely replenish and restore the oceans is at hand and will cost a tiny fraction of the cost of the hundreds of billions being spent on climate change. It can and must begin immediately as reports clearly show the CO2 tipping point for some oceans is as near as 2030, a mere 20 years from today. This is not a deadline by which we must begin to replenish and restore the oceans to deal with the deadly effects of yesterdays CO2; it is the deadline by which we must succeed at the job.

    Every moment of debate and argument, and these debates and arguments have gone on non stop for more than 20 years, forces CO2 acidification and nutrient deprivation to drive once verdant ocean pastures into lifeless seas, or rather seas of toxic slime.

    Replenish and restore the oceans to the condition of health they and we enjoyed 26 years ago. Read more at

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