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Untangling aerosol effects

When it comes to describing how human activities are altering the Earth’s climate, aerosol emissions can tie your tongue in knots. Airborne pollution particles grouped under the “aerosol” heading come in a wide assortment – and some, as this NRCC article explains, tend to absorb sunlight and heat up the atmosphere, while others are more reflective and cooling. On top of that, it’s been thought to make a difference whether this potpourri of pollutants ends up drifting in clear skies or above clouds. Coming in now are the first experimental data that show just how important the effect of cloudiness is.

A study out this week in Nature Geoscience (subscription) uses a new type of satellite data to look at the smoky haze wafting above the southeast Atlantic, mostly from fires in southern Africa, during July-October of 2006 and 2007. The findings confirm what models had suggested: aerosols over cloudier patches of ocean have a net warming effect, but they switch to cooling over unclouded ocean.

Simply put, a veil of aerosols darkens white clouds, but it lightens the dark surface of the sea, and this difference in reflectivity swaps the outcome. The authors, led by Duli Chand of the University of Washington, estimate that 40% cloud cover is the turning point where the southeastern Atlantic’s aerosol cocktail starts adding to greenhouse warming rather than subtracting from it. Cloud cover in the study averaged 48%, and the spatial distribution of clouds and aerosols also overlapped significantly. The overall warming that this produces is about three times what it would be if clouds and aerosols were floating around independently instead of near each other, the group finds.

To pin down the cloud-aerosol relationship, the group used aerosol data from a satellite-based lidar instrument that sends a light signal through the haze, bounces it off clouds, and uses the return time to check where the bounce occurred while also measuring how the light was absorbed. This allows the detector to analyse aerosols in cloudy areas – something that couldn’t be done by previous, so-called passive sensors that rely on reflected sunlight and tend to confuse a layer of clouds with a layer of aerosols.

They suggest their results will be daunting news for climate modellers – actually they use the phrase “a stringent challenge” – because models have trouble with both clouds and aerosols, and getting the global picture right will mean not only doing both well, but putting them in the right places relative to each other.

Anna Barnett


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