"Long before it's in the papers"
January 28, 2015


Aerosols may drive much Arctic warming, scientists find

April 9, 2009
Courtesy NASA
and World Science staff

Dis­cus­sions on glob­al warm­ing usu­ally cen­ter on the heat-trapping green­house gas­es blamed for the trend. But new NASA re­search sug­gests around half the at­mos­pher­ic warm­ing meas­ured in the Arc­tic is due to air­borne par­t­i­cles called aer­o­sols.

Aerosols can influence climate directly by either reflecting or absorbing the sun's radiation as it moves through the atmosphere. The tiny airborne particles enter the atmosphere from sources such as industrial pollution, volcanoes and residential cooking stoves. (Image courtesy NASA Goddard Scientific Visualization Studio)

Emit­ted by nat­u­ral and hu­man sources, aer­o­sols can in­flu­ence cli­mate by re­flect­ing or ab­sorb­ing sun­light. The par­t­i­cles al­so af­fect cli­mate less di­rectly by chang­ing cloud prop­er­ties, such as re­flec­ti­vity. For one type of aer­o­sol, sci­en­tists said, re­duc­tions rath­er than in­creases in its emis­sions seem to have pro­mot­ed warm­ing.

A study led by cli­mate sci­ent­ist Drew Shin­dell of the NASA God­dard In­sti­tute for Space Stud­ies in New York used a com­put­er mod­el to in­ves­t­i­gate how sen­si­tive dif­fer­ent re­gion­al cli­mates are to changes in lev­els of car­bon di­ox­ide, ozone, and aer­o­sols.

The in­ves­ti­ga­tors found that Earth’s mid­dle and high lat­i­tudes are es­pe­cially re­spon­sive to changes in aer­o­sol lev­els. In­deed, the mod­el sug­gests aer­o­sols likely ac­count for 45 per­cent or more of the warm­ing mea­sured in the Arc­tic since 1976. The re­sults are pub­lished in the April is­sue of the re­search jour­nal Na­ture Ge­o­sci­ence.

Though there are sev­er­al types of aer­o­sols, pre­vi­ous re­search in­di­cates two — sul­fates and black car­bon — play lead­ing roles in cli­mate. Both are prod­ucts of hu­man ac­ti­vity. Sul­fates, which come mainly from the burn­ing of coal and oil, scat­ter sun­light and cool the air. Over the past three dec­ades, the Un­ited States and Eu­ro­pe­an coun­tries have passed clean-air laws that have halved sul­fate emis­sions, re­search­ers said.

The mod­els showed that re­gions of Earth that showed the strongest re­sponses to aer­o­sols in the mod­el are the same re­gions that have wit­nessed the great­est real-world tem­per­a­ture in­creases since 1976, in par­tic­u­lar the Arc­tic, Shin­dell said; in the Ant­arc­tic, aer­o­sols play less of a role.

Researchers with the U.S. National Oceanic and Atmos­pheric Ad­mi­nis­tra­tion reported in the April 3 issue of the jour­nal Geo­phys­i­cal Re­search Let­ters that Arc­tic sum­mers may be ice-free in as few as 30 years. That was the lat­est in a series of in­creas­ingly near­er-term fore­casts for Arc­tic melt­ing from va­rious sci­en­tif­ic groups.

The Arc­tic re­gion has seen its sur­face air tem­per­a­tures rise by 1.5 C (2.7 F) since the mid-1970s. In the Ant­arc­tic, sur­face air tem­per­a­ture has in­creased about 0.35 C (0.6 F). That makes sense, Shin­dell ex­plained, be­cause the Arc­tic is near North Amer­i­ca and Eu­rope, highly in­dus­t­ri­al­ized zones that pro­duce most of the world’s aer­o­sols.

“In the mid-lat­i­tudes of the North­ern Hem­i­sphere and in the Arc­tic, the im­pact of aer­o­sols is just as strong as that of the green­house gas­es,” said Shin­dell. “If we want to try to stop the Arc­tic sum­mer sea ice from melt­ing com­pletely over the next few dec­ades, we’re much bet­ter off look­ing at aer­o­sols and ozone” than the green­house gas car­bon di­ox­ide whose emis­sions many en­vi­ron­men­tal­ists are tar­get­ing, he added.

Aerosols tend to be short lived, stay­ing in the at­mos­phere for just days or weeks, where­as green­houses gas­es can per­sist for cen­turies. At­mos­pher­ic chem­ists thus think the cli­mate may re­spond most quickly to changes in aer­o­sol lev­els.

NASA’s up­com­ing Glo­ry sat­el­lite is de­signed to en­hance cur­rent aer­o­sol meas­ure­ment ca­pa­bil­i­ties to help sci­en­tists re­duce un­cer­tain­ties about aer­o­sols by meas­ur­ing the dis­tri­bu­tion and prop­er­ties of the par­t­i­cles.

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Discussions on global warming usually center on heat-trapping greenhouse gases blamed for the trend. But new NASA research suggests around half the atmospheric warming measured in the Arctic is due to airborne particles called aerosols. Emitted by natural and human sources, aerosols can influence climate by reflecting or absorbing sunlight. The particles also affect climate less directly by changing cloud properties, such as reflectivity. For one type of aerosol, scientists said, reductions rather than increases in its emissions seem to have promoted warming. A study led by climate scientist Drew Shindell of the NASA Goddard Institute for Space Studies in New York used a computer model to investigate how sensitive different regional climates are to changes in levels of carbon dioxide, ozone, and aerosols. The investigators found that Earth’s middle and high latitudes are especially responsive to changes in aerosol levels. Indeed, the model suggests aerosols likely account for 45 percent or more of the warming that has occurred in the Arctic during the last three decades. The results are published in the April issue of the research journal Nature Geoscience. Though there are several types of aerosols, previous research indicates two — sulfates and black carbon — play leading roles in climate. Both are products of human activity. Sulfates, which come mainly from the burning of coal and oil, scatter sunlight and cool the air. Over the past three decades, the United States and European countries have passed laws that have halved sulfate emissions, researchers said. The models showed that regions of Earth that showed the strongest responses to aerosols in the model are the same regions that have witnessed the greatest real-world temperature increases since 1976, in particular the Arctic, Shindell said; in the Antarctic, aerosols play less of a role. The Arctic region has seen its surface air temperatures rise by 1.5 C (2.7 F) since the mid-1970s. In the Antarctic, surface air temperature has increased about 0.35 C (0.6 F). That makes sense, Shindell explained, because the Arctic is near North America and Europe, highly industrialized zones that produce most of the world’s aerosols. “In the mid-latitudes of the Northern Hemisphere and in the Arctic, the impact of aerosols is just as strong as that of the greenhouse gases,” said Shindell. “If we want to try to stop the Arctic summer sea ice from melting completely over the next few decades, we’re much better off looking at aerosols and ozone” than the greenhouse gas carbon dioxide whose emissions are targeted by many environmentalists, he added. Aerosols tend to be short lived, staying in the atmosphere for just days or weeks, whereas greenhouses gases can persist for centuries. Atmospheric chemists thus think the climate may respond most quickly to changes in aerosol levels. NASA’s upcoming Glory satellite is designed to enhance our current aerosol measurement capabilities to help scientists reduce uncertainties about aerosols by measuring the distribution and microphysical properties of the particles.