11 November 2011

Aerosols’ additional affect on climate revealed

Aerosol particles in the atmosphere affect the Earth's climate in more ways than we thought, according to new research on their impact on our ecosystems.
Aerosol contamination

Aerosol contamination in northeastern India and Bangladesh. Credit: Wikimedia

ABERYSTWYTH: Aerosol particles in the atmosphere affect the Earth’s climate in more ways than we thought, according to new research on their impact on our ecosystems.

Scientists have previously known that tiny particles suspended in the air, known as aerosols, can affect climate directly by absorbing or reflecting radiation. They can also influence the climate indirectly by seeding clouds that affect the amount of solar radiation reaching the Earth.

Now, new research published in Science today suggests that aerosols can also affect climate due to their slow impact on marine and terrestrial ecosystems, potentially affecting the amounts of carbon dioxide that enter and leave the atmosphere.

“There is a previously ignored impact of aerosols onto climate, through their impacts on the carbon cycle,” said lead author Natalie Mahowald from Cornell University in Ithaca, New York. “This suggests that in the future, cleaning of aerosols due to public health concerns will make reducing carbon dioxide in the atmosphere even more difficult.”

Direct and indirect effects

Aerosols are well known to have a significant influence on climate in a number of ways. The so-called ‘direct’ effect notes the interaction of particles with incoming solar radiation and outgoing long wave radiation.

The ‘indirect’ effect describes how they interact with clouds, changing their properties and seeding new clouds that prevent solar radiation from reaching the Earth. Both these absorbing and reflecting effects can result in a cooling effect on our planet below.

Mahowald suggests that aerosols have an additional climatic impact by adding nutrients to ecosystems and biogeochemical cycles, most notably the carbon cycle.
The effect occurs as aerosols supply nutrients to terrestrial and marine ecosystems, allowing the carbon-fixing organisms, such as photosynthetic plants in these systems to grow and take up additional carbon dioxide from the atmosphere.

Affecting Earth’s ecosystems

Mahowald estimated the amount caused by this interaction is creating an additional, unconsidered cooling effect, by an amount comparable to that of the ‘direct’ aerosol effect.

“Aerosols can impact biogeochemical cycles, especially the carbon cycle (and thus carbon dioxide concentrations) by adding nutrients to different ecosystems, such as nitrogen, phosphorus or iron, or by cooling the planet and shifting precipitation,” she said.

“In the atmosphere, iron is carried mostly in naturally produced aerosols, but the more soluble and bioavailable iron is also likely to be produced by combustion processes, by burning fossil fuel for example,” she added. “Some marine ecosystems are iron limited, and the main way they get new iron is through atmospheric deposition. It turns out that some nitrogen fixing organisms in the ocean probably require lots of iron, and thus the iron and nitrogen cycles are linked.”

New challenge in global warming battle

As aerosols are linked to lung cancer and respiratory problems, global efforts are being set up to reduce aerosol emissions. However this new finding suggests this may have adverse effects on our effort to mitigate climate change.

“We don’t understand these biogeochemical effects very well as they are poorly studied, but they could be quite important. Carbon dioxide has increased by 100 parts per million (ppm) since preindustrial times: this could have been 7 to 50ppm higher without aerosols,” said Mahowald.

Mahowald suggests that humans should work to reduce the aerosols they release into the atmosphere in order to improve public health, with policies already in place in Europe and the U.S. “Hopefully soon developing countries will develop better laws to protect citizens from public health concerns,” she said, adding that this improvement in public health may well make climate change harder to solve, as it may also make ecosystems less able to take up the carbon dioxide humans are emitting.

Importance of timescale

“This research is interesting as it points out an important climate effect of aerosols forcing: aerosol induced changes in biogeochemical cycles, especially the carbon cycle, which in turn affect climate,” said Long Cao, senior research associate at the Carnegie Institution for Science in the U.S., who was not involved in the study.

Cao said that while the finding is not new, the estimates associated with the biogeochemical effect can now be directly compared with previous estimates of ‘direct’ and ‘cloud indirect’ aerosol effects.

“It is also worth noting the importance of timescale here, which also presents an additional challenge,” he added. “When aerosols are introduced into the atmosphere, they perturb the climate quickly through their effect on sunlight and cloud. The biogeochemical effect from aerosol manifests itself much later (probably after several years). Therefore, when we compare the biogeochemical forcing with direct and cloud-related indirect aerosol forcing, we should bear in mind the different timescales.”


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