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Cloud-building plants help cool the atmosphere (but only slightly)

  • 29 Apr 2013, 15:45
  • Roz Pidcock and Freya Roberts

We know that plants absorb carbon dioxide, but are they moderating earth's temperature another way? A new study suggests the cooling effect of tiny particles emitted from plants can offset one per cent of global warming. We asked the authors what this means for slowing the pace of temperature rise.

What are aerosols?

Aerosols are tiny particles that can have a cooling effect on the atmosphere. The particles scatter sunlight directly and stimulate clouds to form, preventing sunlight reaching earth's surface.

Scientists are confident that aerosols offset a substantial portion of human-caused warming, but there is still a lot of uncertainty about how much.

Most aerosols come from human activity, such as vehicle exhausts and wood burning. A much smaller fraction are naturally occurring, mainly through volcanic eruptions.

A new study in Nature Geoscience explains how plants could be another - albeit much smaller - source of natural aerosols through the gases they release.

As co-author Ari Asmi from the University of Helsinki tells us, these so called biological aerosols bolster the effect of aerosols from other sources. He says:

"In very simplified terms, the more aerosol particles you have in air, the greater the cooling effect in the atmosphere".

Plant feedbacks

The researchers collected data from 11 different sites around the world, measuring concentrations of aerosol particles and plant gases together with temperature.

They found that warmer temperatures make plants emit more of particular gases that once in the atmosphere, stick to other aerosol particles.

This increases the number of aerosol particles big enough to trigger a cloud to form, leading to a stronger overall cooling effect. This is an example of what scientists call a negative feedback.

This isn't the first time scientists have proposed the plant aerosol theory. But until now, studies have focussed on single locations, leaving the global effect less well understood. Lead author Pauli Paasonen from the University of Helsinki tells us:

"[T]his is the first study to show that this phenomenon takes place on continental scales."

Global effect

Globally, the authors say the effect is very small - offsetting just one per cent of human-caused warming. In forested areas of Finland, Siberia and Canada the cooling effect can be much larger - up to 30 per cent.

Blue _mountains

The Blue Mountains in Australia,so called because the organic compounds given off by plants create a blue haze. Credit: Amanda Slater.

But Asmi tells us that although the researchers are confident of the mechanism, there is still some uncertainty over the size of the effect because cloud formation is a complex process. He says:

"The method I used to calculate the feedback strength is very much simplified ... I would be confident that there is a cooling effect, but I would be more hesitant about the strength of it -- there is still much to study there."

Paasonen explains a bit more, saying the group's estimates should not be considered as exact but as "order of magnitude estimations."

Rainforests

The new estimate of the global effect of plant aerosols excludes rainforests. Asmi tells us one reason for this is lack of data. He says:

"[We have] only recently ... started to have the necessary international research infrastructure to actually do these kinds of measurements ... We simply do not yet have such comprehensive long time series from rain forests."

Another reason is that temperature variations within rainforests are already very large, so spotting a pattern with increasing temperature is tricky. But as Paasonen tells us, he doesn't expect rainforests to drastically affect the outcome:

"I assume including rain forests would not change the total effect too much ... But, this is something we are about to study in [the] future. There might be surprises, of course."

Aerosols represent one of the biggest uncertainties in climate models, so the new research could help fill in some of the gaps. As Paasonen explains:

"[U]nderstanding this mechanism could help us reduce those uncertainties and make the models better."

 

 

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