In the world of climate modeling, aerosols are troublemakers.

The tiny airborne particles, which come from human sources like burning fossil fuels and biomass as well as natural sources like volcanic eruptions and sea spray, are known to have a cooling effect on the Earth's climate by affecting cloud reflectivity.

Exactly how much they cool, though, is a topic of considerable uncertainty.

This is a problem when climate scientists try to understand the warming effects of carbon dioxide, which they know warms the planet.

"We don't know what the aerosol forcing is, so we don't know what fraction of the carbon dioxide forcing has been offset [by aerosols]," said Ken Carslaw, an atmospheric scientist at the University of Leeds.

A new paper by Carslaw published yesterday in the journal Nature places a sharper focus on what are referred to as natural aerosols and their effects on cloud brightness. As the name suggests, these are the ones that come from natural processes like volcanic eruptions.

Carslaw's research effort was directed at understanding more about the uncertainty in how aerosols cool the planet. He found that a lot of the problem lay in understanding natural aerosols, versus ones from humans.

"We discovered nearly half the forcing uncertainty was caused by these natural aerosols," Carslaw said.

A need to benchmark the preindustrial age
That's not because natural aerosols significantly contribute to climate forcing today. Nowadays, the impacts of human-released aerosols dwarf those of natural ones.

Rather, it's because scientists need to have a base-line understanding to account for the cooling effects of natural aerosols before the industrial age, versus the cooling effects we see today.

"To understand the cooling effect, we want to understand not only what the cooling effect is today but what it was [200] or 300 years ago," said Daniel Murphy, a climate scientist with the National Oceanic and Atmospheric Administration's Earth System Research Laboratory.

The cooling effect of those natural aerosols, before industrialization, is very uncertain.

Knowing that natural aerosols are the ones with the most uncertainty, Carslaw said, provides scientists with a target area to focus on.

"The positive thing is we have a pretty good idea now where the uncertainties are. Once you know what they are, you can kind of go after them," he said.

Carslaw plans to use models to try to learn more about natural aerosol concentrations and their effects on climate in the preindustrial era.

Search for a near-pristine area
Although there is almost nowhere in the world today with what Carslaw called "pristine" aerosol levels, unaffected by human emissions, some parts of the Southern Hemisphere come close, and that is where he plans to look, the researcher said.

NOAA's Murphy said researchers can also work to reduce uncertainty by trying to understand more about how aerosols work in the atmosphere.

Yi Ming, a climate scientist and aerosol expert at NOAA's Geophysical Fluid Dynamics Laboratory, which contributes climate models to the Intergovernmental Panel on Climate Change (IPCC), said the paper's findings underscore the need to focus on an area he and many other researchers thought needed additional emphasis.

"The unique result of this study is that the emphasis is more on natural aerosols as opposed to anthropogenic aerosols," Ming said.

He also said the research could push the IPCC to pay more attention to natural aerosols in the models it uses.

Right now, the IPCC specifies levels of human-caused emissions that go into its models, but not natural ones. This lack of consistency could lead to some of the differences in the models, Ming suggested.

"There's no doubt that the kind of thinking in this paper will help modeling centers better guide their model development effort in the future," he said.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500