Victoria Gill, BBC News 19 Jun 09;
A Norwegian scientist says he has shown how much aerosols influence climate.
Aerosol particles scatter and reflect the Sun's rays - an effect that "masks" global warming.
This study aimed to bring together models and observations of this "direct aerosol effect", to accurately estimate the magnitude of this cooling.
Reporting in the journal Science, climate scientist Gunnar Myhre has found that the effect is weaker than previous studies have estimated.
Dr Myhre, from the Center for International Climate and Environmental Research in Oslo, said this clarifies just how much humans have changed the climate so far.
The pollution particles he studied include industrial aerosols such as sulphates, nitrates found in smoke from burning agricultural waste and black carbon (soot) from diesel engines and other forms of combustion.
"Global models of the emission of these aerosols suggest the cooling effect they have cancels out approximately 10% of the global warming caused by greenhouse gases," explained Jim Haywood, an aerosol researcher from the UK Met Office, who was not involved in this study.
"But satellite methods that detect the amount of aerosols in the atmosphere suggest a cooling effect that cancels out about 20%."
By identifying the source of this discrepancy, Dr Myhre was able to reconcile the two approaches and come up with a more precise estimate - closer to 10%.
This suggests the effect is weaker than the Intergovernmental Panel on Climate Change (IPCC) has estimated.
"Sulphate and organic carbon scatter solar radiation, whereas black carbon, to a large degree, absorbs it," he explained.
"The models take into account that black carbon [emissions] have increased more than the other types of aerosol.
"But this has been difficult to take into account in the observational-based method since we have only observations of the current situation, and not before human activity started."
Dr Haywood praised the research. "Until the publication of this paper, us aerosol researchers have been scratching our heads trying to understand the difference between model-based and observational-based assessments," he said.
"This will have an impact on future climate predictions."
Engineering clouds
But aerosols have not yet revealed all of their climate-influencing secrets.
The particles also modify clouds, increasing the concentration of droplets, and therefore cloud-cover.
Dr Myhre said there was "still a lot of uncertainty about the masking or cooling" caused by this "indirect aerosol effect".
Dr Haywood agreed. "The effect of aerosols on clouds gives us a big headache," he told BBC News. "It creates a big hole in our data-gathering."
He and and his colleagues at the Met Office have already started investigating whether aerosols could be employed to deliberately mask global warming.
In a recent study, he used a climate model to find out what effect the deliberate brightening of clouds, by adding sea-salt particles to increase their reflectivity, would have on global temperatures.
The research team, led by fellow Met Office scientist Dr Andy Jones, found that global warming could be slowed by up to 25 years, but they also found the approach could also have some very detrimental effects.
The most serious of these, they say, is a sharp decrease in rainfall over South America, which would likely accelerate the die-back of the Amazon rainforest, and contribute to the loss of one of the world's major carbon sinks.
"You would have to be very careful about which clouds you choose with this approach," said Dr Haywood.
Dr Myhre stated that the impact of aerosols on the climate would eventually become insignificant, compared to greenhouse gases.
"Aerosols have a very short lifetime and the greenhouse gases have a very long lifetime - more than 100 years for carbon dioxide," he said.
"In the future it's really the greenhouse gases that are the big issue for warming. They will dominate more and more."
