Preindustrial concentrations of atmospheric aerosols were higher than assumed in most modern climate models, a new study suggests.
If it’s right, those models are probably overestimating the amount of cooling that more recent human-generated aerosols are providing to offset the heating effect of greenhouse gases.
Researchers from the University of Leeds, UK, and the University of Washington, US, came to their conclusion after using satellite measurements to compare cloud droplet concentration over the Northern Hemisphere, which is heavily polluted with industrial aerosols, and the relatively pristine Southern Ocean.
The work is described in a paper in the journal Proceedings of the National Academy of Sciences.
“Limitations in our ability to measure aerosols in the early-industrial atmosphere have made it hard to reduce uncertainties in how much warming there will be in the 21st century,” says Leeds’s Daniel McCoy.
“Ice cores provide carbon dioxide concentrations from millennia in the past, but aerosols don’t hang around in the same way. One way that we can try to look back in time is to examine a part of the atmosphere that we haven’t polluted yet.”
The tiny particles that make up aerosols are produced by emissions from cars and industry, as well as natural sources such as phytoplankton and sea spray.
They can directly influence the flow of sunlight and heat within the Earth’s atmosphere and bolster the ability of clouds to reflect sunlight back into space by increasing their droplet concentration. This in turn cools the planet.
The amount of sunlight that is reflected to space is referred to as Earth’s albedo.
The researchers say that droplet number concentration in the Southern Ocean is a suitable proxy for preindustrial concentrations, and that according to their analysis “climate models show systematic underpredictions” of these concentrations, particularly near Antarctica.
“The way that the cloud droplet concentration increases in summertime tells us that ocean biology is playing an important role in setting cloud brightness in unpolluted oceans now and in the past,” says Washington’s Isabel McCoy.
“We see high cloud droplet concentrations in satellite and aircraft observations, but not in climate models. This suggests that there are gaps in the model representation of aerosol-cloud interactions and aerosol production mechanisms in pristine environments.”