Pristine environments offer a window into our cloudy past

A new study uses satellite data over the southern hemisphere to understand the global composition of clouds during the industrial revolution. This research addresses one of the biggest uncertainties in current climate models: the long-term effect of small atmospheric particles on climate change.

Climate models currently include the global warming effect of greenhouse gases, as well as the cooling effects of atmospheric aerosols. The tiny particles that make up these aerosols are produced by man-made sources, such as emissions from cars and industry, as well as by natural sources such as phytoplankton and marine spray.

They can directly influence the flow of sunlight and heat into Earth’s atmosphere, as well as interact with clouds. One of the ways they do this is by reinforcing the ability of clouds to reflect sunlight into space by increasing their droplet concentration. This in turn cools the planet. The amount of sunlight that is reflected in space refers to Earth’s albedo.

However, there has been an extremely limited understanding of how aerosol concentration has changed between early industrial times and today. This lack of information restricts the ability of climate models to accurately estimate the long-term effects of aerosols on global temperatures and how much effect they might have in the future.

Now, an international study led by the Universities of Leeds and Washington has recognized that the remote and pristine parts of the southern hemisphere provide a window into the appearance of the early industrial atmosphere.

The team used satellite measurements of the concentration of cloud droplets in the atmosphere over the northern hemisphere, heavily polluted with today’s industrial aerosols, and over the relatively pristine southern ocean.

They used these measurements to quantify possible changes due to industrial aerosols in Earth’s albedo since 1850.

The results, published today in the magazine. PNAS, suggest that early industrial aerosol concentrations and the number of cloud drops were much higher than is currently estimated in many global climate models. This could mean that human-generated atmospheric aerosols are not having as strong a cooling effect as some climate models estimate. The study suggests that the effect is likely to be more moderate.

Lead co-author Daniel McCoy, a researcher at the Leeds School of Earth and Environment, said: “Limitations in our ability to measure aerosols in the early industrial atmosphere have made it difficult to reduce uncertainties about how much warming there will be in the 21st century.

“Ice cores provide carbon dioxide concentrations of millennia in the past, but aerosols don’t stay the same way. One way we can try to look back in time is to examine a part of the atmosphere that doesn’t we have still polluted.

“These remote areas allow us to glimpse our past and this helps us understand the climate record and improve our predictions of what will happen in the future.”

Co-lead author Isabel McCoy of the Washington Department of Atmospheric Sciences said: “One of the biggest surprises for us was how high the concentration of cloud droplets is in the clouds of the Southern Ocean. How the concentration of cloud increases summer cloud drops “tells us that ocean biology is playing an important role in shaping cloud brightness in uncontaminated oceans now and in the past.

“We see high concentrations of cloud droplets in satellite and aircraft observations, but not in climate models. This suggests that there are gaps in the model representation of interactions between aerosols and clouds and aerosol production mechanisms in pristine environments.

“As we continue to view pristine environments through satellites, aircraft, and ground platforms, we can improve the representation of the complex mechanisms that control the brightness of clouds in climate models and increase the accuracy of our climate projections.”

Co-author Leighton Regayre, research associate at the Leeds School of Earth and Environment, said: “The science behind our climate models is improving all the time. These models address some of the most pressing and complex environmental issues from Scientists at the modern era and the climate have always been frank about the fact that uncertainties exist.

“We are only going to achieve the answers we need to combat global warming by regularly questioning science. Our team used millions of variants of a model to explore all potential uncertainties, the equivalent of having a clinical trial with millions of participants.”

“We hope that our findings, along with studies of the detailed process of aerosol production and aerosol-cloud interactions in pristine environments that our work has prompted, will help guide the development of the next generation of climate models.”

The article “Hemispheric Contrast in Cloud Microphysical Properties Restricts Aerosol Forcing” is published in PNAS, July 27, 2020.