An international research team has now determined that the preindustrial atmosphere contained far more aerosols from fires and slash-and-burn agriculture than indicated by previous studies. Since soot particles have a cooling effect on the Earth, some climate models may now need to be adjusted.
A study recently released in the journal Science Advances investigates the role of soot particles in the Earth’s climate system. For the purposes of the study, an international research team led by first author Pengfei Liu from Harvard University analysed ice cores from the Antarctic and the aerosol particles within them. Researchers from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI), collected some of the cores that the team examined in an extremely remote part of the Antarctic.
Soot particles play an important part in the climate system. Whereas greenhouse gases like CO2 warm the atmosphere because they absorb thermal radiation, the aerosols produced by volcanoes, forest fires and the combustion of fossil fuels have a cooling effect, because they block incoming solar radiation and promote cloud formation. Accordingly, both effects need to be reflected as realistically as possible in climate models to produce reliable forecasts for total future warming.
To test their quality, researchers ‘run’ their computer models using data from the past. If a given model can reproduce the climate data from natural archives – like ice cores, rock or sediments – it should be able to correctly represent all relevant processes. The computers can then extrapolate into the future to create forecasts. But, unlike greenhouse gases, the available data on these cooling aerosols in the preindustrial era is extremely scarce. Accordingly, there is a pressing need to close this gap in our knowledge, so as to further improve the forecasts delivered by climate models.
In order to find out which concentrations of soot particles were present in the preindustrial atmosphere, the team analysed 14 ice cores collected at various sites across the Antarctic, some of them by the AWI. “For a reliable estimate of the relevant aerosol contribution to the climate system, ice cores from extremely remote regions also have to be included in the assessment. Gathering them can pose a major logistical challenge,” says AWI researcher Johannes Freitag. “We drilled one of the cores used in the study on an Antarctic plateau at an elevation of roughly 4,000 metres. To do so, four of us were flown from the AWI’s Kohnen Station to the plateau, where we drilled for the next two weeks, working at -40°C and living in tents. The air was so thin that, when it was time to fly home, the plane couldn’t lift off with the heavy ice core. It was only retrieved two years later, using snowcats, and brought back to Bremerhaven.”
All their hard work ultimately paid off. The analysis of the cores revealed that, at least in the Southern Hemisphere, the amount of soot particles from 1750 to the Industrial Revolution must have been four times higher than indicated by previous studies. Pengfei Liu and his team surmise that people living in South America, Africa and Australia utilised slash-and-burn agriculture much more intensively than previously assumed. In turn, the study shows that the cooling effect of aerosols in the preindustrial era needs to be recalibrated in climate models. “Climate researchers are aware that several models in the current generation tend to overestimate the surface temperature’s sensitivity to greenhouse gases. Until now, we didn’t know why,” says Pengfei Liu. “Our study now offers a potential explanation, which can be used to adapt and improve these models.”
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