Enlarge /. Long lines, so-called "cloud streets", form on the edge of the Antarctic sea ice.
One of the lesser-known scientific complications that make the assessment of man-made climate change a problem is that it's not just about greenhouse gases. Aerosol emissions – for example, tiny atmospheric particles from various sources that scatter sunlight back into space – have offset some of the warming caused by humans. And unlike long-lived greenhouse gases, aerosols are washed out of the atmosphere pretty quickly and don't leave any historical records. This makes the reconstruction of the aerosol level before the industrial revolution a challenge.
In order to improve and review the estimates of previous aerosol values, the researchers have become creative. A new study by Isabel McCoy at the University of Washington takes advantage of the fact that the sky in Antarctica is almost free of man-made aerosol pollution to establish a new pre-industrial baseline.
Aerosols have a cooling influence through direct (scattering solar radiation) as well as indirect (modifying clouds) effects. In this case, the researchers are studying the latter using satellite cloud data. In particular, they calculate the number of cloud droplets per cubic centimeter based on measurements of droplet size and cloud thickness. Since aerosols can act as condensation nuclei around which droplets form, they tend to lead to higher proportions of smaller droplets.
While aerosol pollution from coal combustion and other combustion activities occurs throughout the northern hemisphere, Antarctic atmospheric isolation keeps these aerosols at bay. Using Antarctica as an indicator of pre-industrial aerosol levels, the researchers are applying the satellite data to a number of climate models. First, they compare how well the models match modern cloud droplet concentrations around the world, and then calculate the effects of aerosol pollution over time.
There are some regions in which modern model simulations do not match the satellite data very well. The models generally overestimate droplet concentrations in the northern mid-width hemisphere and underestimate summer droplet concentrations in Antarctica. While the models show only a modest increase in summer, satellite data showed that droplet concentrations in Antarctica are actually as high as in the polluted northern hemisphere – a result that raises eyebrows.
Current (red) and pre-industrial (blue) model simulations of the cloud droplet concentration compared to the satellite data (black). Winter season left, summer right.
How could that be in this “pristine” environment, you ask? It's probably not a disguised coal-fired power station or any other human aerosol source. Aerosols occur naturally and an important source is the phytoplankton and the bacteria in the ocean. They produce dimethyl sulfide – one of the main sources of the distinct smell of sea water and a compound that leads to the formation of tiny sulfur-containing aerosols.
Giant phytoplankton blooms occur in the Antarctic when the sea ice shrinks in summer and life uses the nutrient-rich water. And that creates a lot of aerosols around which cloud droplets can form.
The most interesting thing is what it implies about our estimates of the cooling effects of aerosol pollution. If models substantially overestimate the effects of man-made aerosols on clouds and underestimate the effects of natural aerosols, the calculated effect of human aerosol pollution is too great. If the researchers have calculated the strength of this aerosol cloud effect since 1850, they receive a reduction in energy in the Earth's climate system by 0.6 to 1.2 watts per square meter. (This compensates for part of the energy supplied by greenhouse gases.)
The effects of aerosols were examined using several lines of evidence that go beyond the models. The recalculated value supports what is currently the best estimate, which was derived using all of the evidence, although the range is somewhat narrower. The findings of the study therefore relate more to the climate models, which the researchers believe could use this information to improve the cloud aerosol connection of the models. But it is also a fascinating example of access to the past by examining a place on earth where the industrial revolution – at least in a very specific way – had little impact.
PNAS, 2020. DOI: 10.1073 / pnas.1922502117 (About DOIs).