Aerosols are tiny particles released into the atmosphere by human activities; they have proven negative effects on air quality—damaging human health and agricultural productivity. However, while greenhouse gases cause warming by trapping heat in the atmosphere, some aerosols can have a cooling effect on the climate because the aerosol particles cause more of the sun’s light to be reflected away from the planet. Estimates indicate that aerosols have offset about a third of greenhouse gas-driven warming since the 1950s.

Global and regional-mean temperature responses to identical aerosol emissions in different regions. Identical total annual emissions of sulfate and black carbon aerosol from eight major past, present, and potential future emitting regions (a) result in global-mean cooling spanning a 14x range (b, y-axis), differing substantially in the degree to which that cooling is felt in the emitting region (b, x-axis). Diagonal lines (b) indicate the ratio of regional- to global-mean cooling. Error bars in b capture the standard error. Persad and Caldeira.

However, aerosols have a much shorter lifespan in the atmosphere than the gases responsible for global warming, resulting in an atmospheric distribution that varies by region, especially in comparison to carbon dioxide.

Now, a study by Geeta Persad and Ken Caldeira at the Carnegie Institution for Science finds that the impact these fine particles have on the climate varies greatly depending on where they were released. An open-access paper on their work is published in Nature Communications.

Not all aerosol emissions are created equal. Aerosols emitted in the middle of a monsoon might get rained out right away, while emissions over a desert might stay in the atmosphere for many days. So far, policy discussions have not come to grips with this fact.

—Ken Caldeira

For example, their models show that aerosol emissions from Western Europe have 14 times the global cooling effect that aerosol emissions from India do. Yet, aerosol emissions from Europe, the United States, and China are declining, while aerosol emissions from India and Africa are increasing.

This means that the degree to which aerosol particulates counteract the warming caused by greenhouse gases will likely decrease over time as new countries become major emitters.

—Geeta Persad

The duo also found that there are significant regional differences when it comes to how strongly a country is affected by its own emissions.

For example, aerosols from India cool their country of origin 20 times more than they cool the planet. In comparison, aerosols from Western Europe, the United States, and Indonesia cool their country of origin only about twice as much as they cool the planet—a significant difference in how these emissions are experienced. In many cases, the strongest climate effects of aerosols are felt far from where the aerosols are emitted.

Caldeira and Persad’s work demonstrates that the climate effects of aerosol emissions from different countries are highly unequal, which they say means that policies must reflect this variation.

This work is part of a larger $1.5-million National Science Foundation project with collaborators at UC San Diego and Stanford University that looks at the combined climate, health, and decision-making dimensions of greenhouse gases in comparison to shorter-lived pollutants like aerosols.

Resources

• Geeta G. Persad & Ken Caldeira (2018) “Divergent global-scale temperature effects from identical aerosols emitted in different regions” Nature Communications 9, Article number: 3289 doi: 10.1038/s41467-018-05838-6