Aerosol and greenhouse gas interactions determine frequency of extreme weather events

Pollution will both heat and cool the Earth’s atmosphere in the years ahead, but the overall result won’t be good, according to new research.

An international study suggests atmospheric pollutants, or aerosols, sometimes cause regional cooling by reflecting sunlight back to space at the same time as greenhouse gases are heating things up.

The tension between these two factors will determine the frequency and severity of extreme events such as floods, droughts and heatwaves, but it’s difficult to be specific because assumptions about the nature and concentrations of aerosols vary greatly, largely due to the complex underlying chemistry and physics. 

This leads to very different estimations of future summer weather events. One model suggests as much as a three-fold increase. The researchers talk about an approximate 50% rise this century “under business-as-usual carbon emissions”.{%recommended 7683%}

The work, by American, German and Dutch researchers, was led by Michael Mann from Pennsylvania State University in the US and is published in the journal Science Advances.

Previous studies have demonstrated that severe summer weather events are likely affected by quasi-resonant amplification (QRA) – a phenomenon whereby jetstream patterns showcase large undulations that lock in place, trapping weather systems between them. 

Mann and colleagues had already developed an indirect approach to assessing QRA, which they say produces reliable patterns of summer weather events when compared to historical records. 

In the recent project, they extended their knowledge of QRA to look forward rather than back, using various climate model projections set out in the international framework known as Coupled Model Intercomparison Project Phase 5, or CMIP5. 

While modelling how summer events might look in future as QRA conditions favourable to them increase, the researchers found that when using some simulations these events actually decreased, largely because of aerosol’s cooling effect, which offsets the warming from greenhouse gases. 

However, other models indicated significant increases in conditions that will lead to more extreme summer events.

“Models with amplified Arctic warming yield the most pronounced increase in QRA events,” the researchers write. 

“The projections are strongly dependent on assumptions regarding the nature of changes in radiative forcing associated with anthropogenic aerosols over the next century. 

“One implication of our findings is that a reduction in midlatitude aerosol loading could actually lead to Arctic de-amplification this century, ameliorating potential increases in persistent extreme weather events.”

Ultimately, they say, how summer event intensity and frequency plays out is critically dependent on human behaviours related to fossils fuels and aerosols.