Fossil fuels are changing the weather in more ways than just temperature. Rainfall patterns are shifting between regions, and a new assessment confirms what many atmospheric scientists have suspected – the biggest influence is aerosols released in the burning of coal and petrol.

The world has seen some pretty dramatic shifts in rainfall patterns in recent years, and these have often spelled disaster for populations reliant on the previous norm. Most notably, the tropical rainbelt has shifted southwards, causing hardship in the Americas and droughts in northern Africa that have killed millions. Disentangling the reasons is challenging but essential to predict future changes, giving nations some chance to prepare.

Professor Brian Soden and Dr Eui-Seok Chung of the University of Miami tested several climate models and found it impossible to make sense of observed shifts based on greenhouse gasses or natural changes alone. Only by including aerosols released by human activity, they report in Nature Geoscience, can the bulk of the changes be explained.

Aerosols are fine solid or liquid particles suspended in air. Although some sources of aerosols are as old as the planet, humans have added many new ones, most significantly by burning dirty fuels that release particles in the exhaust.

Water vapor condenses around certain types of aerosols in air that would otherwise be too dry for cloud formation. Aerosols can affect rainfall through both the shortwave radiation they absorb and scatter directly, and by stimulating clouds and changing their properties. Chung and Soden found that almost all their influence on rainfall came through changing the reflectiveness and lifespan of clouds. Limited understanding of how this works has held back regional climate modeling in the past, but the authors think they have taken this forwards.

"Our analysis showed that interactions between aerosol particles and clouds have caused large-scale shifts in precipitation during the latter half of the 20th century, and will play a key role in regulating future shifts in tropical rainfall patterns," said Chung in a statement. 

Aerosols have also affected temperature, counteracting global warming in the North Pacific, while adding to it in the Southern Ocean.

The aptly named Soden added: "Our work helps to understand the mechanisms that drive large-scale shifts in precipitation to better predict how the climate will change in the future."

Looking forward, it seems this will be yet another case of the major costs of human activities falling not on those responsible, but on those who did the least. Human-induced aerosols come overwhelmingly from temperate areas of the Northern Hemisphere, where Europe, North America, and China have burned fuels with abandon. Chung and Soden expect the consequences will be small in temperate zones and greatest where the tropical and subtropical zones meet.