Why cleaner air is actually accelerating global warming

Independent

a day ago

Global warming has picked up pace since around 2010, leading to the recent string of record warm years. Why this is happening is still unclear, and among the biggest questions in climate science today. Our new study reveals that reductions in air pollution – particularly in China and east Asia – are a key reason for this faster warming.
Cleanup of sulphur emissions from global shipping has been implicated in past research. But that cleanup only began in 2020, so it's considered too weak to explain the full extent of this acceleration. Nasa researchers have suggested that changes in clouds could play a role, either through reductions in cloud cover in the tropics or over the North Pacific.
One factor that has not been well quantified, however, is the effect of monumental efforts by countries in east Asia, notably China, to combat air pollution and improve public health through strict air quality policies. There has already been a 75% reduction in east Asian sulphur dioxide emissions since around 2013, and that cleanup effort picked up pace just as global warming began accelerating.
Our study addresses the link between east Asian air quality improvements and global temperature, building on the efforts of eight teams of climate modellers across the world.
We have found that polluted air may have been masking the full effects of global warming. Cleaner air could now be revealing more of the human-induced global warming from greenhouse gases.
In addition to causing millions of premature deaths, air pollution shields the Earth from sunlight and therefore cools the surface. There has been so much air pollution that it has held human-induced warming in check by up to 0.5°C over the last century.
With the cleanup of air pollution, something that's vital for human health, this artificial sunshade is removed. Since greenhouse gas emissions have kept on increasing, the result is that the Earth's surface is warming faster than ever before.
Modelling the cleanup
Our team used 160 computer simulations from eight global climate models. This enabled us to better quantify the effects that east Asian air pollution has on global temperature and rainfall patterns. We simulated a cleanup of pollution similar to what has happened in the real world since 2010. We found an extra global warming of around 0.07°C.
While this is a small number compared with the full global warming of around 1.3°C since 1850, it is still enough to explain the recent acceleration in global warming when we take away year-to-year swings in temperature from natural cycles such as El Niño, a climate phenomenon in the Pacific that affects weather patterns globally.
Based on long-term trends, we would have expected around 0.23°C of warming since 2010. However, we actually measured around 0.33°C. While the additional 0.1°C can largely be explained by the east Asian air pollution cleanup, other factors include the change in shipping emissions and the recent accelerated increase in methane concentrations in the atmosphere.
Air pollution causes cooling by reflecting sunlight or by changing the properties of clouds so they reflect more sunlight. The cleanup in east Asian air pollution influences global temperatures because it reduces the shading effect of the pollution over east Asia itself. It also means less pollution is blown across the north Pacific, causing clouds in the east Pacific to reflect less sunlight.
The pattern of these changes across the North Pacific simulated in our models matches that seen in satellite observations. Our models and temperature observations also show relatively strong warming over the North Pacific, downwind from east Asia.
The main source of global warming is still greenhouse gas emissions, and a cleanup of air pollution was both necessary and overdue. This did not cause the additional warming but rather, removed an artificial cooling that has for a time helped shield us from some of the extreme weather and other well-established consequences of climate change.
Global warming will continue for decades. Indeed, our past and future emissions of greenhouse gases will affect the climate for centuries. However, air pollution is quickly removed from the atmosphere, and the recent acceleration in global warming from this particular unmasking may therefore be short-lived.
Laura Wilcox is a Professor at the National Centre for Atmospheric Science, University of Reading