Cleaner China air ‘fuels North America warming’
China’s rapid reductions in aerosol emissions have exacerbated warming events in the Northeast Pacific Ocean and west coast of North America because of anomalies in atmospheric circulation, a study has found.
While the abatement of rapid aerosol emissions from the burning of fossil fuels has improved air quality in China over the past decade, it has also led to a decrease in the cooling effect that aerosols offer to the Earth’s surface by reflecting solar radiation.
A study used climate modelling to examine how aerosol reduction and resulting local temperature changes in China have influenced warming elsewhere in the world.
“The period of 2010 to 2020 has witnessed the warmest Northeast Pacific sea surface temperatures ever recorded, with several prolonged extreme ocean warming events,” said the team, which included researchers from the United States and Germany, in a paper in Proceedings of the National Academy of Sciences.
The sea surface temperature along the coast from Alaska to California had been “suddenly warming in the past 10 years”, said Zheng Xiaotong, corresponding author and a professor at the Ocean University of China.
These warming events in the Northeast Pacific – dubbed “warm blobs” by researchers – had been accompanied by extreme weather, such as the California drought from 2013 to 2016, which cost billions of dollars in agricultural losses, the paper said.
While year-to-year climate variation could play a role in the marine heating events that led to extreme weather on the west coast of North America, the researchers said it did not fully explain the occurrence of these warm blobs.
Global warming was a “continuous phenomenon” that would not cause such specific warming, Zheng said, adding that there had been a long period of no change, and “then suddenly there was an increase in warming in the past 10 years”. “So, we hypothesised that this phenomenon may be related to some other human factors,” Zheng said.
Using climate models, the researchers found “rapid aerosol abatement in China triggers atmospheric circulation anomalies beyond its source region, driving a substantial mean surface warming” in the Northeast Pacific.
Alongside greenhouseinduced warming and climate variability, the aerosol-induced warming “made the warm blob events more frequent and intense during 2010 to 2020”, affecting biodiversity and causing toxic algal blooms and drought, the paper said.
Aerosols are small particles or liquid droplets suspended in the air that can be emitted as pollution by burning fossil fuels and can counter the warming impact of greenhouse gases.
Beijing’s clean air policies have led to a decrease in pollutant emissions from the burning of fossil fuels in power plants and factories over the past decade, including aerosols such as black carbon – or soot – and sulphate, which have been linked to lung disease and other conditions.
Aerosols can reflect solar radiation back into space, blocking some of the energy from reaching the Earth’s surface and creating a cooling effect. Aerosols can also increase clouds’ ability to reflect solar radiation.
Some aerosols also absorb sunlight, causing temporary cooling on the surface but eventually leading to warming.
Zheng said the local warming effect of the reduction in aerosols in China had been studied previously but its effects on other regions had been overlooked.
The team compared models that reflected China’s aerosol reductions along with models that simulated a plateau, rather than a decrease, in aerosol emissions. They found that a downward trend in aerosol emissions coincided with an increase in mean warming in the Northeast Pacific.
In a simulation where aerosols were the only changed variable and greenhouse gases were fixed at pre-industrial levels, the team found that there was a “continuous cooling” of sea surface temperature in the Northeast Pacific until around 2007, after which a rapid warming began that followed the emissions reductions.
The team said this occurred because of local warming on the coast of Asia intensifying the Aleutian Low – an atmospheric low-pressure patch off the coast of Alaska – and shifting it southwards.
The atmospheric circulation anomaly weakens surface winds and therefore suppresses evaporative cooling in the Northeast Pacific, leading to an increase in sea surface temperature.
China’s increase in aerosol emissions was caused by industrialisation, and Zheng said it was possible that these earlier emissions “greatly slowed down the warming” of the Northeast Pacific in the past.