Climate Change

How much do industrial aerosols contribute to global cooling? New study offers clues

A higher abundance of sulphate aerosols in the preindustrial atmosphere means anthropogenic sulphate has a relatively small cooling effect

By Rohini Krishnamurthy
Published: Tuesday 14 February 2023
Photo: iStock_

Inactive volcanoes contributed 66 per cent of sulphate emissions, known to cool the planet, in the preindustrial era, suggesting they were more abundant than previously estimated, a new study reveals.

The findings suggest that we are likely underestimating sulphate emissions from inactive or non-eruptive volcanoes, probably leading to inaccuracies in future climate projections, the study published in Geophysical Research Letters noted.

Sulphate emissions can be traced back to natural sources such as volcanoes and industrial processes, including oil combustion, traffic emissions and coal burning.

Sulphur dioxide, released from natural as well as anthropogenic sources, reacts with water vapour and other gases in the atmosphere to create sulphate aerosols. They can cool the planet by reflecting sunlight into space, according to the United States National Aeronautics and Space Administration. They also form clouds, which also tend to have a cooling effect. 

“The amount of cooling depends on how abundant sulphate aerosols were in the atmosphere before humans started burning fossil fuels,” Ursula Jongebloed, a graduate student at the University of Washington and one of the study’s authors, told Down To Earth (DTE).

A higher abundance of sulphate aerosols in the preindustrial atmosphere means anthropogenic sulphate has a relatively small cooling effect, the author explained.

But if the preindustrial aerosol is minuscule, she added, anthropogenic sulphate has a large cooling effect.

“This is an interesting study. It argues that underestimating emissions from the preindustrial era leads to overestimating the cooling effect of aerosols seen by global models,” Amit Mishra, assistant professor, Jawaharlal Nehru University, told DTE. He was not involved in the study.

Previous studies estimated that the amount of sulphur dioxide released by volcanoes is smaller than by anthropogenic sources.

It is estimated that anthropogenic aerosols have nullified 60 per cent of human-induced warming in the Arctic, which is warming at a rate almost four times higher than the global rate.

Jongebloed and her colleagues decided to study ice cores in Greenland, which hold records of emissions from North America, Europe and surrounding oceans. 

By studying the ice core layers, the team calculated the levels of sulphate aerosols between 1200 and 1850. 

Their analysis showed inactive volcanoes release up to three times the rate of sulphate emissions than previously believed. 

This new finding is important for global climate models. “Current models underestimate volcanic sulphur emissions from passive degassing (non-eruptive volcanoes) during the preindustrial era,” Jongebloed argued.

The researchers found that higher preindustrial emissions reduced anthropogenic aerosol cooling in the Arctic, estimated by climate models, by up to a factor of two. 

The researchers found a similar result on a global scale as well. However, this was not a part of the paper as they only studied ice cores from the Arctic.

Better estimates of aerosols, according to the experts, can improve global climate models. This is important as this feeds into policies, Mishra highlighted.

Further, climate models do not consider hydrogen sulphide, which escapes from inactive volcanoes. These gases react with oxygen to produce sulphur dioxide within three days, forming sulphate aerosols.  

“Hydrogen sulphide is very difficult to measure, so we don’t have many measurements of it; we can’t observe hydrogen sulphide emissions from satellites, unlike sulphur dioxide,” Jongebloed noted.

Scientists, she added, have previously assumed that hydrogen sulphide contributes less than 1 per cent to the total atmospheric sulphur concentration.

The new study estimates hydrogen sulphide emissions from volcanoes, suggesting that this gas was likely to contribute to atmospheric sulphate in the preindustrial era significantly. 

Jongebloed hopes researchers make more measurements of sulphur dioxide and hydrogen sulphide from inactive volcanoes.

The team also wants to investigate sulphur emissions from other sources, such as phytoplankton (tiny marine algae). They are primary producers known to remove carbon dioxide from the atmosphere.

Marine phytoplankton emits dimethylsulphide, an organic sulphur-containing compound that produces sulphur aerosols, according to studies.

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