Account for Volcanic Sulphur Emissions to Model Climate More Accurately

Volcanoes leak a surprisingly high amount of their atmosphere- and climate-changing gases in their quiet phases, according to a new study, which is asking scientists to account for these leaks in their climate models to make them more accurate. Volcanoes draw plenty of attention when they erupt. But the research, led by the University of Washington (UW), US, analysed a Greenland ice core to show that volcanoes quietly release at least three times as much sulphur into the Arctic atmosphere than estimated by current climate models. The study has implications for a better understanding of the Earth's atmosphere and its relationship with climate and air quality.

It is published in the journal Geophysical Research Letters. "We found that on longer timescales the amount of sulphate aerosols released during passive degassing is much higher than during eruptions," said first author Ursula Jongebloed, a UW doctoral student. "Passive degassing releases at least 10 times more sulphur into the atmosphere, on decadal timescales, than eruptions, and it could be as much as 30 times more," said Jongebloed. According to the international study, the team analyzed layers of an ice core from central Greenland to calculate levels of sulphate aerosols between the years 1200 and 1850. The authors wanted to look at the sulphur emitted by marine phytoplankton, which was previously believed to be the biggest source of atmospheric sulphate in pre-industrial times.

"We don't know what the natural, pristine atmosphere looks like, in terms of aerosols," said senior author Becky Alexander, a UW professor. "Knowing that is a first step to better understanding how humans have influenced our atmosphere," said Alexander. The team deliberately avoided any major volcanic eruptions and focused on the pre-industrial period, when it's easier to distinguish the volcanic and marine sources, the study said. "We were planning to calculate the amount of sulphate coming out of volcanoes, subtract it and move on to study marine phytoplankton," said Jongebloed. "But when I first calculated the amount from volcanoes, we decided that we needed to stop and address that," said Jongebloed.

The location of the ice core at the centre of the Greenland Ice Sheet records emissions from sources over a wide swath of North America, Europe and surrounding oceans, the study said. While this result applies only to geologic sources within that area, including volcanoes in Iceland, the authors expect it would use elsewhere, the study said. "Our results suggest that volcanoes, even in the absence of major eruptions, are twice as important as marine phytoplankton," Jongebloed said. The discovery that non-erupting volcanoes leak sulphur at up to three times the rate previously believed is important for efforts to model past, present and future climate, the study said. Aerosol particles, whether from volcanoes, vehicle tailpipes or factory chimneys, block some solar energy.

If the natural levels of aerosols are higher, that means the rise and fall of human emissions - peaking with the acid rain of the 1970s and then dropping with the Clean Air Act and increasingly strict air quality standards - have had less of an effect on temperature than previously believed, the study said. "There's sort of a 'diminishing returns' effect of sulphate aerosols, the more that you have, the less the effect of additional sulphates," said Jongebloed. "When we increase volcanic emissions, which increases the baseline of sulphate aerosols, we decrease the effect that the human-made aerosols have on the climate by up to a factor of two," said Jongebloed. That means Arctic warming in recent decades is showing more of the full effects of rising heat-trapping greenhouse gases, which are the main control of Earth's average temperature, the study said.

"It's not good news or bad news for the climate," said Jongebloed of the result. "But if we want to understand how much the climate will warm in the future, it helps to have better estimates for aerosols," said Jongebloed. Better estimates for aerosols can improve global climate models. "We think that the missing emissions from volcanoes are from hydrogen sulphide," said Alexander, referring to the gas that smells like rotten eggs. "We think that the best way to improve these estimates of volcanic emissions is to really think about the hydrogen sulphide emissions," said Alexander.