Sulphur dioxide (SO2) is an aerosol that cools the atmosphere, can cause acid rain, and, at low altitudes, causes respiratory problems in humans. Volcanoes are natural sources of the gas and often we associate these emissions with dramatic eruptions. In reality, however, most of the gas is emitted when they are not erupting – through their everyday puffing and steaming.
‘Daily volcanic emissions are a constant input to the Earth’s atmosphere,’ says Simon Carn, lead author of a new report on global volcano emissions and associate professor at Michigan Tech in Houghton, Michigan. ‘These can impact climate in more subtle ways than a big eruption.’
In order to get a good idea of how much SO2 is being regularly released, Carn and his team have used satellite data look at the respiration of 91 active volcanoes across the globe from 2005 to 2016.
They found that 20 to 25 million tons of SO2 are being emitted annually, considerably higher than a previous estimate of made in the 1990s of 13.4 million tons. However, Carn believes that this doesn’t mean that volcanoes are emitting more, only that measurements are becoming more accurate:
‘The satellite measurements we used cover the entire globe, and so we were able to measure emissions from many more volcanoes than any previous study.’
The global range of the satellite data also brought some surprises. Strong SO2 pulses were detected from volcanoes that had been completely off the radar to most volcanogists. ‘Many volcanoes were known to be strong emitters of gas,’ he says, ‘but a few in remote parts of countries like Indonesia and Tonga were new discoveries.’
While the collective SO2 has stayed the same, changes can be seen at an individual level. This is because emissions of SO2 are mainly related to the supply of fresh magma. ‘A volcano’s variable magma supply would cause its emissions to wax and wane over time,’ says Carn. ‘Meanwhile, volcanoes with a continuous supply of magma have roughly constant emissions.’ Because of the gas and magma relationship, changes in SO2 can also be used to forecast potential eruptions.
Human activity produces twice the amount of SO2 as volcano emissions, mostly by the burning of fossil fuels – particularly solid fuels such as coal. Over the past few decades, however, stricter policies and changes to industry have brought about a steady decline of human emissions of the gas. As this continues, volcano monitoring will continue to demonstrate the natural background levels of SO2.