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The town's rapid industrial growth in the 1980s led to hazardous waste build-up, contaminated water and soil with mercury, arsenic and sulphates.

Because sulphate aerosols only last a couple of years in the atmosphere compared to the decades that carbon dioxide lasts, stratospheric aerosol injection is only seen as a short-term method.

They identified geochemical "fingerprints" of various microorganisms, including those that must have used sunlight for energy, metabolised sulphate and probably also produced methane.

Further investigations traced this back to a phase of strong methane formation by microorganisms known as archaea in water with a low sulphate content.

Once polluted regions in India or the Middle East improve their air quality by abandoning fossil fuels and transitioning to green energy sources, they too will lose their shield of sulphates, scientists said.