In June, 1991, something surprising happened to the Earth. Mount Pinatubo, in the Philippines, erupted. The first surprise was that it was thought to be a mountain, not a volcano. In fact, pressure built up over centuries beneath this dormant volcano caused the second largest eruption of the 20th Century, spewing vast amounts of white ash and sulphates as high as the stratosphere – 10 km above the Earth’s surface.

Around 15-17 million tonnes of this volcanic material spread into a lazy haze covering much of the globe. During the following 15 months, scientists discovered a second surprise: this particle cloud had formed a protective sun-shield, reflecting a significant proportion of the sun’s rays back into space. As a result, the average global temperature that year dropped by 0.6C. And for some researchers, that raised an interesting possibility. Could we do this on purpose, deliberately producing artificial clouds to reduce global warming?

In truth, we already know the action needed to reduce man-made climate change: a rapid reduction in greenhouse gas emissions, replacing fossil fuels with renewable energy. According to the recent Intergovernmental Panel for Climate Change (IPCC) report, the world has just 12 years in which to cut emissions by half, and 32 years (by 2050) to get net emissions down to zero. However, far from the required dramatic decline, the latest figures show global carbon dioxide emissions actually going up. “Now more than ever, unprecedented and urgent action is required by all nations,” urged the UN’s Emissions Gap Report in November. And some think that artificially engineered reflective clouds may be one important solution.

If you like this, you may also enjoy:

Currently around 30% of the sun’s rays that reach Earth are reflected back to space by white surfaces, largely our polar ice. Sea ice reflects sunlight better than any known natural surface, bouncing around 90% back up; at the other end of the scale, the dark open ocean reflects just 6% of sunlight and absorbs 94%. With Arctic sea ice rapidly declining, replacing one with the other is expected to see global warming accelerate. Unless, that is, we find another white surface that can do the job.