It has long been known that sulphur dioxide aerosols can reflect sunlight back into space. On a large scale, this has tended to come from volcanic eruptions. The explosion of the island of Krakatoa in 1815 led to the following year, 1816, becoming known in Europe as ‘the year without a summer’. More recently, it is estimated that the eruption of Mount Pinatubo in the Philippines in 1991, the second largest eruption of the 20th century, sent around 18 million tonnes of SO2 into the stratosphere. Temperatures in the troposphere – the atmospheric layer closest to the earth – dropped by about 0.5°C as a result for about two years afterwards.