evidence: Charles Keeling spent two years taking measurements in Antarctica and above the Mauna Loa volcano in Hawaii but reported that even in this short period, CO2 levels had risen. He concluded that the oceans weren't absorbing greenhouse gases being pumped out by industry. Instead, emissions were driving levels of CO2 higher. "It was a seminal discovery. For the first time, scientists knew that the oceans weren't going to absorb all this carbon dioxide," says Mike Hulme at the Tyndall Centre for climate change research at the University of East Anglia. [...] To mainstream scientists, evidence that warming was down to human activity was becoming too big to ignore. While scientists uncovered evidence for the greenhouse effect and warming it was producing, others pointed to different processes impacting on global climate. Volcanos, for example, blast millions of tonnes of sulphur dioxide into the atmosphere that form aerosol particles which reflect sunlight back into space. The 1991 eruption of Mount Pinatubo in the Phillipines sent about 20m tonnes of the gas into the atmosphere, leading to a global cooling of around 0.5C a year later. Scientists now believe that the warming experienced in the early 20th century can largely be explained by the lack of volcanic activity.