Earth 'will heat up by 4C' by the year 2100, scientists predict

Rise is twice that considered dangerous by climate scientists

The Earth’s temperature could increase by 4C by 2100, scientists have warned.

Temperature rises resulting from unchecked climate change might be at the severe end of the scale of current predicitions, according to a new scientific study.

The author of the study, professor Steven Sherwood, said that unless emissions of greenhouse gases were reduced, the planet would heat up by a minimum of 4C by 2100, twice the level regarded as dangerous by climate scientists.

The research, based on a new climate model taking greater account of cloud changes, shows that as the planet gets warmer, fewer clouds will form. This means that less sunlight is reflected back into space, driving temperatures up further still.

Professor Sherwood, of the University of New South Wales in Australia, told The Guardian: "This study breaks new ground twice: first by identifying what is controlling the cloud changes and second by strongly discounting the lowest estimates of future global warming in favour of the higher and more damaging estimates.

"4C would likely be catastrophic rather than simply dangerous," Sherwood said. "For example, it would make life difficult, if not impossible, in much of the tropics, and would guarantee the eventual melting of the Greenland ice sheet and some of the Antarctic ice sheet" - causing sea levels to rise by many metres as a result.

The research, which is published in the journal Nature, is described as a “big advance” in terms of alleviating uncertainty about the true impact of global warming. Hideo Shiogama and Tomoo Ogura, at Japan's National Institute for Environmental Studies, said the explanation of how fewer clouds form as the world warms was "convincing", and agreed this indicated future climate would be greater than expected. But they said more research was needed to narrow down the projections of future temperatures.

Scientists measure the impact of greenhouse gases on the Earth’s climate by estimating the temperature rise that would be caused by a doubling of carbon dioxide in the atmosphere compared with pre-industrial levels, which is likely to happen within 50 years according to current trends. 

Despite decades of research attempting to narrow uncertainties, these estimates have previously spanned roughly 1.5 to five degrees, but the new research estimated a narrower window between 3C and 5C by focusing on an area of uncertainty: cloud formation.

Researchers worked to ensure that computer climate models accurately represented the way clouds form in the real world. When water evaporates from the oceans, the vapour can either rise over nine miles to form rain clouds that reflect sunlight, or it can drift back down without forming clouds. Climate models that incorporated both of these potentialities predicted significantly higher future temperatures than those only including the nine-mile-high clouds.

"Climate sceptics like to criticise climate models for getting things wrong, and we are the first to admit they are not perfect," Sherwood told The Guardian. "But what we are finding is that the mistakes are being made by the models which predict less warming, not those that predict more."

He added: "Sceptics may also point to the 'hiatus' of temperatures since the end of the 20th Century, but there is increasing evidence that this inaptly named hiatus is not seen in other measures of the climate system, and is almost certainly temporary."

However, Sherwood also told The Guardian that his team’s cannot conclusively rule out the fact that future temperature rises may lay at the lower end of projections. "But," he said, for that to be the case, "one would need to invoke some new dimension to the problem involving a major missing ingredient for which we currently have no evidence. Such a thing is not out of the question but requires a lot of faith."

He added: "Rises in global average temperatures of [at least 4C by 2100] will have profound impacts on the world and the economies of many countries if we don't urgently start to curb our emissions."

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