From The Guardian UK: http://www.theguardian.com/environment/2013/dec/31/planet-will-warm-4c-2100-climate
New climate model taking greater account of cloud changes indicates heating will be at higher end of expectations
The Guardian, Tuesday 31 December 2013
Temperature rises resulting from unchecked climate change will be at the severe end of those projected, according to a new scientific study.
The scientist leading the research said that unless emissions of greenhouse gases were cut, the planet would heat up by a minimum of 4C by 2100, twice the level the world’s governments deem dangerous.
The research indicates that fewer clouds form as the planet warms, meaning less sunlight is reflected back into space, driving temperatures up further still. The way clouds affect global warming has been the biggest mystery surrounding future climate change.
Professor Steven Sherwood, at the University of New South Wales, in Australia, who led the new work, said: “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 told the Guardian. “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“, with sea levels rising by many metres as a result.
The research is a “big advance” that halves the uncertainty about how much warming is caused by rises in carbon emissions, according to scientists commenting on the study, published in the journal Nature. 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 challenges lay ahead to narrow down further the projections of future temperatures.
Scientists measure the sensitivity of the Earth’s climate to greenhouse gases by estimating the temperature rise that would be caused by a doubling of CO2 in the atmosphere compared with pre-industrial levels – as is likely to happen within 50 years, on current trends. For two decades, those estimates have run from 1.5C to 5C, a wide range; the new research narrowed that range to between 3C and 5C, by closely examining the biggest cause of uncertainty: clouds.
The key was to ensure that the way clouds form in the real world was accurately represented in computer climate models, which are the only tool researchers have to predict future temperatures. When water evaporates from the oceans, the vapour can rise over nine miles to form rain clouds that reflect sunlight; or it may rise just a few miles and drift back down without forming clouds. In reality, both processes occur, and climate models encompassing this complexity predicted significantly higher future temperatures than those only including the nine-mile-high clouds.
Continue reading at: http://www.theguardian.com/environment/2013/dec/31/planet-will-warm-4c-2100-climate