Intermediate 
According to a new scientific study, temperature rises caused by uncontrolled global warming could be at the high end of current estimates. The scientist who led the research said that, unless emissions of greenhouse gases are reduced, the planet will heat up by a minimum of 4C by 2100. This is twice the level the worlds governments consider to be dangerous.
The research indicates that fewer clouds form as the planet warms, which means less sunlight is reflected back into space. This forces temperatures up even higher. The way clouds affect global warming has been the biggest mystery in the study of future climate change.
Professor Steven Sherwood, at the University of New South Wales in Australia, who led the new work, said that the study broke new ground in two ways. First, it identified what controls the cloud changes and, second, it rejected the lowest estimates of future global warming and favoured the higher and more damaging estimates.
4C would 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, with sea levels rising by many metres as a result.
The research reduces 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. Experts at Japans National Institute for Environmental Studies said the explanation of how fewer clouds form as the world warms was convincing and agreed that this indicated future climate change would be greater than expected.
Scientists measure the sensitivity of the Earths 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  which is likely to happen within 50 years. For two decades, those estimates have run from 1.5C to 5C:  wide range. The new research narrowed that range down to between 3C and 5C, by closely examining the biggest cause of uncertainty: clouds.
Computer climate models are the only tool researchers have to predict future temperatures and it was important to make sure that the way clouds are formed was represented accurately in those models. 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 happen and climate models that included the second possibility predicted significantly higher future temperatures than models that only included the nine-mile-high clouds.
Climate sceptics like to criticize climate models for getting things wrong and we are the first to admit they are not perfect, said Sherwood. But what we are finding is that the mistakes are being made by the models that 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 hiatus is not seen in other measures of the climate system and is almost certainly temporary.
Global average air temperatures have increased quite slowly since a high point in 1998, which was caused by the ocean phenomenon El Nin_o. But, observations show that heat is continuing to be trapped in increasing amounts by greenhouse gases, with over 90% disappearing into the oceans. Furthermore, a study in November 2013 suggested the pause may be mainly an illusion  a result of the lack of temperature readings from polar regions, where warming is greatest.
Sherwood accepts his teams work on the role of clouds cannot definitely rule out that future temperature rises will be at the lower end of projections. But, for that to be the case, there would need to be some major missing ingredient for which there is currently no evidence. He added that a 4C rise in global average temperatures would have a serious impact on the world and the economies of many countries if emissions were not reduced.