Global average temperatures are on track to rise at least 4 degrees Celsius by 2100, and twice that by 2200 if carbon dioxide emissions are not reduced, according to a new study suggesting the global climate is more sensitive than previously believed.

According to its authors, the research solves one of the great mysteries surrounding climate sensitivity: the role of cloud formation on global warming.

"Our research has shown climate models indicating a low temperature response to a doubling of carbon dioxide from preindustrial times are not reproducing the correct processes that lead to cloud formation," said lead author from the University of New South Wales' Center of Excellence for Climate System Science Steven Sherwood.

"When the processes are correct in the climate models the level of climate sensitivity is far higher. Previously, estimates of the sensitivity of global temperature to a doubling of carbon dioxide ranged from 1.5°C to 5°C. This new research takes away the lower end of climate sensitivity estimates, meaning that global average temperatures will increase by 3°C to 5°C with a doubling of carbon dioxide."

The key to this new estimate is real world observations of water vapor's role in cloud formation. When water vapor is taken up by the atmosphere via evaporation, updraughts can rise up to 15 km to form clouds that then result in heavy rain, or just a few kilometers before they return to the surface again, never having formed rain clouds. These less ambitious updraughts reduce total cloud cover because they draw more vapor away from the higher-up regions where clouds are formed.

Together, the scientists realized that climate models indicating a limited global temperature response to carbon dioxide fail to include sufficient levels of this lower-level process, instead simulating nearly all updraughts as rising to 15 km where they go on to form clouds. This increase in clouds means an increase reflection of sunlight, causing the climate to become less sensitive to atmospheric carbon dioxide.

Such a scenario is not the one seen in real world observations, however, and once the models were corrected to match what is seen, they produced cycles that take water vapor to a wider range of heights, leading to fewer clouds and a warmer climate.

"Climate [skeptics] like to criticize climate models for getting things wrong, and we are the first to admit they are not perfect, but what we are finding is that the mistakes are being made by those models which predict less warming, not those that predict more," Sherwood said. "Rises in global average temperatures of this magnitude will have profound impacts on the world and the economies of many countries if we don't urgently start to curb our emissions."

The study was published in the journal Nature.