The Earth’s climate is far more sensitive to carbon dioxide emissions than previously thought, heightening the likelihood of a 4C temperature rise by 2100, new Australian-led research of cloud systems has found.
The study, published in Nature, provides new understanding on the role of cloud formation in climate sensitivity – one of the key uncertainties in predictions of climate change.
Report authors Steven Sherwood, Sandrine Bony and Jean-Louis Dufresne found climate models which show a low global temperature response to CO2 emissions do not factor in all the water vapour released into the atmosphere.
Models typically simulate water vapour as rising to 15km and forming clouds, rather than updraughts of water vapour that rise only a few kilometres and pull away the cloud-forming vapour. This prediction of cloud cover is important because clouds reflect sunlight, lessening the impact of global warming.
In reality, the study found, water vapour is distributed to different heights in the atmosphere, causing fewer clouds to form as the climate warms.
In turn, this increases the amount of sunlight entering the atmosphere, making the level of warming far more sensitive to heat-trapping gases such as CO2.
As a result, the world can expect a temperature increase of “at least” 4C by 2100 if, as predicted, there is a doubling of CO2 in the atmosphere. This could then rise by more than 8C by 2200.
This is beyond the lower range of predictions and double the 2C limit, compared with pre-industrial times, agreed by countries to prevent the impact of runaway climate change. By comparison, average temperatures have risen 0.8C over the past 100 years.
“Mixing inferred from observations appears sufficiently strong to imply a climate sensitivity greater than 3C for a carbon dioxide doubling,” the report states. “This is significantly higher than the currently accepted lower bound of 1.5C, thereby constraining model projections toward relatively severe future warming.”
The issue of how sensitive the climate is to CO2 was a contentious one following the release of the latest climate report in September by the Intergovernmental Panel on Climate Change (IPCC). Some sceptics have claimed a so-called warming “pause” over the past 15 years shows the climate does not react strongly to high concentrations of carbon emissions, which continued to escalate to a new average high of 393.1ppm in 2012.
“Climate sceptics like to criticise 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,” said Professor Sherwood, lead author of the report at the UNSW centre of excellence for climate system science.
“Our research has shown climate models indicating a low temperature response to a doubling of carbon dioxide from pre-industrial times are not reproducing the correct processes that lead to cloud formation.
“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 1.5C to 5C,” he said.
“This new research takes away the lower end of climate sensitivity estimates, meaning that global average temperatures will increase by 3C to 5C with a doubling of carbon dioxide.
“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.”
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