Scientists have warned that the world’s climate is warming faster than feared because previous predictions were too “optimistic” and overestimated the cooling impact of clouds.
A study published in the journal Nature found increasing levels of carbon dioxide will lead to thinner ocean clouds and reduce their cooling impact, causing temperature rises of at least three degrees Celsius over the course of the century.
The team of scientists said the findings show some climate models have been too “optimistic” and previous estimates of a minimum temperature rise of only 1.5°C could now be discounted.
The optimistic models did not properly assess the impact of water evaporation, which sometimes rises only a short distance into the atmosphere and causes updraughts that reduce cloud cover, the study found.
”These models have been predicting a lower climate sensitivity but we believe they’re incorrect,” said Professor Steven Sherwood, from the University of New South Wales, lead author of the paper.
The debate comes as Australia marked its hottest year since reliable recordings began in 1910.
The world’s driest continent also recorded its hottest day, hottest month, hottest winter’s day and hottest summer.
Globally, according to figures released in December by the United States National Climatic Data Centre, 2013 was set to be the fourth hottest year in 134 years of records behind 2010, 2005 and 1998.
Meteorologists said it was the hottest year on record for a non-El Niño year.
According to the Nature report global average temperatures will rise at least 4.0°C by 2100 and potentially more than 8.0°C by 2200 if carbon dioxide emissions are not reduced.
The new research found global climate was more sensitive to carbon dioxide than most previously estimated.
“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 Professor 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 5.0°C.
According to the Nature article the key to this narrower but much higher estimate can be found in the real world observations around the role of water vapour in cloud formation.
Observations show when water vapour is taken up by the atmosphere through evaporation, the updraughts can either rise to 15km to form clouds that produce heavy rains or rise just a few kilometres before returning to the surface without forming rain clouds.
When updraughts rise only a few kilometres they reduce total cloud cover because they pull more vapour away from the higher cloud forming regions.
The researchers found climate models that show a low global temperature response to carbon dioxide do not include enough of this lower-level water vapour process.
Instead they simulate nearly all updraughts as rising to 15 km and forming clouds.
When only the deeper updraughts are present in climate models, more clouds form and there is an increased reflection of sunlight.
However, real world observations show this behaviour is wrong.
When the processes in climate models are corrected to match the observations in the real world, the models produce cycles that take water vapour to a wider range of heights in the atmosphere, causing fewer clouds to form as the climate warms.
This increases the amount of sunlight and heat entering the atmosphere and, as a result, increases the sensitivity of our climate to carbon dioxide or any other perturbation.
The result is that when water vapour processes are correctly represented, the sensitivity of the climate to a doubling of carbon dioxide, which will occur in the next 50 years, means we can expect a temperature increase of at least 4.0°C by 2100.
“Climate sceptics 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,” said Professor Sherwood.
“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,” he added.
You can find the Nature article here.