The warming of the Earth's surface, caused by greenhouse gases, is not the only influence on our changing climate. It now appears that increasing levels of carbon dioxide in our atmosphere, completely independent of any global warming, have a significant impact as well.

New research published in Nature Geoscience focusing on the tropics has revealed that by increasing carbon dioxide levels in the atmosphere we are changing its overturning circulation. This overturning circulation helps form clouds and rain systems, directly affecting the location and amount of rainfall.

“This finding adds another layer of complexity to geo-engineering proposals,” said an author of the paper and a chief investigator with the Centre of Excellence for Climate System Science at the University of New South Wales, Prof Steven Sherwood.

“It shows that cooling the Earth with giant mirrors in space or with sulphate aerosols in our atmosphere, as has been proposed in some quarters, is not a complete solution to climate change."

Lead author Dr Sandrine Bony from Laboratoire de Météorologie Dynamique agreed, saying the focus of geo-engineering has often been too narrow.

“We already knew geo-engineering approaches would not prevent the acidification of our oceans but now we find that carbon dioxide – even without any warming of the Earth's surface - has a very powerful effect on the distribution and intensity of rainfall,” Dr Bony said.

“This is another significant hurdle to for geo-engineers to overcome.”

Using a wide range of climate models and a numerical weather prediction model, the researchers tested how tropical climates would change if sea surface temperatures remained constant and carbon dioxide increased in the atmosphere.

It very quickly became apparent that around 50% of the tropical circulation changes projected under climate change by the end of the century were independent of the warming of the planet. Carbon dioxide alone was driving a large part of the resulting precipitation changes in the models.

This change in rainfall came about because the additional carbon dioxide affects the rate at which the atmosphere can radiate heat out to space. This is known as radiative cooling.

This cooling of the atmosphere affects the strength of atmospheric vertical motion, which is the ability of masses of air to rise and fall in large circulations. Such movement directly controls the regional formation and distribution of clouds and rain over oceans and land.

The current research, which looked only at the tropical regions, showed increased carbon dioxide levels change the distribution of rain over oceans and land throughout the tropics.

Prof Sherwood speculated further research might find the effect of carbon dioxide on rainfall to be more pronounced at higher latitudes, particularly in the Northern Hemisphere. If so, it creates an additional obstacle for geo-engineering solutions.

“I doubt we will ever have a feasible geo-engineering method to actually remove carbon dioxide from the atmosphere. Now this research suggests that those geo-engineering methods which aim to cool the Earth would not be as effective as we had hoped either,” Prof Sherwood said.

“It highlights that we need to combat global warming by reducing our emissions at the source.”

Paper: Robust direct effect of carbon dioxide on tropical circulation and regional precipitation.

For more information or to arrange and interview with Professor Steve Sherwood contact Media Manager Alvin Stone. Phone: (02) 9385 8953. Mobile: 0418 617 366. Email: alvin.stone@unsw.edu.au