January 31, 2013
Greenhouse Gases Affect Rainfall Differently Than Natural Global Warming
April Flowers for redOrbit.com - Your Universe Online
An international group of scientists has shown that global warming from greenhouse gases affects global rainfall patterns differently than global warming from solar heating. Using computer model simulations, the research team showed that global rainfall has increased less over the present-day warming period than during the Medieval Warm Period between 950 and 1250 AD despite the fact that temperatures today are higher than during that time.
The findings of this study were published in today´s issue of the journal Nature.
The scientists examined global precipitation changes over the last millennium and predicted changes through the end of the 21st century. They compared natural changes from solar heating and volcanism with changes from man-made greenhouse gas emissions. Using an atmosphere-ocean coupled climate model that simulates realistically both past and present-day climate conditions, they found that for every degree rise in global temperature, the global rainfall rate since the Industrial Revolution has increased by about 40% less than during the Earth´s past warming phases.
"Our climate model simulations show that this difference results from different sea surface temperature patterns. When warming is due to increased greenhouse gases, the gradient of sea surface temperature (SST) across the tropical Pacific weakens, but when it is due to increased solar radiation, the gradient increases. For the same average global surface temperature increase, the weaker SST gradient produces less rainfall, especially over tropical land,” explained Bin Wang, professor of meteorology at the International Pacific Research Center, University of Hawaii at Manoa.
"Adding long-wave absorbers, that is heat-trapping greenhouse gases, to the atmosphere decreases the usual temperature difference between the surface and the top of the atmosphere, making the atmosphere more stable," added the report´s coauthor Jian Liu from the Chinese Academy of Sciences. "The increased atmospheric stability weakens the trade winds, resulting in stronger warming in the eastern than the western Pacific, thus reducing the usual SST gradient — a situation similar to El NiÃ±o."
On the other hand, solar radiation heats the planet's surface by increasing the usual temperature difference between the surface and the top of the atmosphere without weakening the trade winds. This heating warms the western Pacific. The eastern Pacific, on the other hand, remains cool from the usual ocean upwelling.
"While during past global warming from solar heating the steeper tropical east-west SST pattern has won out, we suggest that with future warming from greenhouse gases, the weaker gradient and smaller increase in yearly rainfall rate will win out," concludes Wang.