May 1, 2008

Researchers Predict Climate Will Cool in Coming Decade

Scientists at Germany's Kiel University predict the Earth's temperature will stay roughly the same in the coming decade as the planet's natural climate cycle enters a cooling phase.  

The researchers developed computer models for their analysis, which suggest the cooling phase will counter warming from greenhouse gas emissions.

But thereafter, the researchers expect temperatures to begin rising quickly by about 2020.

The forecast centers around the ocean's natural temperature cycles, called the Atlantic Multidecadal Oscillation (AMO), which is closely associated with the warm currents that carry heat from the tropics to European shores.

While the cause of the oscillation is not yet understood, the cycle appears to occur about every 60 to 70 years.   Some believe it may partially explain the temperature increases early last century and subsequent cooling in the 1940s.

Climate scientists say the research could help societies plan better for the future.

"One message from our study is that in the short term, you can see changes in the global mean temperature that you might not expect given the reports of the Intergovernmental Panel on Climate Change (IPCC)," Noel Keenlyside of the Leibniz Institute of Marine Sciences at Kiel University told BBC News.

While Keenlyside's projection diverges from other computer models for about 15-20 years, the curves come back together and temperatures rise beyond that time.

"In the long term, radiative forcing (the Earth's energy balance) dominates. But it's important for policymakers to realize the pattern," he said.

Climatic event modeling of the oceans is challenging as data is lacking about some of the key processes, such as the meridional overturning circulation (MOC), that carries heat northwards in the Atlantic.  Only recently have researchers started routinely deploying mobile floats and tethered buoys that over time will indicate how this circulation is changing.

In lieu of direct measurements of the MOC, the Kiel University team used historical data over the past 50 years from the Labrador Sea, where warm water gives up its heat to the atmosphere before sinking lower into the ocean.

Combining this data with existing global warming models, the team generated model results that mimicked recent temperatures observed over the north Atlantic, western Europe and North America.  The model projects a weakening of the MOC and a resulting cooling of north Atlantic waters going forward, which should keep temperatures in check around the world, similar to the warming and cooling associated with El Nino and La Nina in the Pacific.

"We have to take into account that there are uncertainties in our model; but it does suggest a plateauing of temperatures, and then a continued rise," said Dr Keenlyside.

While the projection may come as news to those who think warming temperatures are a permanent phenomenon, scientists are not surprised by the model's predictions.

"We've always known that the climate varies naturally from year to year and decade to decade," Richard Wood from the UK's Hadley Centre told BBC News.

"We expect man-made global warming to be superimposed on those natural variations; and this kind of research is important to make sure we don't get distracted from the longer term changes that will happen in the climate (as a result of greenhouse gas emissions)."

However, Dr Wood warns that modeling of this kind is new, and that once data can be obtained directly from the depths of the Atlantic it may change the view of how the AMO works and its implications for the global climate.

Michael Schlesinger, a researcher at the University of Illinois and the scientist who first characterized the AMO in 1994, said the new model was "very exciting".

"No doubt we need to have more data from the deep ocean, and we don't have that at present," he told BBC News.

"But imagine the payoff of knowing with some certainty what the next 10 years hold in terms of temperature and precipitation - the economic impacts of that would be significant."

The research was published in the May 1 issue of the journal Nature.  A summary of the report can be viewed at http://www.nature.com/nature/journal/v453/n7191/full/nature06921.html.


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Leibniz Institute of Marine Sciences

Hadley Centre

University of Illinois