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September 22, 2010

Sudden ocean cooling likely contributed to Northern Hemisphere global warming pause in mid-20th century

The hiatus of global warming in the Northern Hemisphere during the mid 20th century may have been due to an abrupt cooling event centered over the North Atlantic around 1970, rather than the cooling effects of tropospheric pollution, a new study shows.


David W.J. Thompson, an atmospheric science professor at Colorado State University, is the lead author of the paper in the Sept. 23 edition of Nature. Co-authors include John M. Wallace, a University of Washington atmospheric sciences professor, John J. Kennedy at the Met Office and Phil D. Jones of the University of East Anglia, both in the United Kingdom.


The international team of scientists discovered an unexpectedly abrupt cooling event that occurred between roughly 1968 and 1972 in Northern Hemisphere ocean temperatures. The research indicates that the cooling played a key role in the different rates of warming seen in the Northern and Southern hemispheres in the middle 20th century.


“We knew that the Northern Hemisphere oceans cooled during the mid 20th century, but the sudden nature of that cooling surprised us,” Thompson said.


While the temperature drop was evident in data from all Northern Hemisphere oceans, it was most pronounced in the northern North Atlantic, a region of the world ocean thought to be climatically dynamic.


“Accounting for the effects of some forms of natural variability – such as El Niño and volcanic eruptions – helped us to identify the suddenness of the event,” Jones said.


The different rates of warming in the Northern and Southern Hemispheres in the middle 20th century are frequently attributed to the larger buildup of tropospheric aerosol pollution in the rapidly industrializing Northern Hemisphere. Aerosol pollution contributes to cooling of the Earth’s surface and thus can attenuate the warming due to increasing greenhouse gases.


But the new paper offers an alternative interpretation of the difference in mid-century temperature trends.


“The suddenness of the drop in Northern Hemisphere ocean temperatures relative to the Southern Hemisphere is difficult to reconcile with the relatively slow buildup of tropospheric aerosols,” Thompson said.


“We don’t know why the Northern Hemisphere ocean areas cooled so rapidly around 1970. But the cooling appears to be largest in a climatically important region of the ocean,” Wallace said.


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For more information, contact Wallace at 206-543-7390 or wallace@atmos.washington.edu