Ocean cooling contributed to mid-20th century global warming hiatus

LOS ANGELES, Sept. 22 (Xinhua) -- An abrupt cooling event centered over the North Atlantic around 1970 might play a role in the hiatus of global warming in the Northern Hemisphere during the mid-20th century, a new study suggests.

"We knew that the Northern Hemisphere oceans cooled during the mid-20th century, but the sudden nature of that cooling surprised us," said David W. J. Thompson, an atmospheric science professor at Colorado State University, who led an international team on the subject.

The team 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.

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 Nino and volcanic eruptions - helped us to identify the suddenness of the event," said team member Phil D. Jones of the University of East Anglia in the United Kingdom.

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 study 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," said John M. Wallace at the University of Washington who also took part in the study appearing on Wednesday in the journal of Nature.

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