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

Date:
September 23, 2010
Source:
Colorado State University
Summary:
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.

Iceberg in the icefjord near the city of Ilulissat in Greenland. 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.
Credit: iStockphoto/Anders Peter Amsnæs

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, according to a new paper appearing Sept. 22 in Nature.

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David W. J. Thompson, an atmospheric science professor at Colorado State University, is the lead author on the paper. Other authors are John M. Wallace at the University of Washington, and 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 Nino 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.


Story Source:

The above story is based on materials provided by Colorado State University. Note: Materials may be edited for content and length.


Journal Reference:

  1. David W. J. Thompson, John M. Wallace, John J. Kennedy and Phil D. Jones. An abrupt drop in Northern Hemisphere sea surface temperature around 1970. Nature, September 22, 2010 DOI: 10.1038/nature09394

Cite This Page:

Colorado State University. "Ocean cooling contributed to mid-20th century global warming hiatus." ScienceDaily. ScienceDaily, 23 September 2010. <www.sciencedaily.com/releases/2010/09/100922132002.htm>.
Colorado State University. (2010, September 23). Ocean cooling contributed to mid-20th century global warming hiatus. ScienceDaily. Retrieved November 24, 2014 from www.sciencedaily.com/releases/2010/09/100922132002.htm
Colorado State University. "Ocean cooling contributed to mid-20th century global warming hiatus." ScienceDaily. www.sciencedaily.com/releases/2010/09/100922132002.htm (accessed November 24, 2014).

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