COLORADO, US/ 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.

David W. J. Thompson, an atmospheric science professor at Colorado State, 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.

Oceans cooling, climate changing

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, published in Nature, 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.

"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.

Mid of the Century

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 build up 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,” ended Wallace.