The mid-twentieth century saw the progressing warming of the globe come to an abrupt and for a time inexplicable hiatus. According to new research this temporary cooling was at least partially the result of a sudden cooling event centred over the North Atlantic between 1968 and 1972.
“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 and lead author on the paper being published in the latest issue of the journal Nature.
The different rates of warming across Earth’s two hemispheres in the middle of the 20th century had often been attributed to a larger build-up of tropospheric aerosol pollution in the fast-industrializing Northern Hemisphere. With more advanced countries located in the Northern Hemisphere compared to the Southern, scientists assumed that this was an adequate explanation.
But the new paper offers a new interpretation based on new information.
The temperature drop was noticed throughout the data collected from the Northern Hemisphere it was most obvious in the northern North Atlantic, a region of the oceans thought to be climatically in flux.
“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.
The paper was also authored by 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.
“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.