A cold snap in northern oceans around 1970 may have caused a dip in world temperatures that briefly interrupted a trend of global warming, scientists said on Wednesday.
Many experts had previously explained a slight global cooling around 1970 as a side-effect of a slow build-up of sun-dimming air pollution from factories, power plants and cars that cleared up in later years with stricter air pollution laws.
But scientists in the United States and Britain said an examination of temperature records revealed "a rapid drop in Northern Hemisphere sea surface temperatures of about 0.3 degree Celsius (0.5 F) between about 1968 and 1972".
The cooling was most in the North Atlantic. Reasons for the chill were unclear but it coincided with a sudden inflow of cold water from the Arctic.
"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," they said of their conclusions in the journal Nature.
They found the drop after excluding other natural factors such as long-term shifts in ocean currents and temperatures or volcanic ash that can block sunlight.
David Thompson, lead author of the study at Colorado State University, said: "We knew that the Northern Hemisphere oceans cooled during the mid 20th century, but the sudden nature of that cooling surprised us."
GREAT SALINITY ANOMALY
Some sceptics say breaks in a warming trend of the 21st century cast doubt on findings by the U.N. panel of climate scientists that climate change is man-made. The magazine Newsweek ran a story in April 1975 entitled "The Cooling World".
Phil Jones, a co-author of the study at Britain's University of East Anglia, said the ocean cooling coincided with a little-understood flow of fresh water from the Arctic into the North Atlantic, known as the "Great Salinity Anomaly".
He said that pulse seemed to come from natural variations. "No one is postulating that the 'Great Salinity Anomaly' has any relationship to warming or the greenhouse effect from humans," he told Reuters.
Mark Maslin, Director of the Environment Institute at University College London, said in a comment that the findings were a "stark warning of what may happen in the future with larger releases of fresh water into the North Atlantic."
Sea ice is made of fresh water. Arctic Ocean summer ice shrank this year to its third smallest since satellite records began in the 1970s.
The U.N. panel of climate scientists says average world temperatures have climbed about 0.7 degree Celsius since the Industrial Revolution ushered in wide use of fossil fuels that emit heat-trapping carbon dioxide.
Jones, at the centre of a furore over e-mails hacked from the University of East Anglia in late 2009, was reinstated this year after reviews cleared him of suspicions of exaggerating evidence in favour of global warming.
Thursday's paper is the first he has since published in Nature. "Maybe it will get them thinking," he said, asked how climate sceptics would react to his involvement in a paper highlighting a cause of cooling, rather than warming.
© 2013 Thomson/Reuters. All rights reserved.