According to scientists a cold snap in northern oceans around 1970 may have caused a dip in world temperatures that briefly interrupted a trend of global warming.
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."