Though climate change has been warming the oceans, several temperature dips have also occurred. And now scientists have found that one such cold spell, which happened around 1970, was more dramatic than previously thought.

Over the course of the 20th century, the Earth's surface has warmed by more than 0.9 degrees Fahrenheit (0.5 degrees Celsius), according to the United Nations' Intergovernmental Panel on Climate Change. However, the warming in the oceans, particularly in the Northern Hemisphere, has been less straightforward.

World average sea surface temperatures plummeted around 1945 and then again around 1970, before continuing to warm, data has shown. The first dip is believed to be an anomaly, caused by problems with the instruments used to measure it. New research indicates, however, that the second cold snap was both quite real and quite significant.

The study found that between 1968 and 1972, the sea surface temperature of the Northern Hemisphere dropped by about 0.54 degrees F (0.3 degrees C), a change so dramatic that it cannot be explained by natural cycles in ocean temperature or cooling caused by aerosols, small particles of pollution, which can reflect sunlight and cool Earth's surface.

Unlike data for the earlier drop, evidence of cooling temperatures around 1970 exists in numerous measurements of sea surface temperatures. The researchers liken the speed of the cooling to "abrupt climate change," which the IPCC describes in a 2007 report as a regional change of several degrees Celsius within several decades. On the other end of the scale, climate change caused by astronomical factors, such as changes in the Earth's orbit, progresses over thousands of years, according to the report.

The most marked cooling during this period occurred in the North Atlantic Ocean, and the authors speculate that it may be connected to a decrease in salinity (the salt content of the ocean) there in the late 1960s and early 1970s. This is the only potential cause the researchers offer.

Meanwhile in the Southern Hemisphere, surface temperatures rose steadily during the second half of the century, according to the report. Researchers were already aware that the Southern Hemisphere had warmed more than the Northern Hemisphere during this time, however, this new study revealed the sudden nature of the gap between the two hemispheres, according to study researcher David Thompson, an atmospheric scientist at Colorado State University.

Most previous work "is based on 'smoothed' versions of the data, and thus the suddenness of the difference around 1970 was largely overlooked," he told LiveScience in an email.  

Thompson and the other researchers used a method that reduced year-to-year variation in temperatures caused by variation in air circulation; the El Nino Southern Oscillation, a periodic ocean−atmosphere fluctuation that affects weather around the globe; and volcanic eruptions, which like humans, spew potentially cooling aerosols into the atmosphere. These short-term phenomena can obscure significant events, such as this sudden, northern cooling, the researchers wrote in the Sept. 23 issue of the journal Nature.