Despite the long term warming trend seen around the globe, the oceans have cooled in the last three years, scientists announced today.

The temperature drop, a small fraction of the total warming seen in the last 48 years, suggests that global warming trends can sometimes take little dips.

In the last century, Earth's temperature has risen about 1 degree Fahrenheit (0.56 degrees Celsius). Most scientists agree that much of the warming in the past 50 years has been fueled by the burning of fossil fuels and other human activities.

"This research suggests global warming isn't always steady, but happens with occasional 'speed bumps,'" said study co-author Josh Willis, a researcher at NASA's Jet Propulsion Laboratory. "This cooling is probably natural climate variability. The oceans today are still warmer than they were during the 1980s, and most scientists expect the oceans will eventually continue to warm in response to human-induced climate change."

Rising seas

Regardless of the cooling trend observed since 2003, average sea levels have continued to rise. The rising of sea level occurs due to the thermal expansion of the oceans from the heating and chunks and runoff from melting ice sheets and glaciers.

"The recent cooling episode suggests sea level should have actually decreased in the past two years," Willis said. "Despite this, sea level has continued to rise. This may mean that sea level rise has recently shifted from being mostly caused by warming to being dominated by melting. This idea is consistent with recent estimates of ice-mass loss in Antarctica and accelerating ice-mass loss on Greenland."

In a previous study, researchers reported that in parts of the Antarctic, 84 percent of glaciers have retreated over the past 50 years in response to a warmer climate. But the melting glaciers are not the reason for the cooling.

The amount of ice and water from the melting glaciers is very small compared to the overall temperature of the oceans, Willis told LiveScience.

Ocean temperatures have been through dips like this before.

There have been substantial decadal decreases, said study co-author John Lyman of NOAA's Pacific Marine Environmental Laboratory. "Other studies have shown that a similar rapid cooling took place from 1980 to 1983. But overall, the long-term trend is warming."

Implications

Determining the amount of heat oceans store is important for determining the amount of total energy absorbed from the sun and energy reflected back.

"The capacity of Earth's oceans to store the sun's energy is more than 1,000 times that of Earth's atmosphere," Lyman said. "It's important to measure upper ocean temperature, since 84 percent of the heat absorbed by Earth since the mid-1950s has gone toward warming the ocean. Measuring ocean temperature is really measuring the progress of global warming."

Researchers have not yet identified the cause of ocean cooling in the last three years but hope that further studies will clarify this anomaly.

Some say it could be due to events such as volcanic eruptions, but the reasons need to be looked at still, Willis said.

The study is detailed in the current issue of the journal Geophysical Research Letters.

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