News in Science

Deep oceans may mask global warming

Heat sink Deep areas of Earth's oceans may absorb enough heat that warming of the surface pauses for as long as a decade, a new study has found.

The joint US and Australian study shows that hiatus periods, when the warming of Earth's surface slows, may be a relatively common phenomenon linked to La Niña-like conditions.

While excess energy entering our climate system might not always warm the surface, the total amount of heat is still increasing, the researchers report in the latest issue of Nature Climate Change.

"In our model we found that most of the heat is going into the deep ocean at those times when the net temperature of the surface is flat," says study co-author Julie Arblaster, senior research scientist at the Bureau of Meteorology.

"These hiatus periods, or slow down periods, can happen from time to time even when there's additional energy coming into the system," says Arblaster.

The study illustrates one reason why global temperatures do not simply rise in a straight line, says lead author Dr Gerald Meehl from the National Center for Atmospheric Research.

"We will see global warming go through hiatus periods in the future, however, these periods would likely last only about a decade or so, and warming would then resume," says Meehl.

The latest hiatus was during the decade of 2000-2009, when the average surface temperature of the Earth showed little increase, and even a slight decrease.

At the same time, measurements of the energy entering and leaving the top of the atmosphere found a total of roughly one watt per metre square was retained in Earth's climate system.

Climate models

To find out where the heat was going, the researchers ran simulations using climate system models able to portray complex interactions within the environment. After supercomputers ploughed through the calculations, they found eight hiatus periods across the coming century to further investigate.

The simulations showed that during hiatus decades ocean temperatures above 300 metres were cooler than at other times, but waters below 300 metres warmed by around 18 per cent more than at other times.

A regional pattern appeared in the simulations, which was similar to what we observe in La Niña events — lower sea surface temperatures in the tropical Pacific, and warmer temperatures in the oceans 35 degrees north and south of the equator.

These patterns are related to changes in ocean circulation, says Arblaster.

"We found that heat is moving down due to strengthened circulation in the Pacific Ocean and weakened circulation in the North Atlantic."

As circulation decreases in the North Atlantic, less cooler water, such as from melted sea ice enters the deep sea, explains Arblaster.

"But more study is needed to see what leads to such changes [in ocean circulation] occurring."

Dr Ming Feng, an oceanographer from the CSIRO who was not involved in the study, says it is "interesting research."

"This is a modelling exercise that shows a plausible explanation for why there is a cooling effect when heat is still entering the system," Ming says.