The physical science section of the Fifth Assessment Report by the Intergovernmental Panel on Climate Change (IPCC), which was published on 27 September, reports that over 90% of the extra energy accumulated in the Earth’s climate system from 1971 to 2010 is stored in the oceans as heat, while less than 10% is stored in the atmosphere.
“This is the first time that the change in the oceans’ temperature has received such a wide focus in an IPCC report. It’s because it’s only now that we have enough measurements and observations on the seas,” says Professor of Meteorology Timo Vesala, who participated in writing of the report.
“The surface area of the seas is greater than that of the continents, the seas are deep and the heat capacity of water, which refers to its capacity to retain and store heat, is significantly higher than that of air,” Vesala explains.
You can easily see the difference on a beach in August, for instance. Air temperatures can fluctuate by about 20 degrees in a single 24-hour period but the sea temperature remains more or less the same.
Globally, the temperature of the atmosphere also fluctuates more than that of the seas. Therefore, because a cool year may follow a hot one, we cannot show that the climate is getting warmer on the basis of samples from only one decade.
If the warming of the Earth were measured through the total amount of heat in the seas and atmosphere, however, the rising trend would become more evident even over a decade.
“If we had lots of money and began measuring climate change from scratch, we’d build a much more extensive observation network for the seas,” says Vesala.
The heat balance between the sea and the atmosphere may also fluctuate naturally. From time to time the seas release a considerable amount of their heat into the air. This happens, for instance, during hurricanes or the El Niño phenomenon, as ocean currents in the Eastern Pacific change.
“From the viewpoint of an ocean, a small change in the heat balance causes the temperature of the atmosphere to fluctuate significantly because the seas store about nine times more heat than does the atmosphere,” says Vesala.