IT CONTAINS most of the world’s land and 90 per cent of its people, but that is not why the northern hemisphere is consistently hotter than its southern counterpart. It turns out that ocean circulation is to blame.

The temperature disparity was first recorded by early 16th century explorers, who noticed icebergs floating in the southern hemisphere at latitudes where they wouldn’t have been in the north. The northern hemisphere is currently 1.5 °C warmer on average than the southern hemisphere.

To find out what is going on, Georg Feulner and colleagues at the Potsdam Institute for Climate Impact Research in Germany used climate models to simulate what would happen if the North Atlantic heat pump, a northward-moving mass of warm water and part of the global thermohaline circulation, was turned off. Having controlled for factors such as land mass, they found that the temperature gap almost disappeared.

The heat pump is driven by cold, salty water in the north Atlantic sinking and pulling warmer surface water from the tropics to replace it. This warm water then releases heat into the atmosphere. As there is no returning flow of warm water to the south, the warmer air is trapped in the north.


This mechanism explains 90 per cent of the disparity, with differences in the amount of light reflected from the poles responsible for the rest, says Feulner, who presented the work at the European Geosciences Union meeting in Vienna this month.

“It is fascinating that a detailed study of the reasons for this hemispheric contrast has not been done until now,” says Eric Wolff of the British Antarctic Survey. The results could also help explain the more extreme temperature differences between the two hemispheres when Earth was coming out of the last glacial period, he adds.

This article appeared in print under the headline “Central heating keeps northern hemisphere hot”