Ocean pump keeps northern hemisphere hot

By Rebecca Summers - From New Scientist

The northern hemisphere 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”

"We have found a new trigger to explain the start-up of the Atlantic current system during the greenhouse-icehouse climate transition: During the warm climate, buoyant fresh water flooded out of the Arctic and prevented the ocean-sinking that helps power the conveyor. We found that the Arctic-Atlantic gateway closed due to tectonic forces, causing a dramatic increase in North Atlantic salinity. This caused warming of the North Atlantic and Europe, and kickstarted the modern circulation that keeps Europe warm today," says David Hutchinson, researcher at the Department of Geological Sciences, Stockholm University, and lead author of the article published in Nature Communications.

The team of scientists, from the Bolin Centre for Climate Research, used a combination of geophysical data and climate modelling to show that the freshwater transport through the Arctic-Atlantic gateway plays a critical role in controlling the overturning circulation.

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"Not only did deep water start forming in the Atlantic Ocean, it also stopped forming in the North Pacific at the same time, which matches geological evidence. We were surprised to find that our computer simulations can explain both of these changes due to salty ocean currents connecting the Pacific to the Atlantic. Our study is the first to show that these two events are linked, which is very exciting," says Hutchinson.

The climate at this time was very warm, with atmospheric CO₂ levels two to three times the present day levels, and this contributed to extremely fresh Arctic waters. The study begs the question of whether in a future warm world, in which the Arctic may again be very fresh, the sinking in the Atlantic may cease again, which may dramatically alter the climate of Europe. Without the Atlantic conveyor belt, Europe can experience both colder winters and hotter and drier summers, making a more extreme and inhospitable climate.

"Our study helps to bridge the gap between climate modeling and geological observations of the deep past. We hope this will inspire further research from both communities on the deep circulation of the ocean," says Hutchinson.