Why does the Northern Hemisphere have more temperature variations than?

By Rebecca Summers

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”

Global climate is the largest spatial scale.   We are concerned with the global scale when we refer to the climate of the globe, its hemispheres, and differences between land and oceans. Energy input from the sun is largely responsible for our global climate.   The solar gain is defined by the orbit of Earth around the sun and determines things like the length of seasons.   The distribution of land and ocean is another import influence on the climatic characteristics of the Earth.   Contrasting the climate of the Northern Hemisphere, which is approximately 39% land, with the Southern Hemisphere, which only has 19% land, demonstrates this (see the table below).   The yearly average temperature of the Northern Hemisphere is approximately 15.2C, while that of the Southern Hemisphere is 13.3C. The presence of the water reduces the annual average temperature.   The land reduces the winter average temperature while increasing the average temperature during summer.   As a result, the annual amplitude of the seasonal temperature is nearly twice as great for the Northern Hemisphere. The Northern Hemisphere has a large variation in the monthly mean temperature. The land absorbs and loses heat faster than the water.   Over land, the heat is distributed over a thin layer, while conduction, convection and currents mix the energy over a fairly thick layer of water. Soil, and the air near it, therefore follow radiation gains more closely than water.   For this reason, continental climates have a wider temperature variation.   We observed this in Chapter 3 by comparing the seasonal cycles of temperatures for different regions of the globe.

The average temperatures of the Northern Hemisphere and Southern Hemisphere for winter, summer and the year.   The Annual Range is give as well as the differences between the Hemispheres.   Differences between the Hemispheres are caused by the differences in the distribution of land and water.

In Earth's present-day climate, the annually-averaged surface air temperature in the Northern Hemisphere (NH) is ? 1.5°C higher than in the Southern Hemisphere (SH). This interhemispheric temperature difference has been known for a long time, and scientists have pondered over its origin for centuries. Frequently suggested causes include differences in seasonal insolation, the larger area of tropical land in the NH, albedo differences between the Earth's polar regions, and northward heat transport by the ocean circulation. Here we systematically assess the origin of the interhemispheric temperature difference. To this end we combine an analysis of climatological data as well as observations of the Earth's energy budget with simulations using a coupled climate model. We find that the interhemispheric temperature difference is predominantly caused by meridional heat transport in the oceans, with an additional contribution from the albedo difference between Antarctica and the Arctic.