Modelling high-latitude greenhouse warming

A focus point for the research of the centre is the so-called polar amplification, i.e., the effect that when climate warms or cools, the changes are generally much more drastic at high latitudes than they are closer to equator. Temperatures measured in ice core bore holes in Greenland show, for instance, that central Greenland temperatures during the Last Glacial Maximum were about 20 degrees lower than today while the tropics were only a couple of degrees cooler. Computer simulations of how climate will evolve during the 21st century demonstrate that the Arctic climate changes will be both faster and greater than at lower latitudes. This effect is active throughout the Arctic and affects both sea ice in the Arctic Ocean and permafrost in Arctic land areas.

Temperature evolution during the 21st century according to 5 climate models. The thick lines are global mean values while the thin lines denote Arctic means (60°-90°N). All models project a significantly amplified warming in the Arctic.

Arctic climate feedbacks

An important cause of this effect has to do with the local energy budget: When climate warms, ice and snow melts away and much of the solar energy previously reflected by these bright surfaces will be absorbed, contributing to an amplification of the warming. Such an effect, where a warming yields a change in another part of the climate system (the ice in this case), which then responds by enhancing or reducing the initial warming, is called a feedback mechanism. There is a host of Arctic feedbacks, involving not only physical but also chemical and biological components. A warming may, for example, lead to new vegetation, not previously native to a certain latitude, gradually replacing the existing vegetation. This can lead to changes in the reflection of sunlight at the surface and changes in the exchange of heat or water vapor between land and atmosphere and, in turn, to further climate changes.

Atmospheric transport effects

It is, however, not only such local high-latitude effects that contribute to polar amplification of climate change. Also heat transport in the atmosphere and oceans from low to high latitudes will change when climate changes, thereby leading to differences in the warming. As long as equatorial areas are warmer than polar areas, the atmosphere will try to reduce this difference by “mixing” the air masses. This yields wave motions in the atmosphere, seen at the ground as the passing highs and lows that give us changes in weather from week to week. If Earth warms, the atmospheric content of water vapor will increase, and the waves will transport more of it polewards. In atmospheric energetics, water vapor is a form of energy, and an increased poleward energy transport will both dampen the warming near the equator and amplify it near the poles. This mechanism of communication between low and high latitudes is one we have worked with intensively at the centre using both very simple models and GCMs.

Read the paper "Polar amplification as a preferred response in an idealized aquaplanet GCM" or contact Peter L. Langen. (The original publication is available at - login needed)