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Cold temperature inversions actually help warm the Arctic

As the Earth warms, the change seen at high northern latitudes is greater than …

Cold temperature inversions actually help warm the Arctic
Image courtesy of earthobservatory.nasa.gov

When many people think about climate change, their thoughts turn to the Arctic, and for good reason. As the Earth warms, the change seen at high northern latitudes is greater than any other part of the planet—a phenomenon known as Arctic amplification. The process is thought to largely be a result of albedo feedbacks; snow and ice are much more reflective than open ocean or the land surface, so as snow and ice melt away (there's been a well-documented decline in Arctic sea ice), the amount of solar radiation that is absorbed increases dramatically. This acts as a positive feedback—melting ice leads to greater warming, which leads to even more melting ice, etc.

Other research has shown that other factors contribute to this amplification as well, and a paper published this week in Nature Geoscience identifies another that may be important. It’s a little counter-intuitive, but it has to do with atmospheric temperature inversions that actually trap cold air near the surface.

In general, the temperature of the atmosphere decreases with increasing altitude, but there are exceptions. In temperature inversions, warmer air ends up on top of a colder layer. That configuration is inherently stable—cold air doesn’t want to rise, and warm air doesn’t want to sink. This stagnation of atmospheric mixing allows air pollution to accumulate over some cities and create smog.

Temperature inversions can form in several ways. During the Arctic winter, they form because the extreme cold at the surface cools the lower atmosphere, setting up a stable inversion. In order to investigate how this affects Arctic amplification, researchers used a climate model to simulate future trends while manipulating the strength of temperature inversions in the Arctic. They found that weaker inversions (less stable) led to a smaller amplification of warming, while stronger inversions (more stable) resulted in more drastic warming. It appears we can add atmosphere temperature inversions to the list of reasons why the Arctic warms so much more rapidly than the rest of the planet.

It may seem like cold air sitting stagnant on the surface should counteract warming rather than enhance it, but that’s not the case. A well-mixed atmosphere conveys heat from the surface to the top of the atmosphere, where it can be radiated to space. Since temperature inversions create stable, stratified conditions, the efficiency of that heat transport decreases. That means that what little heat is emitted by the cold Arctic surface can accumulate in the lower atmosphere, raising temperatures there. This matches observations of milder Arctic winters.

This information provides one more piece of the puzzle as we keep close watch on changes in the Arctic, where warming can affect the rest of the planet in a number of ways. One prominent example: massive amounts of carbon are locked up in the Arctic permafrost. If the permafrost melts, that carbon will be released as methane (which is around 20 times more potent than CO2 as a greenhouse gas) to the atmosphere. 

It’s only very recently that the capability to simulate this feedback has been added to climate models—model projections in the last Intergovernmental Panel on Climate Change report could not account for it. It’s clear that understanding the climatic behavior of the Arctic is crucial to projecting future warming, so the progress is welcome.

Nature Geoscience, 2011. DOI: 10.1038/NGEO1285  (About DOIs).

Listing image by Image courtesy of earthobservatory.nasa.gov

Channel Ars Technica