Another feedback - a "layered cake" atmosphere

Researchers find temperature feedback magnifying climate warming in Arctic

Physics,

3 February, 2014

A team of researchers with the Max Planck Institute in Germany, has found that temperature feedback in the Arctic is causing more warming in that region than sea ice albedo. In their paper published in the journal Nature Geoscience, the team describes how plugging data into a computer simulation revealed a "layered cake" atmosphere that traps heat over the polar cap.

Scientists have known for several years that temperatures in the Arctic are rising faster (due to  global warming) than for the rest of the planet—for the most part, most climatologists have attributed this to sea ice albedo—a feedback system where a small rise in temperature leads to melting of ice and snow. Less ice and snow means less heat is reflected back into space, which means more warming occurs, and so on. In this new effort, the researchers suggest that while sea ice albedo is causing temperatures to rise, it's second to temperature feedback in overall impact.

To gain a better perspective on why Arctic temperatures are increasing so much, the researchers turned to highly sophisticated and data intensive climate computer models. Their model showed a cap of cold layered air hovering over the Arctic, holding in the heat. The researchers believe their simulation accurately portrays what actually exists in the real Arctic.

Normally, they explain, changing weather patterns (such as thunderstorms) in other parts of the world keep atmospheric air churning, which in turn allows heat closer to the ground to be moved higher, allowing some of it to escape into space. Things are very different in the Arctic—there is very little churning, which means that warm air close to ground (just one to two kilometers thick) remains where it is, trapped by a heavy layered atmosphere.

The simulation also helps to explain why Arctic warming is more pronounced in the winter than during other seasons—even less mixing of the air in the atmosphere occurs because the air is so cold.

The team reports that their simulations show that the temperature feedback that occurs in the Arctic is causing more average temperature increase than sea ice albedo, the second most critical factor in causing warming. They have not used their findings to try to predict what sort of overall impact increasing Arctic temperatures might have on the rest of the planet, however, if the polar cap will melt completely, or if it does, when it might occur.

Explore further: Three-decade decline in reflectivity of Arctic sea ice

More information: Arctic amplification dominated by temperature feedbacks in contemporary climate models, Nature Geoscience (2014) DOI: 10.1038/ngeo2071

Abstract
Climate change is amplified in the Arctic region. Arctic amplification has been found in past warm and glacial periods, as well as in historical observations and climate model experiments. Feedback effects associated with temperature, water vapour and clouds have been suggested to contribute to amplified warming in the Arctic, but the surface albedo feedback—the increase in surface absorption of solar radiation when snow and ice retreat—is often cited as the main contributor. However, Arctic amplification is also found in models without changes in snow and ice cover. Here we analyse climate model simulations from the Coupled Model Intercomparison Project Phase 5 archive to quantify the contributions of the various feedbacks. We find that in the simulations, the largest contribution to Arctic amplification comes from a temperature feedbacks: as the surface warms, more energy is radiated back to space in low latitudes, compared with the Arctic. This effect can be attributed to both the different vertical structure of the warming in high and low latitudes, and a smaller increase in emitted blackbody radiation per unit warming at colder temperatures. We find that the surface albedo feedback is the second main contributor to Arctic amplification and that other contributions are substantially smaller or even opposeArctic amplification.

Journal reference: Nature Geoscience 

Alaska's Arctic icy lakes lose thickness

The ubiquitous shallow icy lakes that dominate Alaska's Arctic coastal plain have undergone a significant change in recent decades.

BBC,

3 February, 2014

These lakes, many of which are no more than 3m deep, melt earlier in the season and retain open water conditions for much longer.

And 20 years of satellite radar also now show that far fewer will freeze right through to the bottom in winter.

The results of the space-borne survey are published in The Cryosphere.

What is happening to the lakes is an example of how land ice is following the pattern of diminishing sea ice in the region, say scientists.

"The decline after 2006 is quite sharp," explained Dr Cristina Surdu from the University of Waterloo, Ontario, Canada. "This is another piece in the puzzle of climate change in the region.

"We're seeing warmer air temperatures; we're seeing sea-ice extent decreasing; and we're seeing a general greening of the Arctic with the treeline moving north. The lakes are part of that story."

Surdu's and colleagues' research focussed on an area near Barrow, the largest settlement on Alaska's North Slope.

It encompassed more than 400 lakes that individually have surface areas up to 60 sq km but which are about, in the main, just 1.5m deep.

The lakes tend to be completely ice free in summer for a short period of roughly 8-10 weeks and then freeze up again with the onset of colder temperatures in Autumn.

The team used radar data from Europe's ERS satellites in the 1990s and 2000s to track the progression of the freeze-up.

Orbital radar can sense the presence of liquid water on the ground even when it is covered by tens of centimetres of ice. The energy of the signal scattered back to the satellites is significantly different when some water is retained.

And what the scientists could see over the course of the study period was that a smaller fraction of the lakes' ice was freezing right through to the bed in winter.

From 1991 to 2011, the fraction of grounded ice present by the end of the freeze-up in April had declined by 22%. Modelling work suggests this is equivalent to the lakes' ice caps being reduced in thickness by 18-22cm.

From 1992 to 2011, the studied lakes experienced a 22% reduction in grounded ice (dark blue)

The change in the lakes' behaviour almost certainly reflects the warmer conditions in the region. The mean air temperature in Barrow, for example, increased by 1.7C in the first decade of the 21st Century. But it also reflects shifts in precipitation.

"Snow is a very important factor in all this because it is an insulator," explained Dr Surdu.

"If it falls at the beginning of the ice season, it slows down the thickening of the ice on these lakes; whereas, if it falls at the end of the ice season, it helps retain the ice because it insulates that ice from warming temperatures.

"But what we're actually seeing is more snowfall at the beginning of the ice season and so the precipitation is working against the ice."

The presence of more liquid water underneath the lake ice is likely to have a number of impacts. One is to change the lakes' ecology.

For people, some impacts, such as the availability of more fresh water, will be beneficial. But other impacts will cause difficulties. An example would be the "ice roads" driven across the lakes in winter by the trucks that supply Barrow and neighbouring communities on the coastal plain. Continued thinning could limit the roads' use in future.

The warmer conditions in the lakes also have important climate feedbacks by disturbing the underlying permafrost and transferring more heat back into the atmosphere in autumn. These effects work to amplify the changes already under way.

The study used European Space Agency satellites that are no longer in operation. But Esa is about to launch new radar spacecraft under the EU's Copernicus-Sentinel programme.

With the promise of two identical radar satellites flying at the same time, it should make studies such as this one far easy to conduct. The Sentinels will return more data more frequently.