Saturday round-up: All quiet on the northern seas

The Arctic may become more serene, as a slowly warming sea stabilises atmospheric circulation, reducing the frequency of "polar lows", with the progress of climate change. Image © Science/AAAS

Highly-waterproofed sailors used to fighting howling winds and avoiding icebergs in the world’s northernmost oceans are set to have their struggles eased by climate change. That’s because, not only is Arctic ice area decreasing, but severe North Atlantic storms will decrease in frequency as the world warms.

These storms are small and therefore difficult to observe, Matthias Zahn and Hans von Storch from Germany’s GKSS Research Centre write in top journal Nature this week, but still dangerous. “Accompanied by strong winds and heavy precipitation, these often explosively developing cyclones – termed polar lows – constitute a threat to offshore activities such as shipping or oil and gas exploitation,” they say.

Polar lows, or ‘Arctic hurricanes’, start as low level airflows that are built up by air circulation caused by large temperature differences between different levels of the atmosphere. They occur every winter, and while there are weaknesses in direct historical measurement, a previous simulation based on climate data from 1948–2006 showed an average of 56 polar lows per year.

Zahn and von Storch compared these simulations against what data was available, and found they at least were “generating realistic statistics of polar lows”. Consequently they used a similar method, and combined them with scenarios of how the the world will change in the future produced by the Intergovernmental Panel on Climate Change.

In the A2 scenario, where economic growth per person and technological change are fragmented and comparatively slow, they find an average of 17.6 polar lows per winter for the period 2070-2099. In the A1B scenario, which describes rapid economic growth and a balanced use of energy sources, this figure falls to 16.9. The B1 scenario, which describes a similar world to the A1B scenario, but with less focus on industry and more on clean technology, would see 22.7 polar lows per season.

This fall in polar storms from the 20^th century is a distinct contrast from much of the rest of the world, which is predicted to see an increased frequency of extreme weather events like heatwaves or intense precipitation caused warmer climate. “Our results provide a rare example of a climate change effect in which a type of extreme weather is likely to decrease, rather than increase,” von Storch and Zahn write.

 

The German scientists attribute the fall to slower circulation in the atmosphere, caused by a fall in the difference between the temperatures in the sea and the lowest level of the atmosphere, known as the troposphere. “The North Atlantic is expected to experience a comparatively moderate warming, whereas tropospheric air temperature is expected to warm more quickly than the global average air temperature in the Arctic,” they write. “This tendency of a more slowly warming ocean leads to increased vertical stability, which is less favourable for the formation of polar lows.”

 

The 2010 minimum sea ice extent, shown in white, compared to the 1979-2000 median average, shown by the orange outline. Credit: National Snow and Ice Data Center, Boulder.

Shipping in the Arctic also continues to get easier because of the falling area covered by ice in summer. The ice area looks to have reached its third lowest ever minimum this month at 4.76 million square kilometres, higher only than 2008 and the record low year 2007. Although 2009 saw 340,000 square kilometres more ice at its minimum than this year, 2008 and 2007 had 240,000 and 630,000 square kilometres less ice, respectively. “We are still looking at summers with an ice-free Arctic Ocean in perhaps 20 to 30 years,” said Mark Serreze, director of University of Colorado at Boulder’s National Snow and Ice Data Center. You can track the progress of Arctic ice coverage at this link.

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