Latest Arctic sea ice science
The decline in Arctic sea ice is caused by human activity, researchers have concluded, but there’s also a large role for the Arctic weather in determining the amount of ice that survives each year’s melt season.
That’s the conclusion of two new research papers which examine the issue of how Arctic sea ice is responding to climate change, which continues to attract media coverage and online discussion.
Over the past few weeks, as the sea ice melt season has begun, sea ice extent (a measurement which is roughly equivalent to area) has been higher than it has been for some time – at levels approaching the 1979 – 2000 average. This has given the usual speculation about how the sea ice will behave this year which accompanies the start of the melt season added impetus.
Image - NSIDC Arctic sea ice extent daily 070512 (note)
Sea ice extent neared the average towards the end of April. Source: NSIDC
Arctic ice is never static – every year it grows and shrinks with the seasons. But even with the yearly cycle of ice there are long-term trends which suggest the arctic ice cap is shrinking – over the past several decades both the maximum extent of the ice in winter and the minimum ice extent in summer have shrunk.
Two new studies published last week in the journal Geophysical Research Letters examine the overall trend of below-average sea ice extent, and explore one of the leading factors determining year-to-year variability in sea ice extent.
Long term trends and short term changes
The first of these studies looked at what things are most likely to explain recent observations of sea ice retreat in the Arctic. The authors compiled a record of sea ice behaviour over the last sixty years and tried to map it against different factors influencing the sea ice, to see what was causing the decline. This study was able to discount a combination of internal variability and natural feedbacks in the sea ice system as the cause, and found the most likely explanation for the decline in sea ice over the period to be the near linear rise in carbon dioxide concentrations which happened at the same time.
The second study examines year-on-year changes in the ice, and confirms previous studies which asserted the significant role played by the arctic weather in determining how much ice grows in winter and survives in summer.
The new research finds that anticyclonic areas (high pressure systems of sinking air) are associated with low sea ice extent. The influence of these weather systems was demonstrated clearly in 2007: the summer during which the lowest sea ice extent ever was recorded was characterised by strong anticyclonic conditions. The authors go on to project that, had these same wind patterns materialised in the summers of 2010 and 2011, Arctic sea ice extent would have reached record lows in these years too.
Is the arctic ice cycle changing?
Finally, scientists at the National Snow and Ice Data Centre have suggested an intriguing idea based on their observations of the Arctic sea ice: a shift to the right.
Image - NSIDC Arctic sea ice extent 020412 (note)
Sea ice extent over the last 5 years indicating a slight shift to the right. Source: NSIDC
An emerging shift to the right?
As if mapping the long term trends in sea ice wasn’t complicated enough, scientists are now wondering if the ice seasonal behaviour might be changing as well as the long term pattern of ice extent decline.
The moment when the Arctic ice reaches its maximum for the year varies within a six week period, usually between mid-February and the end of March. As temperatures begin to warm in the Arctic, March is usually a month of overall ice loss. However, in recent years March has seen sea ice continuing to grow, with the largest ice extent has been occurring later on in the season, and scientists are beginning to speculate about why this might be.
Recent analysis from NSIDC says:
“… even with so much variability, there is a small trend towards later maximum ice extents. This year’s maximum ice extent continued that trend, occurring 12 days later than average […]
“It is not clear why the maximum ice extent would happen later, given that in general, ice extent is decreasing. One possibility is that the lower winter ice extents might make it easier for ice to continue growing later in the season. With lower winter extents, a late cold snap or northerly wind could spread ice southward over ocean that would normally be ice-covered at that point.”
If nothing else such theories demonstrate that understanding how the region is changing as the climate changes is a more complicated task than just looking at sea ice extent. But as scientists get to grips with the detailed picture of the changes in the arctic, these most recent studies continue to support the conclusion that Arctic sea ice is in decline due to the changing climate at the top of the planet.