Latest Arctic sea ice science
- 09 May 2012, 11:36
- Freya Roberts
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.
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.
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.