[Vision2020] Arctic Sea Ice "An Icy Retreat, " Dirk Notz, Max-Planck-Institute for Meteorology

[Ted Moffett]

http://www.realclimate.org/index.php/archives/2010/07/an-icy-retreat/#more-4469
 An icy retreat 26 July 2010 -

Guest Commentary by Dirk Notz, MPI
Hamburg<http://www.mpimet.mpg.de/en/mitarbeiter/dirk-notz/>

*Dirk Notz* is head of the research group “Sea ice in the Earth System” at
the Max-Planck-Institute for Meteorology in Hamburg.

It’s almost routine by now: Every summer, many of those interested in
climate change check again and again the latest data on sea-ice evolution in
the Arctic. Such data are for example available on a daily basis from the US
National Snow and Ice Data Center <http://nsidc.org/arcticseaicenews/>. And
again and again in early summer the question arises whether the most recent
trend in sea-ice extent might lead to a new record minimum, with a sea-ice
cover that will be smaller than that in the record summer of 2007.

However, before looking at the possible future evolution of Arctic sea ice
in more detail, it might be a good idea to briefly re-capitulate some events
of the previous winter, because some of those are quite relevant for the
current state of the sea-ice cover. The winter 2009/2010 will be remembered
by many people in Europe (and not only there) as particularly cold, with
lots of snow and ice. Not least because of the sustained cold, some began to
wonder if global warming indeed was real.

Such questioning of global warming based on a regional cold period of course
neglects the crucial difference between weather and climate, with the former
being the only thing that we as individuals will ever be able to experience
first hand. A single regional cold spell has not a lot to do with climate –
let alone with global climate. This becomes quite obvious if one instead
considers the mean temperature of the entire globe during the last 12
months: this period was, according to the GISS data, the warmest 12-month
period since the beginning of the records 130 years ago. Regarding sea ice,
it was particularly important that temperatures in parts of the Arctic were
well above average for most of the winter. This was directly experienced by
some members of our working group during a field experiment at the West
Coast of Greenland.

The initial plan of this field experiment was to study the growth and decay
of sea ice in great detail throughout an entire winter. In particular, we
wanted to focus on the evolution of very young sea ice that had just formed
from open water. Therefore, we wanted to start our measurements just before
initial ice formation, which usually takes place in mid-November, at least
according to past experience of the local Greenlandic population. Hence, we
traveled to our measuring site close to the Greenlandic settlement of
Upernavik in early November to put out our measuring buoys. We were hoping
that ice formation would start shortly after we had put out the instruments
such that they were protected from storms and waves. However, with
temperatures that were often more than 10°C above the long-term mean, sea
ice was nowhere to be seen. Even in January, there were days on end with
above 0°C temperature and heavy rain fall. Finally, in February a stable ice
cover formed, which of course remained relatively thin and which hence had
melted completely by mid May.

The fact that it was sometimes warmer at our measurement site at the West
Coast of Greenland than it was in Central Europe at the same time surprised
us quite a bit. However, some recent studies indicate that such a
distribution of relatively high temperature in parts of the Arctic and
relatively low temperature in Northern and Central Europe and parts of the
US might become somewhat more wide-spread in the future. While the Arctic
has always shown large internal variability that lead to large-scale shifts
in weather patterns, in the future the ongoing retreat of Arctic sea ice
might cause those weather patterns to occur more often that allow for
Northerly winds to bring cold air from the Arctic to the mid-latitudes.
Hence, it is quite possible that because of the retreat of Arctic sea ice,
some smaller parts of the Northern Hemisphere will experience pronounced
cold spells during winter every now and then. The mean temperature of the
Northern Hemisphere will nevertheless increase further, and the export of
cold air from the Arctic of course leads to warm anomalies there.

But let’s return to the evolution of Arctic sea ice. Because of relatively
high temperatures, Arctic sea-ice extent remained well below the long-term
mean for most of the preceding winter. However, in March temperatures
suddenly dropped for a couple of weeks, in particular in parts of the
Barents Sea and in parts of the Beaufort Sea. This in turn lead to the
formation of a thin ice cover in these regions, which caused a marked
increase in observed sea-ice extent. For the measurement of this extent, it
doesn’t matter at all how thick the ice is: any ice, however thin,
contributes to sea-ice extent. Therefore, only considering a possible
“recovery” of just the *extent *of Arctic sea ice always remains somewhat
superficial, since sea-ice extent contains no information on the thickness
of the ice. A much more useful measure for the state of Arctic sea ice is
therefore the total sea-ice volume. However, for its estimation one
additionally requires information on the overall distribution of ice
thickness, which we have not been able to measure routinely in the past.
While this will hopefully change in the future because of the successful
launch of the Cryosat 2 satellite a couple of weeks ago, at the moment we
unfortunately must rely on judging the current state of the Arctic sea-ice
cover mostly by its extent.

Because of the very low thickness of much of the Arctic sea ice, it wasn’t
too surprising that at the end of the winter, sea-ice extent decreased
rapidly. This rapid loss lead up to the lowest June sea-ice extent
<http://nsidc.org/arcticseaicenews/2010/070610.html>since the beginning of
reliable observations. After this rapid loss of the very thin ice that had
formed late in winter, the retreat slowed down substantially but the ice
extent remained well below the long-term mean. Currently, the ice covers an
area that is slightly larger than the extent in late July of the record year
2007. However, this does not really allow for any reliable projections
regarding the future evolution of Arctic sea ice in the weeks to come.

The reason for this is mostly that sea ice in the Arctic has become very
thin. Hence, in contrast to the much thicker ice of past decades, the ice
now reacts very quickly and very sensitively to the weather
patterns<http://nsidc.org/arcticseaicenews/2010/072010.html>that are
predominant during a certain summer. This currently limits the
predictability of sea-ice extent significantly. For example, in 2007 a
relatively stable high-pressure system formed above the Beaufort sea,
towards the north of North America, leading to rapid melting of sea ice
there. If again such stable high pressure system forms in the Arctic
throughout the coming weeks, we might well experience a sea-ice minimum that
is below the record minimum as observed in 2007. However, if the summer
should turn out to be colder than during the previous years, a sea-ice
minimum similar to that observed in 2009 would not be too surprising. Hence,
at the moment all that remains is to wait – and to check again and again the
latest data of Arctic sea-ice extent.

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Vision2020 Post: Ted Moffett
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