Arctic sea ice low – what does it really mean?
- 13 Sep 2011, 17:00
- Verity Payne
Arctic sea ice has
hit the headlines over the last week. Last week the Polar Science
Center of the Applied Physics Laboratory at the University of
Washington found that Arctic sea ice volume has reached a new record
minimum in 2010. This week, following an analysis of satellite
data, researchers at the University
of Bremen announced that Arctic sea ice extent has reached a
'historic low' in 2011. Meanwhile, the National Snow and Ice
Data Center (NSIDC), who normally release details of Arctic sea
ice minima, has yet to determine that this year's summer minimum
Arctic sea ice extent has been reached.
All of these different sources and claims can get pretty
confusing. Just how do these different groups measure sea ice
coverage? And why are they not all saying the same thing at the
same time?
Arctic populations have been determining sea ice coverage
visually for many years, so there are some some written records of
ice extent over the last century. However, it is only since the
launch of microwave energy-detecting satellites in the 1970s that
scientists have been able to reliably quantify sea ice coverage.
The satellites detect changes in microwaves emitted from the
planet's surface. Ice, with its crystalline structure, emits more
microwave energy than open seawater, so scientists can determine
sea ice versus ocean coverage from the microwave data.
Some confusion arises from how the researchers present their
data. Sometimes we hear about Arctic sea iceextent, other times sea
icevolume, and sometimes even sea icethickness. It is important to
know which of these parameters is being discussed.
We have explained a bit about the difference between sea ice
volume and sea ice extent in this blog. Here, we look at the
question in a bit more detail.
The parameter that we tend to hear most about is Arctic sea ice
extent - this is a measurement of the area of Arctic Ocean with at
least 15% sea ice, and is determined from the satellite
measurements discussed above. The National Snow and Ice
Data Center (NSIDC) and IJIS
(the International Arctic Research Center, working in co-operation
with the Japan Aerospace Exploration Agency) post regular updates
on Arctic sea ice extent.
The differences between results from separate research groups
can arise as the groups may measure the microwaves using different
frequency microwave sensors, and can reconstruct the ice coverage
using different algorithms. This is why the sea ice reconstructions
from researchers at the University of Bremen and the NSIDC differ
slightly. The Bremen researchers use data from sensors that can
study smaller area, but are more susceptible to interference
melting or storm activity. The NSIDC sensors pick up a larger area,
so are less detailed, but are not so susceptible to the effects of
melting and storm activity. The two techniques do give
good agreement though, and tend to show the same overall
trends.
The change in Arctic sea ice extent, 1979-2010. The graph
shows the ice extent in March and September, as compared to the
average sea ice extent for the whole period (the grey dotted line).
Source:
CryoClim
Arctic sea ice volume features less frequently in the news, but
is another important parameter. The volume of sea ice is obviously
more difficult to assess than the extent of sea ice, but it can be
determined in two ways. It is either generated by combining
computer modelling and the sea ice coverage determined by microwave
satellite data (Pan-Arctic Ice Ocean Modeling and Assimilation
System, PIOMAS - at the Polar Science Center), or more directly
from altimetry measurements made by the CRYOSAT-2
satellite launched in 2010. The Polar Science Center posts regular
updates on sea ice volume
here.
Arctic sea ice volume anomaly from PIOMAS updated once a
month. Daily Sea Ice volume anomalies for each day are computed
relative to the 1979 to 2010 average for that day of the year. The
trend for the period 1979- present is shown in blue. Shaded
areas show one and two standard deviations from the trend. Error
bars indicate the uncertainty of the monthly anomaly plotted
once per year. Source:
Polar Science Center
Arctic sea ice thickness can also be determined using the CRYOSAT-2
satellite, or from the PIOMAS
combination of modelling and microwave satellite
reconstructions.
Although confusing, the fact that different research groups use
different means of measuring the state of Arctic sea ice and may
thus make slightly different assessments of this year's summer ice
coverage minimum at slightly different times is relatively
unimportant. The key point is that all of these different
techniques show the same scenario - Arctic sea ice is declining,
and that natural variation alone cannot explain it. Around half of
the decline in Arctic sea ice since 1979 can be
attributed to increases in greenhouse gases.
The media tend to focus on the Arctic sea ice minimum in
September, and whether it has broken any records. However, this is
misleading and draws attention away from more important phenomena.
For example, as Real Climate highlighted in a recent
blogpost, it is the major loss of Arctic sea ice during the
early summer months of this year that will have the biggest
climatic impact. Early summer ice-loss prevents the Arctic from
remaining cool throughout summer, since the reflective ice is no
longer present, allowing the sun to reach the open seawater and
heat it. The warm seawater could delay the formation of ice during
Autumn.
The early summer sea ice loss did, however, lead to much
speculation in the blogosphere about the approaching September
minimum, and whether it would break the record set by 2007's
abnormally low sea ice extent. The unusual low of 2007 has since
been ascribed to a very specific set of weather conditions, which
exacerbated the increase in melting associated with global warming.
Interestingly, as pointed out in an interesting
blogpost at Climate Progress, similar weather conditions were
present at the start of this summer, but the weather has not been
so conducive to melting towards the end of summer, yet the Arctic
sea ice extent has continued to hover around similar levels to
those seen in 2007.