The National Snow and Ice Data Center has recently
released its data set of Arctic sea ice coverage for the 2016 Northern
Hemisphere Summer. These figures show that on 10th September 2016
Arctic sea ice reached the second lowest annual extent ever recorded.
The release of this data is a greatly anticipated metric for the health of the
Arctic region for a current year and is one of the most widely published pieces
of research on the Arctic. Evidence of a further decline in Arctic sea ice has received
global coverage in a variety of formats, including reports in national
newspapers, e.g. The Washington Post and The Guardian, and reanalyses in climate blogs. Climatic impacts relating to Arctic sea ice
are varied, including an important albedo feedback mechanism. Hence it is vital
this issue is reported and interpreted appropriately.
What the data tells us: The headline statistic is the minimum
extent of sea ice in the Arctic Ocean. Sea ice extent is defined as: total area
of the Arctic Ocean with at least 15% sea ice coverage – hence it is measured
in units of area (km2).
The NSIDC, in collaboration with NASA, use microwave measurements from the
Defense Meteorological Satellite Program (DMDP). This data is ongoing since
November 1978.
It is typically used to compare
the minimum daily value recorded each year which is found at the end of the
summer melt season. Hence the annual minima are normally reached in September,
before sea ice growth recovers over the winter. On 10th September,
Arctic sea ice extent reached a daily value of 4.14 million square kilometres –
the lowest value for 2016. As Figure 1 below shows, this is the second lowest
annual minima recorded (2016 is represented by the blue line). Only in one other year has there been a lower value of minimum Arctic sea ice extent – in
2012 where it reached 3.39 million square kilometres.
This graph indicates the decline of Arctic sea ice extent, as the last five
years have all been significantly lower than the 1981 – 2010 average, displaying
a decline beyond the scope of natural variability.
A further trend revealed in the
2016 data is the slow growth of Arctic sea ice during October. As Figure 1
shows, the most recent observations of Arctic sea ice extent are currently at
the lowest ever recorded value for this time of year - indeed an extent of less
than 2012. This indicates anomalously slow growth of sea ice over October.
Longer term (Anthropocene) trends: It is important to put this data
in context of longer term trends of Arctic sea ice extent. In 2013, the IPCC report synthesised research on this subject, with emphasis on the period of
satellite observations (i.e. since 1978) to clarify the trend. When analysed on
a decadal timescale (Figure 2), the results show an increasing decline in
summer sea ice extent over time, although there is a much less pronounced
change in winter maximum extent. The 2016 daily minimum falls within the
uncertainty values for the most recent three years represented in the figure
(2009 – 2012, black line on Figure 2), indicating a continuation of this observed
trend throughout the Anthropocene.
Figure 2, source: 2013 IPCC WG1 (pp.235), Figure 4.2. This shows “decadal averages of sea ice extent in the
Arctic”.
Analysis of Arctic sea ice extent
satellite observations can also be undertaken, at a higher temporal resolution:
the annual scale, as shown in Figure 3. This figure is also taken form the 2013
IPCC report (updated from Cosimo and Nishio 2008), and represents changes in sea ice extent
as annual anomalies from the November 1978 to December 2012 average. This graph
shows that, despite interannual variability, there is a clear decline in annual
Arctic sea ice extent.
In the IPCC report this annual data
is fitted with red linear regression line (Figure 3). This results in a
linear decline in Arctic sea ice extent of 3.8 ± 0.3 % per decade. However, as
Figure 1 also shows, observations indicate that Arctic sea ice is not declining
at a linear rate on this timescale. In fact, Arctic sea ice extent appears to
be declining by a greater amount each decade. Hence a linear analysis may
appear inappropriate.
Figure 3, source: 2013 IPCC WG1 pp. 331. This
shows “anomalies in Arctic sea ice extent from satellite passive microwave observations”.
Sea Ice Albedo Feedback: Importance of Arctic sea ice extent is
frequently referred to in relation to the Sea Ice – Albedo feedback mechanism. Hence
I will offer a brief explanation of the mechanisms of this widely established feedback.
Theory suggests that changes in the Earth’s surface albedo (i.e. “the fraction
of solar radiation reflected by a surface or object” IPCC 2013 pp. 1448) will cause a resultant change in surface temperature. Sea ice has a
significantly higher albedo than ocean water, hence a decrease in sea ice
extent will theoretically result in an increase of average Arctic surface
albedo and a subsequent increase of short wave radiation absorbed in the Arctic
region. This will drive an increase in temperature and further loss of sea ice,
completing the positive feedback. This positive feedback has been demonstrated in
climate model experiments and is widely believed to be a fundamental feedback in the global climate system.
Conclusion: To conclude this post, recent data shows furthering of
a well observed pattern of climate change during the Anthropocene – a decline
in Arctic sea ice extent. One of the mechanisms driving this loss is a positive
albedo feedback. The decline of Arctic sea ice extent appears to be increasingly
nonlinear, possibly as a result of this feedback mechanism.
Interesting Links: Here are some links to other blogs which have
covered the release of Arctic sea ice data:
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