Climate change impacts and vulnerability in Europe 2016

Changes in the climate system
3.3.2 Arctic and Baltic sea ice
Key messages
• The extent and volume of the Arctic sea ice has declined rapidly since global data became available, especially in summer.
Over the period 1979–2015, the Arctic has lost, on average, 42 000 km 2 of sea ice per year in winter and 89 000 km 2 per
year by the end of summer.
• The nine lowest Arctic sea ice minima since records began in 1979 have been the September ice cover in each of the last
nine years (2007–2015); the record low Arctic sea ice cover in September 2012 was roughly half the average minimum
extent of 1981–2010. The annual maximum ice cover in March 2015 and March 2016 were the lowest on record, and the
ice is also getting thinner.
• The maximum sea ice extent in the Baltic Sea shows a decreasing trend since about 1800. The decrease appears to have
accelerated since the 1980s, but the interannual variability is large.
• Arctic sea ice is projected to continue to shrink and thin. For high greenhouse gas emissions scenarios, a nearly ice-free
Arctic Ocean in September is likely before mid-century. There will still be substantial ice in winter.
• Baltic Sea ice, in particular the extent of the maximal cover, is projected to continue to shrink.
Relevance
Observed changes in the extent of Arctic sea ice provide
evidence of global warming. Reduced polar sea ice will
speed up global warming further (Hudson, 2011) and
several studies have also suggested causal links between
the sea ice decline and summer precipitation in Europe,
the Mediterranean and East Asia (Simmonds and
Govekar, 2014; Vihma, 2014). Reduced Arctic ice cover
may also lead to increases in heavy snowfall in Europe
during early winter (Liu et al., 2012).
The projected loss of sea ice may offer new economic
opportunities for oil and gas exploration, shipping,
tourism and some types of fisheries Most of these
activities would increase the pressure on, and the risks
to, the Arctic environment.
Past trends
In the period 1979–2015, the sea ice extent in the Arctic
decreased by 42 000 km 2 per year in winter (measured
in March) and by 89 000 km 2 per year in summer
(measured in September) (Figure 3.8), which, based
on historical records, is likely unprecedented since the
14th century (Halfar et al., 2013). The maximum sea ice
extent in March 2015 and March 2016 were the lowest
on record. Arctic sea ice loss is driven by a combination
of warmer ocean waters and a warmer atmosphere,
including an earlier onset of summer surface melt
(Collins et al., 2013). In contrast, Antarctic sea ice has
reached record high levels in recent years, but the
expansion of the Antarctic sea ice has been less than half
of the loss of Arctic sea ice (Parkinson, 2014).
Changes in Arctic sea ice may trigger complex feedback
processes. Warming and a longer melt season result
in increased solar heat uptake by the ocean, which
delays the autumn refreeze (Stammerjohn et al., 2012).
However, a warmer atmosphere means that there are
more clouds and, in summer, these reflect sunlight,
thus representing a negative feedback mechanism.
Even so, some evidence suggests that winter regrowth
of ice is inhibited by the warmer ocean surface (Jackson
et al., 2012). Thinner winter ice leads to more heat loss
from the ocean and a warmer atmosphere, and hence
thicker cloud cover, which inhibits the escape of heat to
space (Palm et al., 2010), which is a positive feedback
mechanism.
The minimum Arctic sea ice cover at the end of the
melt season in September 2012 broke all previously
observed records. All years since 2002 have been below
the average for 1981–2010 (Figure 3.8). Comparison
of recent sea ice coverage with older ship and aircraft
observations suggests that summer sea ice coverage
may have halved since the 1950s (Meier et al., 2007).
Since more reliable satellite observations started in 1979,
summer ice has shrunk by 10 % per decade (Comiso
et al., 2008; Killie and Lavergne, 2011). Between 1979 and
2011, the reduction of sea ice has significantly reduced
albedo, corresponding to an additional 6.4 ± 0.9 W/m 2
of solar energy input into the Arctic Ocean region since
1979. Averaged over the globe, this albedo decrease
corresponds to a forcing that is 25 % of that due to the
change in CO 2 during this period (Pistone et al., 2014).
The Arctic sea ice is also getting thinner and younger,
as less sea ice survives the summer to grow into
92 Climate change, impacts and vulnerability in Europe 2016 | An indicator-based report