My latest Mind and Matter column in the Wall
Street Journal is about the retreat of Arctic Sea Ice and what it
means:
This week probably saw the Arctic Ocean’s sea ice reach its
minimum extent for the year and begin to expand again, as it
usually does in mid-September. Given that the retreat of Arctic ice
has become a key piece of evidence for those who take a more
alarmed view of global warming, it’s newsworthy that 2012’s melt
was the greatest since records began in 1979, with sea ice in the
Northern Hemisphere shrinking to about 1.3 million square miles, or
about half the 1979-2008 average.
As this column has sometimes pointed out ways in which the
effects of global warming are happening more slowly than predicted,
it is fair to record that this rate of decline in Arctic sea ice is
faster than many predicted. Although an entirely ice-free Arctic
Ocean during at least one week a year is still several decades away
at this rate, we are halfway there after just three decades.
Arctic melts on this scale have happened before, however. Svend
Funder of the Danish Museum of Natural History and his colleagues
recently studied the northern coast of Greenland, where
the land-fast sea ice never breaks up, even in a year like this.
Yet evidence of wave action in the past (indicating open waters)
and waterlogged driftwood show that for 2,500 years in the
“Holocene Optimum” period, when Arctic summer temperatures were two
to four degrees Celsius warmer than today, the summer melt of the
Arctic Ocean routinely left half as much ice as this year. [A quote
from Funder’s paper: “Multiyear sea ice reached a minimum between
~8500 and 6000 years ago, when the limit of year-round sea ice at
the coast of Greenland was located ~1000 kilometers to the north of
its present position.” hat tip, see here]
Another study, by Jørgen Berge and colleagues from the
University Centre in Svalbard, Norway, and other institutions, discovered a downward migration of
egg-carrying amphipod crustaceans that enables them to recolonize
Arctic ice from ice-free areas using deep currents. They say this
implies that some animals are well adapted to the seasonal loss of
ice. [“From an evolutionary perspective, this may have been a
successful adaptive strategy in a more seasonally ice-covered
Arctic, as experienced several times during the past 12,000 years,
some reports indicate that the Arctic Ocean was void of summer sea
ice as late as 8000 years ago.”]
In the Holocene Optimum there was no collapse of the polar-bear
population or “point of no return.” The extent of Arctic summer sea
ice then increased steadily, reaching a maximum during the very
recent so-called Little Ice Age of 1500-1850. Potential
confirmation that this was an unusually icy epoch comes from a
newly published study by Durham University (in Britain) of the
genetics of Arctic foxes on Iceland.
Greger Larson and his colleagues found that
the remains of 17 Arctic foxes in Iceland from the ninth to the
12th centuries shared a single genetic signature, while the modern
Icelandic fox population has five different genetic types. During
the cold centuries, they infer, genetically diverse Arctic foxes
from the Eurasian continent apparently reached Iceland via sea
ice.
Anecdotal evidence suggests that the extent of summer Arctic sea
ice then shrank after 1850, before expanding in the 1960s. Clearly,
the Arctic Ocean’s sea ice is both more variable and more
vulnerable to warming than expected. But is the current rapid
retreat caused only by warming? At least some of it might be caused
by soot from dirty, coal-fired power stations. Some scientists have
noticed that the decline in Arctic sea ice
correlates better with the rapid growth of coal consumption in
China than it does with global temperature. As the argument goes:
Soot falling on white ice darkens it, which results in faster
melting in summer sun.
Correlation does not always mean causation, but if soot is
contributing to sea-ice melt, then it is moderately good news,
because cleaning up soot emissions from power stations could be
both cheaper and quicker than cutting carbon-dioxide emissions.
There’s also the puzzling fact that Antarctic sea ice shows no sign of summer retreat, and the
current winter’s peak extent is well above average. The
sea-dominated Southern Hemisphere is certainly warming more slowly
than the land-dominated Northern Hemisphere, but it has still been
warming. If warming is supposed to be “global,” shouldn’t sea ice
retreat at both ends of the world? [The models cannot
account for the difference: see here: “The negative SIE trends in most of the
model runs over 1979 – 2005 are a continuation of an earlier
decline, suggesting that the processes responsible for the observed
increase over the last 30 years are not being simulated
correctly.”]