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volume were yet more shocking: measurements of ice thickness
showed that there was only a quarter of the Arctic ice in 2012 that
there had been in 1979. The ice that remained was much thinner—
much of it only a skim one season old, by contrast with the abun-
dance of thick, multi-year ice that there used to be.
The decline was not smooth. Some years showed more rapid
declines than others. Some, indeed, showed increases (there has been
one, as we write 101 )—while yet others saw steep plunges in ice areas
and volumes. But the overall trend is clear. Could it be, though, a
longer-term blip? We know that climate and ice amount can vary over
timescales from years to millions of years. As ever, in attempting to
understand the Earth's behaviour, we go back to the past to see
whether the seemingly extraordinary is (geologically) an everyday
occurrence, and thus whether today's melt might only be a prelude to
the regrowth of ice to normal levels in the next few decades.
What does the long-term history of Arctic ice tell us? An array
of evidence has been painstakingly collected from the Arctic
regions. Some evidence comes from the present-day shorelines of
landmasses—Greenland, Arctic Canada and Alaska, Spitzbergen, and
elsewhere. Some has been collected, as borehole cores of deep-sea
sediment layers, from the floors of the Arctic Ocean, where ice-
breakers have accompanied the drilling-ship to allow safe passage.
Some evidence has been collected from the ice itself, as annual snow-
layers have, year by year, piled up on the landmasses adjoining the Arc-
tic Ocean and then been compressed and hardened into ancient ice.
The story is clear enough. 102 Arctic sea ice is ancient. There is evi-
dence of perennial ice from 13 million years ago, when grains of iron
oxide from Siberia began to appear within the sediments accumulat-
ing on the Arctic sea floor. This is a clue that the sea ice had changed
from seasonal to perennial in the Arctic, for these specific mineral
grains could only be plausibly carried so far by being rafted on
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