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persistent multi-year ice. Since that time, this surface ice layer has
been highly sensitive to climate change, waxing and waning as the
Earth has successively cooled and warmed. During the coldest phases
of the Ice Ages of the last 3 million years it may have become a solid
armour, perhaps reaching several hundred metres thick. When the
warm interglacials came, it shrank back, at times virtually disappear-
ing altogether—as it seems to have done in the warmest part of the
last interglacial phase, some 125,000 years ago.
The history of Arctic ice in the current, Holocene, interglacial phase
may be tracked, for instance, by searching for bones of bowhead
whales along Arctic coastlines, because these follow the sea-ice front
to escape their main predators, the killer whales (although it is a risky
tactic, for whole schools of bowheads can be trapped and killed by
enveloping ice if they linger too long as winter sets in). Such evidence
shows that the early part of this interglacial was a little warmer than
present in the Arctic, with reduced ice cover. Since then, the ice (and
the bowhead whales, and the Inuit who hunted them) has been spread-
ing back southwards.
This trend has now been sharply reversed, and the current rapid
shrinking of the ice has no parallel in the last few thousand years. It is
a striking anomaly, one of the clearest indications of human modifi-
cation of climate—and it includes a strong reinforcing mechanism,
for open water absorbs more of the Sun's heat than reflective ice,
ratcheting temperatures yet higher. The computer models are being
rapidly revised, and we can expect an ice-free Arctic Ocean in sum-
mer by mid-century, perhaps earlier.
As carbon dioxide levels continue their inexorable upward trend, it
is becoming more likely that the Arctic ice, once lost, will not return
for thousands of years, and will be a crucial part of a permanently
(as far as we are concerned) warmer ocean system. What will be
the effects?
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