Geoscience Reference
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below the icecap surface, what was a continuous network of air
passages closes off into myriad tiny air bubbles that, from then on
(and no matter how much more compression takes place) are simply
locked into the ice.
Drill into that ice, as teams of scientists have done in both Green-
land and Antarctica (and in other ice masses too), and one obtains a
record of air that reaches back almost 870,000 years (with an aim to
try to obtain air a million years old) from beneath the centre of Ant-
arctica. That is possible—but about the limit, because the ice is not
permanent. It flows under its own weight out to the edge of the con-
tinent to break into icebergs, which then melt, releasing (with faintly
audible 'pops') air from the time of the mammoths and Neanderthals
back into the atmosphere.
The fossil record of air reveals an even more remarkable stability—
or at least a stable oscillation between two modes. About 180 parts
per million (ppm) in glacial phases, and about 280 ppm in interglacial
phases. Even at these longer timescales, the Earth's carbon regulation
systems is—again, when left to itself—a very finely regulated
machine.
Now of course that machine has been pushed, with unprecedented
speed, into a new gear. There is an iconic curve that shows the climb
of carbon dioxide in the atmosphere since 1958, painstakingly and
determinedly produced by the late, remarkable scientist Charles Keel-
ing (a singularly stubborn man, to carry on taking measurements,
year by year, that others originally told him were valueless). It starts at
under 320 ppm and now, not much more than half a century on,
stands at 400 ppm. To see the effect in proper—that is properly
geological—context, though, it needs to be grafted on to the almost-
million-year ice core record. At this scale it is a cliff: a straight vertical
line terminating now in an atmosphere the Earth last experienced in
the Pliocene Epoch, before the Quaternary Ice Ages. It is, very likely,
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