Oceans in Crisis - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 175

does not even appear as a blip on the rising curve of carbon dioxide
concentrations.
There are larger movements of carbon dioxide out in the natural
environment. Each year some 60 billion tonnes are fixed as plant mat-
ter, and an equivalent amount rots and decays to release the same
amount into the atmosphere. The process is geographically asym-
metrical. There is more land, and therefore more plant matter in the
northern hemisphere, and so the Earth 'breathes' annually; these
'breaths' are seen as the annual rises and falls of carbon dioxide in the
rising trend. Further, each year some 90 billion tonnes dissolves into
the oceans and—at least until a couple of centuries ago—the same
amount was exhaled. This is all part of a grand, finely functioning,
and delicately poised global carbon cycle. Despite these huge inflows
and outflows of carbon—and the even larger reservoirs from which
they are derived (the oceans, for instance, store within them over 45
thousand billion tonnes of carbon)—there has been a quite extraordi-
nary long-term stability of atmospheric carbon dioxide for the thou-
sands of years before the Industrial Revolution that is nothing short
of amazing (or at least we find it so). Perhaps it was not quite a natural
stability, given the possible effects of early agriculture, but neverthe-
less it shows how finely regulated the land/ocean carbon cycle nor-
mally is, when left to itself.
We know this because air—pure, pristine air from the past—can
be fossilized. Not in normal rock—at least, not as far as we know—
but in ice. The layers of snow that form every year over the vast
ancient icecaps of Greenland and Antarctica (and also the some-
what more short-lived icecaps of the Himalayas, Andes, and other
mountain ranges) are initially light and fluffy, being full of air. As
more layers of snow fall on them they become compressed, and
much of the air is squeezed out—but not all. As fluffy snow con-
verts into firm snow and then into ice, tens to hundreds of metres