Peak Ice: The Dance of the Ice Sheets
For a long time, the conventional wisdom was that the maximum extent of glaciers during the last ice age was reached in about 16,000 BC. Then it was realized that better calibration of the rate of production of carbon-14, the main radioactive clock we have for that period, required that the date of the maximum be pushed back to 19,000 BC. Now Peter Clark and colleagues, writing in a recent issue of Science, provide further clarity. They show that the maximum extent of the ice lasted from about 24,500 to about 17,000 BC.
Plants and animals don’t grow underneath ice sheets, so datable evidence for them means no ice at the site of observation. Assemble several thousand of these dates and you get a picture of the changing extent of the ice sheets. The picture is sharper for some ice sheets than for others, but cumulatively the evidence for several thousand years of near-stability is fairly impressive.
It contrasts with some other guides to the palaeoclimate. Oxygen isotopes in ocean-floor microfossils are a guide (unfortunately) to two variables: sea level (or the volume of the ice sheets, which amounts to the same thing) and water temperature. The heavier of the two common isotopes of oxygen tends to stay behind when molecules of liquid H2O evaporate from the ocean surface, so (a) if the light molecules accumulate in the ice sheets instead, the oceanic oxygen gets relatively heavier, but (b) this effect is less marked when it is warmer because now the heavier molecules evaporate more readily.
Disentangling these two controls on oxygen isotopes is a major challenge, but accepted pictures of deep-ocean temperature show a quite sharp minimum at about 17,000 BC. What Clark and colleagues have shown, in contrast, is that the ice volume was near to its maximum, equivalent to a sea level about 128 m lower than today’s, for several thousand years before that. This agrees with calculated variations of the solar radiation regime due to changes in the Earth’s orbit. The calculations are reliable, and show a broad minimum of Northern Hemisphere radiation centred near 20,000 BC. The concentration of atmospheric CO2 also varies only moderately, between 185 and 200 parts per million, during their suggested glacial-maximum span.
This is evidence that the climate can be stable in more than one state. On graphs which span tens or hundreds of thousands of years, the last ten thousand years, the Holocene, stand out as a time of little change. I am not saying that the climate has not changed during this time. We know, for example, that it was a bit warmer in its earlier than its later part, and the cool spell known as the Little Ice Age, which bottomed out about 1850 AD, is well documented. But the Holocene climate was much less variable than the irregularly cooling climate during the preceding hundred thousand years of the ice age. Now Clark and colleagues have shown that peak ice can be stable as well, in the sense of looking rather flat on a graph (but in a much cooler way). We would naturally like to know why.
Another interesting point is that different ice sheets reached their maxima at different dates. The big standout in this respect is West Antarctica, where the maximum was as late as 12-13,000 BC, but the big northern ice sheets, now long gone except for Greenland, had maxima up to a few thousand years apart, reminding me of the smaller glaciers during the Little Ice Age, which peaked as early as the late 17th century in some places and as late as the early 20th century in others. Evidently, as they paced through their stately dance, neither the Little Ice Age glaciers nor the ice-age ice sheets were much good at keeping in step.
Regional variations are therefore a fact of the global climate, about which it is nevertheless legitimate to make on-average statements. One implication is that we should not pay much attention to people who point out correctly that some parts of the world are not warming very much at the moment, but argue wrongly from this that global warming is a myth.
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