Orbits, CO2, and
the Next Ice Age
Two centuries ago, climatologists were more concerned about the next ice age than they were about global warming. Svante Arrhenius, who first estimated the climate sensitivity to atmospheric CO2 in 1896, was interested in explaining the cause of the last ice age. From moraines it was known that the landscape had undergone repeated assault from massive ice sheets. The timing was not known very well, because moraines are difficult to date, especially without carbon-14 dating, and they document only the coldest times, while the warmest times that we’re interested in, the interglacials, leave no trace. So the landscape offered no information about the longevity of warm climate intervals such as ours, only an ominous drumbeat of glaciations in the past.
The 1970s saw the development of time records from the ratio of oxygen-18 to oxygen-16 in ocean sediments (Chapter 5). It was found that the Earth spent most of the past million years or so in a glacial climate state. Interglacial climate stages, the most recent ones anyway, generally lasted for 10 millennia, while glacial states might last ten times as long. The orbital theory of climate explained that the duration of a warm interval was deter-