Background
The authors contend that "ice-core evidence from previous interglaciations indicates that forcing by orbital-scale changes in solar radiation and greenhouse-gas concentrations should have driven earth's climate significantly toward glacial conditions during the last several thousand years," and that "the hypothesized reason most of this cooling did not occur is that humans intervened in the natural operation of the climate system by adding significant amounts of CO2 and CH4 to the atmosphere, thereby offsetting most of the natural cooling [that otherwise would have occurred] and fortuitously producing the climatic stability of the last several thousand years."  If true, how did humans do it?  Ruddiman et al. attribute the anomalous increase in atmospheric CO2 to massive early deforestation of Eurasia, while they link the anomalous CH4 increase to the introduction of irrigation for rice farming in southeast Asia, as well as to increases in biomass burning and the development of animal husbandry.

What was done
Based on the periodicities and phases of the natural cycles of CO2 and CH4 that are revealed in the 400,000-year Vostok ice core, Ruddiman et al. first determined that the air's CO2 concentration should have fallen to 240-245 ppm, whereas it gradually rose to a level of 280-285 ppm, just before the start of the Industrial Revolution, while the air's CH4 concentration rose to approximately 700 ppb when it should have fallen to about 450 ppb.  Then, based on the IPCC sensitivity estimate of a 2.5°C temperature increase for a doubling of the air's CO2 content, they calculated that the supposedly anthropogenic-induced CO2 and CH4 anomalies should have produced an equilibrium warming of approximately 0.8°C on a global basis and 2°C in earth's polar regions.

What was learned
On the basis of these calculations, the authors conclude that "without any anthropogenic warming, earth's climate would no longer be in a full-interglacial state but well on its way toward the colder temperatures typical of glaciations," and that "an ice sheet would now be present in northeast Canada, had humans not interfered with the climate system."

What it means
If correct, the overdue-glaciation hypothesis indicates that in the absence of anthropogenic contributions of CO2 and CH4, the climate today would be, in the words of Ruddiman et al., "roughly one third of the way toward full-glacial temperatures," which also suggests that the extra CO2 we are currently releasing to the atmosphere via the burning of fossil fuels may well be what's keeping us from going the rest of the way.  Hence, even if the IPCC is correct in their analysis of climate sensitivity and we are wrong in suggesting the sensitivity they calculate is way too large, the bottom line for the preservation of civilization and much of the biosphere is that governments ought not interfere with the normal progression of fossil fuel usage, for without more CO2 in the atmosphere, we could shortly resume the downward spiral to full-fledged ice-age conditions.  Ought we not be doubly careful, therefore, as the United States indeed is, in not rushing forward to implement the Kyoto Protocol or anything like it?  We certainly think so.