Key aspects of past variability

This and the next two chapters are concerned with Earth-system history over the last ~420 000 years, with special emphasis on the last ~130 000 years, the period since the beginning of the last interglacial. In the present chapter, we are concerned with the changes that are associated with the last four glacial cycles, up to the end of the last glacial maximum (LGM). For much of this period, the mean state of the climate and hence Earth-system components linked to climate, differed greatly from that in the recent past. Our main goal is to elucidate the combination of forcings and feedbacks responsible for the major changes observed, with a view to understanding better the sequences and synergies arising from their interactions, especially during periods of rapid change.

The high-latitude records show temperature variability to have been much greater during glacial periods than during interglacials, including the Holocene. By contrast, hydrological variability in lower latitudes has been extreme even during the Holocene. Variability of changing amplitudes, in both temperature and hydrology, is the norm throughout the period. This initial observation is of outstanding importance, for it tells us that even if we discount the likelihood of anthropogenic effects on climate, this does not dispose of future climate change. Scientists and policy makers across the whole spectrum, from the most sceptical about the impact of increasing atmospheric greenhouse-gas concentrations to the most firmly convinced, cannot afford to ignore all the evidence for natural climate variability.