Are There Any Satisfactory Geologic Analogs for a Future Greenhouse Warming?

Abstract

There have been numerous attempts to propose past warm time periodsas “analogs” for a future greenhouse warming. In thispaper it is argued that, although paleoclimate studies may provideimportant insights into process operating in the climate system,there may be no warm time period that is a satisfactory past analogfor future climate. The future greenhouse warming may represent aunique climate realization in earth history. This conclusion is basedon the following considerations: 1 ) comparisons with Holocene (9000BP) or Eemian climates (120 000 BP) may be inappropriate becausemuch of the variations in these climates can be explained in termsof seasonal rather than mean-annual forcing it has yet to be demonstratedthat increased warmth for these intervals involved mew annual temperatureincreases that were globally synchronous, 2) comparisons with olderand warmer climates (ex., Pliocene, Eocene, or Cretaceous) can bemisleading because these warm periods had reduced polar ice cover,whereas future air temperatures will be very warm, but ice sheetswill persist because of their large thermal inertia. Due to the differenttime scales for the atmosphere, deep ocean, and ice sheets, thissignificant nonequilibrium component to the future climate responseis probably very different than the long time-averaged picture representativeof past warm periods. Furthermore, changes in geography have probablysignificantly modified the atmosphere and ocean circulation duringthe earlier warm periods resulting in regional climates significantlydifferent than what might occur in the future. It is therefore suggestedthat future discussions on geologic analogs be restricted to studyof processes operating in the climate system and that continued useof the term for past warm time periods be abandoned.