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

There have been numerous attempts to propose past warm time periods as “analogs” for a future greenhouse warming. In this paper it is argued that, although paleoclimate studies may provide important insights into process operating in the climate system, there may be no warm time period that is a satisfactory past analog for future climate. The future greenhouse warming may represent a unique climate realization in earth history. This conclusion is based on the following considerations: 1 ) comparisons with Holocene (9000 BP) or Eemian climates (120 000 BP) may be inappropriate because much of the variations in these climates can be explained in terms of seasonal rather than mean-annual forcing it has yet to be demonstrated that increased warmth for these intervals involved mew annual temperature increases that were globally synchronous, 2) comparisons with older and warmer climates (ex., Pliocene, Eocene, or Cretaceous) can be misleading because these warm periods had reduced polar ice cover, whereas future air temperatures will be very warm, but ice sheets will persist because of their large thermal inertia. Due to the different time scales for the atmosphere, deep ocean, and ice sheets, this significant nonequilibrium component to the future climate response is probably very different than the long time-averaged picture representative of past warm periods. Furthermore, changes in geography have probably significantly modified the atmosphere and ocean circulation during the earlier warm periods resulting in regional climates significantly different than what might occur in the future. It is therefore suggested that future discussions on geologic analogs be restricted to study of processes operating in the climate system and that continued use of the term for past warm time periods be abandoned.