MODELING PLIOCENE WARMTH

Modeling studies relevant to the question of Pliocene warmth are reviewed. Although significant regional climate variations may be due to Pliocene changes in orography and ocean circulation, these changes do not appear sufficient to explain inferred globally increased warmth. Climate modeling studies for a doubling of CO2 suggest high-latitude temperature increases approximately comparable to inferred warming during the Pliocene. I suggest that such a CO2 increase may be primarily responsible for the Pliocene temperature increase. Downdraw of CO2 since the Pliocene is consistent with some geochemical indications of enhanced weathering in the Pleistocene. However, additional proxy testing of this hypothesis is required. The CO2 models also predict tropical sea-surface warming of 2-3-degrees-C and winter temperature warming in central Eurasia (50-70-degrees-N) of approximately 4-10-degrees-C - predictions that distinguish CO2 from some other mechanisms. Enhanced scrutiny of Pliocene records from the above regions may prove a valuable test of the CO2 hypothesis. Finally, although a CO2 increase may be responsible for Pliocene warmth, this time interval should not be cited as a direct analog for a future greenhouse warming, as the future climate will have a significant nonequilibrium response because of the rapid rate of change of forcing. Despite these limitations, enhanced study of Pliocene climates may be very valuable as an independent source of validation of CO2-doubling simulations.