CLIMAP SSTs re-revisited

Since the 1976 publication of the CLIMAP ice age sea surface temperature (S ST) reconstruction showing a 1-2 degrees C tropical cooling a substantial d ebate has arisen as to whether tropical SSTs may instead have been 4-5 degr ees colder than present. Herein I review the arguments for large SST variat ions and question a number of key findings, particularly the validity of ic e-age coral SST estimates and "down-projecting" tropical snowline changes t o the surface. GCM results indicate that an intermediate solution requiring similar to 2.5 degrees C warm pool cooling is consistent with most quantit ative low elevation surface land data and is small enough to allow the pers istence of tropical biota in the ocean during glacial times. The proposal r educes estimated ice-age climate sensitivity (for a doubling of CO2) from a ''high-end" sensitivity of about 4.5 degrees C (for a 5 6 degrees C tropic al cooling) to a "mid-range" sensitivity of about 3.0 degrees C for a 2.5 d egrees C warm-pool decrease.