Foraminiferal faunal estimates of paleotemperature: Circumventing the no-analog problem yields cool ice age tropics

The sensitivity of the tropics to climate change, particularly the amplitud e of glacial-to-interglacial changes in sea surface temperature (SST), is o ne of the great controversies in paleoclimatology. Here we reassess faunal estimates of ice age SSTs, focusing on the problem of no-analog planktonic foraminiferal assemblages in the equatorial oceans that confounds both clas sical transfer function and modern analog methods. A new calibration strate gy developed here, which uses past variability of species to define robust faunal assemblages, solves the no-analog problem and reveals ice age coolin g of 5 degrees to 6 degrees C in the equatorial current systems of the Atla ntic and eastern Pacific Oceans. Classical transfer functions underestimate d temperature changes in some areas of the tropical oceans because core-top assemblages misrepresented the ice age faunal assemblages. Our finding is consistent with some geochemical estimates and model predictions of greater ice age cooling in the tropics than was inferred by Climate: Long-Range In vestigation, Mapping and Prediction (CLIMAP) [1981] and thus may help to re solve a long-standing controversy. Our new foraminiferal transfer function suggests that such cooling was limited to the equatorial current systems, h owever, and supports CLIMAP's inference of stability of the subtropical gyr e centers.