Late Pliocene foraminiferal Mg/Ca and delta (super 18) O records from Ocean Drilling Program Hole 806B in the western equatorial Pacific (WEP) reveal warm pool climate evolution during the onset of Northern Hemisphere glaciation, 3.1-2.3 Myr B.P. Mg/Ca data indicate an average late Pliocene sea surface temperature (SST) of 27.8 degrees C, a small long-term cooling of 0.3 degrees C between 3.1 and 2.3 Ma, and a glacial-interglacial (G-I) SST range of 2 degrees C throughout this time interval. For comparison, Pleistocene SSTs at this site over the last 0.9 Myr average 27.7 degrees C with a G-I range of 3 degrees C. Orbital-scale variability in Hole 806B SSTs during the late Pliocene occurs predominantly at approximately 100 ka, in contrast to foraminiferal delta (super 18) O records, which show a dominant 41 kyr period. Variability at a 41 kyr period, out of phase with local annual insolation changes driven by obliquity, is also observed in the new WEP SST record. The WEP SST record suggests that an approximately 3 degrees C equatorial Pacific SST zonal gradient prevailed during the late Pliocene, compatible with a weaker Walker circulation. Adjustment of Hole 806B SSTs for past changes in seawater Mg/Ca suggests that SSTs higher than 30 degrees C prevailed at 3 Myr B.P., followed by a progressive cooling of the warm pool through the late Pliocene. The characteristics of late Pliocene tropical climate evolution suggest that atmospheric greenhouse gas forcing played a major role in driving the observed G-I SST changes.