Estimates of Pliocene Tropical Pacific Temperature Sensitivity to Radiative Greenhouse Gas Forcing

Abstract

The Western Equatorial Pacific (WEP) warm pool, with surface temperatures >28 °C and a deep thermocline, is an important source of latent and sensible heat for the global climate system. Because the tropics are not sensitive to ice-albedo feedbacks, the WEP's response to radiative forcing can be used to constrain a minimum estimate of Earth system sensitivity. Climate modeling of pCO 2 -radiative warming projections shows little change in WEP variability; here we use temperature distributions of individual surface and subsurface dwelling fossil foraminifera to evaluate past variability and possible radiative and dynamic climate forcing over the Plio-Pleistocene. We investigate WEP warm pool variability within paired glacial-interglacial (G-IG) intervals for four times: the Holocene-Last Glacial Maximum, ~2, ~3, and ~4 Ma. Our results show that these surface and subsurface temperature distributions are similar for all G-IG pairs, indicating no change in variability, even as pCO 2 -radiative forcing and other boundary conditions changed on G-IG timescales. Plio-Pleistocene sea surface temperature (SST) distributions are similar to those from the Holocene, indicating WEP SSTs respond to pCO 2 -radiative forcing and associated feedbacks. In contrast, Plio-Pleistocene subsurface temperature distributions suggest subsurface temperatures respond to changes in thermocline temperature and depth. We estimate tropical temperature sensitivity for the mid-Pliocene (~3 Ma) using our individual foraminifera SST data set and a previously published high-resolution boron isotope-based pCO 2 reconstruction. We find tropical temperature sensitivity was equal to, or less than, that of the Late Pleistocene.