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

The Western Equatorial Pacific (WEP) warm pool, with surface temperatures >28 degrees 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, similar to 2, similar to 3, and similar to 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 (similar to 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. Plain Language summary The western tropical Pacific Ocean is a major source of heat and moisture for the global climate system. Understanding what the western tropical Pacific Ocean looked like during the past is important to constraining future global climate behavior. Here we reconstruct surface ocean conditions over the last five million years. We find the thermocline, a subsurface layer where temperature rapidly decreases with depth, was deep five million years ago and gradually cooled and shoaled to present day. We also find surface ocean temperatures from three million years ago respond to carbon dioxide in a similar manner as in the last few thousand years.