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The West Pacific Warm Pool (WPWP)'s response to increasedpCO₂during the Pliocene is a key model validation target. Different temperature proxies show different trends: The foraminiferal Mg/Ca sea surface temperature (SST) record shows Pliocene WPWP temperatures ~1.2°C cooler than today (Wara et al., 2005,https://doi.org/10.1126/science.1112596), whereas a TEX₈₆study finds a cooling trend and claims the Pliocene WPWP was warmer than today (Zhang et al., 2014,https://doi.org/10.1126/science.1246172). We focus on understanding biases in Mg/Ca data as the best way to constrain the temperature of the Pliocene WPWP. The strongest nonthermal controls on foraminiferal Mg/Ca are Mg/Ca of seawater and dissolution. Dissolution, which imparts a cool bias to Mg/Ca temperatures, depends on Δ[CO₃²⁻], the difference from the carbonate ion concentration needed for calcite saturation. Thus, Pliocene proxy discrepancies might stem from varying Δ[CO₃²⁻] over time. To constrain the effect of changing dissolution on the Mg/Ca data, we collected benthic foraminiferal B/Ca data (a proxy for Δ[CO₃²⁻]) from the WPWP spanning 0–5.5 Ma. We find no long‐term trend in Δ[CO₃²⁻], but variations above and below the threshold of foraminiferal dissolution yield an ~0.4°C cold bias when averaged over the middle to early Pliocene. Changes in seawater Mg/Ca create an ~0.6°C cold bias in the Pliocene Mg/Ca data. After accounting for these biases, we find that the Pliocene WPWP was ~0.1°C cooler than the late Holocene, ranging from −0.5°C to +0.5°C including all uncertainties. Our reconstruction shows a much lower east‐west temperature gradient in the Pliocene tropical Pacific than today, supporting a permanent El Niño‐like “El Padre” state.

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 ofpCO₂‐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 aspCO₂‐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 topCO₂‐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‐basedpCO₂reconstruction. We find tropical temperature sensitivity was equal to, or less than, that of the Late Pleistocene.