Clumped Isotope Temperatures and Entrainment Explain Glacial to Recent Tropospheric Structure over the West Pacific Warm Pool

A controversial problem has been to understand how reconstructed tropical ocean temperatures can be compatible with the extent of low-latitude glaciers from the Last Glacial Maximum (LGM) to present, which appears to require a lapse rate that is steeper than the moist adiabat. Here we report this problem can be resolved by a combination of new estimates of ocean surface temperatures that allow us to confidently enhanced warming in the western tropical Pacific warm pool since the LGM, and by including the process of entrainment of drier air during convection in the region. Clumped isotope data indicate the magnitude of warming since the LGM was 3-5°C, consistent with Mg/Ca-based estimates and larger than estimated using transfer functions, alkenones, and in a recent proxy synthesis. We demonstrate that mean environmental lapse rates are steeper than moist adiabatic during the recent and LGM. Using a radiative-convective model, we show that both modern and glacial snowlines can be explained if entrainment during deep convection is taken into account, a process constrained here using extensive data for the warm pool. Furthermore, temperature changes smaller than ~3°C in the warm pool are excluded unless LGM relative humidity values were outside of the range associated with deep convection in the modern. Our new clumped isotope data not only addresses a longstanding problem in LGM climate and emphasizes the essential role of entrainment for establishing the magnitude of temperature changes in convective regions, but also implies a higher equilibrium climate sensitivity than included in the current generation of models.