Making Sense of Climate Change Since the Late Miocene

Abstract

Over the past 66 million years (Myr), the Earth transitioned from a warm, ice-free greenhouse climate of the early Cenozoic to a cold icehouse climate with glaciated poles of the late Cenozoic. The late Miocene (~11.6-5.3 million years ago) represents one of the two long-term cooling steps, although the Earth overall was still significantly warmer than the present day. The important climate components, for example, ocean temperature and temperature gradients, and the key climate forcing, greenhouse gas levels, are not well constrained for this critical interval. My dissertation research focuses on providing a better understanding of climate changes from the late Miocene to the present. I have led the investigation on (1) reconstructing the thermal history of the warm pool for the past 10 Myr and deciphering high latitude amplification, (2) building the spatial pattern of surface temperature change and connecting the paleo-warming pattern to the future, and (3) refining alkenone-CO2 proxy to better reconstruct atmospheric CO2 levels in the geologic past.

First, we present new multi-proxy (Mg/Ca and TEX86) multi-site paleotemperature records over the last 10 Myr from the Western Pacific Warm Pool (WPWP)—the warmest end member of the global ocean that is uniquely important in the global radiative feedback change. Compiling our new data with existing records reveals a persistent extratropical response pattern in the Pacific in which high latitude (~50°N) temperatures increase by ~2.4°C for each degree of WPWP warming. This Pacific high latitude amplification is also evident in model outputs of millennium-long climate simulations with quadrupling atmospheric CO2, therefore providing a strong constraint on the future equilibrium response of the Earth System to warming.

Second, we developed a novel regression-based technique that removes time variations and allows past climates to be directly compared with future climates: long (10 million years), globally distributed paleoclimate SST time series were regressed onto the SST record from the WPWP. Using this new approach, we have identified a nearly stationary pattern of warming similar to that in some equilibrated model simulations under high CO2 conditions. These data help us to define an "equilibrium" warming pattern different from the transient pattern of warming that characterizes the past 160-170 years, illuminating our potential future path of the “pattern effect”.

Finally, we constrained uncertain parameters used in alkenone-CO2 proxy via a Markov Chain Monte Carlo Bayesian inference approach. The alkenone-CO2 estimated from the Bayesian-determined parameters agree well with the ice-core record, validating our new approach. Applying these inferred parameters to late Eocene and early Oligocene, we obtained consistent CO2 estimates among different sites that are in line with boron-derived CO2 and ice sheet model prediction.