Orbital-Scale Global Ocean Sea Surface Temperatures Coupling With Cryosphere-Carbon Cycle Changes Over the Past 4 Million Years

Changes in the thermal conditions of the ocean surface, the interface for air-sea exchange, are critical for understanding global climate and environmental change. Here we explore the evolution of sea surface temperature (SST) and the meridional SST gradient (STG) at orbital timescales since 4 million years ago (Ma), along with interactions between SSTs, the cryosphere, and the global carbon cycle. We observe orbital eccentricity and obliquity influences on SST evolution and infer that SST changes may have played a key role in atmospheric CO₂ and cryosphere changes through key climate transitions in the past 4 Ma. We find a major equator-to-pole STG increase in the Northern Hemisphere (NH) close to the initiation of major NH glaciation (at ∼2.7 Ma). In addition, we find substantial increases in the obliquity sensitivity (S_obl) of NH STG at ∼2.7 Ma and in Southern Hemisphere (SH) STG at ∼1 Ma, which may be responses to important expansions of NH and SH ice sheets, respectively. Phase analysis shows that SST changes typically lead global ice volume changes throughout the last 4 Ma. SST changes also lead atmospheric CO₂ changes since ∼1.5 Ma, which indicates that SST changes either drove, or directly reflect, processes that changed ocean-atmosphere carbon exchange and, thus, atmospheric CO₂ concentrations. Overall, our study emphasizes that SST changes were a critical component of climate change throughout the last 4 Ma.