Role of Mid‐ and High‐Latitude, Southern Hemisphere Processes for Ushering in the Middle Pleistocene Transition

Abstract Glacial‐interglacial cycles increased in length from about 41 to 100 thousand years during the Middle Pleistocene Transition (MPT) about 1.5 to 0.8 million years ago. This took place in step with strong global cooling after several million years of weaker cooling. Such strong MPT cooling likely set the stage for longer ice age cycles by, for example, facilitating Northern Hemisphere ice sheet growth. Here we use simulations with a newly developed Earth System Model of Intermediate Complexity together with sea surface temperature reconstructions to evaluate possible causes of the MPT cooling. In particular, we concentrate on the effects of changes in a. Southern Hemisphere Westerly Wind strength and position, b. Sea ice and iceberg export from the Antarctic shelf, c. Dust fertilization of the Southern Ocean and d. Northern Hemisphere ice sheet extent. We find that weakened, equatorward‐shifted Southern Hemisphere Westerly Winds in combination with increased sea ice and iceberg export from the Antarctic Shelf can explain much of the strong MPT cooling. Our simulations suggest that these changes caused the cooling by decreasing heat and carbon exchange between the upper and deep Southern Ocean and increasing deep recirculation in the ocean, leading to deep ocean isolation, carbon storage and atmospheric CO 2 decrease. Associated mean ocean warming together with benthic δ 18 O data imply large MPT sea level drop and ice sheet growth. Dust‐driven iron fertilization further enhanced CO 2 drawdown while Northern Hemisphere ice sheet growth contributed mainly to regional cooling while promoting land biomass reduction that releases CO 2 to the atmosphere.