Guest Seminar: The other ice age CO2 problem: Did the carbon cycle drive climate?

Speaker

Dr. Mathis Hain
Assistant Professor
UC Santa Cruz

Abstract

Forty years have passed since the discovery that atmospheric CO₂ levels were roughly 30% lower during the peak of the last ice age, some 20 thousand years ago, and the question what natural carbon cycle change(s) caused this glacial-to-interglacial CO₂ change has animated scientific discourse ever since; the classical ice age CO₂ problem. In this talk I argue that we now know what it takes to explain why CO₂ changed systematically over the recurring ice age cycles of the late Pleistocene, roughly the last one million years, but that we have made frustratingly little progress in understanding the role of CO₂ climate forcing in driving ice age climate cycles. Recent developments using the boron isotope pH proxy to extend the CO₂ record further back in time suggest that enhanced glacial-stage CO₂ drawdown was in part responsible for the Mid-Pleistocene Transition (MPT) from ~40-kyr to 100-kyr ice age cycles. Further, the carbon cycle change at the MPT can be attributed to enhancement of glacial-stage dust-borne iron fertilization of the Southern Ocean, whereas glacial/interglacial CO₂ change prior to the MPT was dominated by orbitally-paced overturning changes deep ocean overturning. These results suggest a central role of CO₂ climate forcing and carbon cycle feedbacks in the waxing and waning of Earth’s ice sheets before, during and after the Mid-Pleistocene climate transition.