Environmental Science and Engineering Seminar

The present is a precarious moment in Earth's history. Records of the dramatic amplification of climate fluctuations over the past 3 Myr indicate that an Ice Age is imminent, except that the recent man-induced rise in atmospheric CO2 levels is inducing global warming. What will happen over the next several decades? Answers from a reductionist approach using models based on the laws that govern climate variability have significant uncertainties. There is "deep dissatisfaction with the ability of our models to inform society about the pace of warming, how this warming plays out regionally, and what it implies for the likelihood of surprises" (Palmer and Stevens, 2019, PNAS). An empirical or holistic approach based on records of past climates is also inadequate because "… we still lack a unified mechanistic understanding that links changes in Earth's orbit to the Ice Ages" (Hodell, 2016, Science). That criticism of the hypothesis that polar glaciers wax and wane in response to local Milankovitch forcing stems from questionable assumptions: that the ocean obligingly provides fresh water for glaciers, and that the atmosphere passively transports that water from low to high latitudes. This draws attention to the global structure of Milankovitch forcing. The two main components of this driver of the atmosphere and ocean, precession and obliquity, pose the following questions.

1. How does precession, which merely redistributes sunlight over the course of a year without changing the annual average, induce a 20 Kyr recurrent signal whose cold phase in tropical Pacific sea surface temperature is at a peak when perihelion coincides with the southern, not northern summer solstice? Why is the ITCZ north of the equator?
2. How do 40 Kyr obliquity oscillations, which merely redistribute sunlight spatially without changing the global average, induce 40 Kyr oscillations in globally averaged temperature? Why is the phase of that signal such that tropical SST is at a minimum when sunlight is intense in low latitudes?
3. Could the alternating warming and cooling trends of the saw-tooth signal of the past 0.8 Kyr, and the preceding cooling trend from 3 to 1 Myr, be a natural (as opposed to forced) climate mode with feedbacks sustaining the trends and thresholds reversing them? If so then the saw-tooth would be present in the absence of Milankovitch forcing which serves as pacemaker for the thresholds, lending regularity to an otherwise irregular signal.

This seminar proposes that the available observational and theoretical studies of past and present climates provide tentative answers to these questions – "a unified mechanistic understanding" of Ice Ages – and identify a strategy for improving climate models by means of a marriage of holistic and reductionist approaches. One tentative conclusion: rising CO2 levels are interfering with the oceanic heat budget and, over the course of several decades could restore the warm conditions of the "permanent" El Niño of 3 Myr ago.