Climate Science Speaker Series

Jerry McManus, Lamont-Doherty Earth Observatory

Rapid climate changes characterized the last ice age and deglaciation, with dramatic warming following the coldest intervals in the northern hemisphere. The repeated pattern of alternating temperature swings revealed in ice cores from Greenland and Antactica suggest a bipolar see-saw of heat redistribution by a dynamical component of the Earth system such as the large scale Atlantic Meridional Overturning Circulation (AMOC). Computer model simulations support this possibility, yet direct evidence for these changes in deep ocean circulation has been difficult to obtain. We have examined multiple geochemical and isotopic tracers of the deep circulation throughout the last ice age from rapidly accumulating sediments in the North Atlantic Ocean.  They document the systematic association of variations in AMOC and abrupt climate change through the glaciation and deglaciation.  Diminished AMOC accompanied the millennial northern coolings, including the cold, stadial, portions of so-called “Dansgaard-Oeschger (DO) events” as well as the extreme stadial “Heinrich events” associated with catastrophic iceberg discharges. Perhaps most notably, rapid increases in AMOC, in the form of surges in the depth and export of North Atlantic Deep Water from the North Atlantic Basin, accompanied the dramatic northern warmings that punctuated the ice age, underscoring the important potential role of internal Earth systems in climate change.



As global anthropogenic emissions of greenhouse gases continue to rise, there is an increasing risk of serious disruptions in ecosystems due to global warming. As a consequence, research on climate engineering (CE) is receiving growing attention, also among climate scientists. But, even basic CE research using Earth System Models (ESMs) raises a series of ethical questions that need to be considered. Also, CE carries a risk of serious side effects, e.g., concerning the hydrological cycle.

Climate engineering can be divided into Greenhouse Gas Removal and Radiation Management (RM) techniques. RM here refers to the deliberate modification of either incoming solar radiation or outgoing terrestrial radiation. We will review the basic principles of four proposed RM techniques – stratospheric sulfur injections, marine sky brightening, cirrus cloud thinning and desert brightening. We then present recent robust results concerning changes to the hydrological cycle from multi-model ESM experiments within the Geoengineering Model Intercomparison Project (GeoMIP).

We show that the changes in the hydrological cycle depend strongly on which RM technique is applied. For instance, cirrus cloud thinning influences the hydrological cycle in a distinctly different way than techniques that reduce incoming solar radiation. We will demonstrate how that finding can be explained from atmospheric energy budget considerations.