AGS-PRF: The Effect of Carbon-Dioxide (CO2) Ramping Rate on the Atlantic Meridional Overturning Circulation and its Coupled Climate Implications

The Atlantic Meridional Overturning Circulation (AMOC), often drawn schematically as a conveyor belt, plays a key role in Earth's climate by transporting heat northward throughout most of the Atlantic Ocean. In addition to accounting for perhaps a quarter of the northward heat transport in the Northern Hemisphere, AMOC is important for the uptake of carbon dioxide (CO2) by the ocean and for the cycling of oxygen and nutrients for the ocean biosphere.

The sinking motion in the conveyor belt happens in the cold, high salinity and hence relatively dense waters of the northern North Atlantic, and studies going back to the 1960s suggest that AMOC could weaken, or even collapse, if the water in the sinking branch becomes less dense as climate warms. There is evidence for AMOC collapse during the last ice age as a consequence of fresh water entering the northern North Atlantic as the continental ice sheets melted, and there is also evidence that AMOC has slowed over the last century.

Interest in AMOC is motivated in part by arguments that it has a "tipping point", meaning that it can collapse for some level of CO2 increase or freshening of the surface ocean (which can occur as a consequence of a warming climate), and if that happens the collapsed AMOC state will be self-sustaining and hence permanent.

Work performed here considers the weakening of AMOC caused by CO2 increases in long simulations of the Community Earth System Model (CESM), focusing in particular on how the rate of CO2 increase affects the AMOC response. The work differs from previous studies which have generally focused on the magnitude, rather than the rate, of CO2 increase, or on the magnitude of freshwater input.

The work is informed by the idea of "rate-induced tipping points" which has recently gained attention in the dynamical systems literature. Preliminary results show strong rate dependence in long CESM simulations, both in the amount and abruptness of AMOC weakening and in its subsequent recovery. Issues addressed in the work include the physical mechanisms that set the pace and magnitude of the weakening and the impacts of weakening on the climate of the North Atlantic region.

The work is of societal as well as scientific interest given the potential climatic impacts of abrupt AMOC slowdown or collapse. AMOC heat transport has sizeable effects on climate, and consequences of a weaker AMOC would likely include colder winters in Western Europe and reduced rainfall in both Western Europe and the Amazon. In addition, the Principal Investigator of this award contributes to efforts at the host institution, the University of Washington, to mentor and retain students and early-career scientists and increase the scientific literacy of the general public.

One such activity is the Scientist-Teacher Workshop series organized by the UW Program on Climate Change, which seek to help high school teachers develop curricula on climate science.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.