Clarifying past AMOC and climate sensitivity with paleoclimate archives

The abrupt transient centennial to millennial coolings of the North Atlantic region first described in Greenland ice core isotope records by Hans Oeschger and Willy Dansgaard are now widely recognized as recurrent, abrupt weakenings of the Atlantic meridional overturning circulation (AMOC). A hierarchy of climate models has shown that freshening in the North Atlantic can trigger AMOC collapse or condition unforced AMOC variability.  Yet, the existence of a freshwater trigger is largely untested for most events, and it is also uncertain whether a reduction in freshwater fluxes was necessary to permit the recovery of AMOC.  Decadally resolved stalagmite oxygen isotope records from coastal caves in NW Iberia record changes in the δ¹⁸O of the surface eastern North Atlantic which are highly sensitive to the freshwater balance.   Careful refinement of stalagmite carbon isotope proxies to correct for in-cave fractionation effects, has provided an indicator of the abrupt temperature changes caused by AMOC in this region.  These dual indicators resolve the detailed phasing of freshening and past AMOC variations. In stalagmite records spanning late MIS 7 and MIS 6, nearly half of the abrupt coolings have no significant freshening event within a hundred years of the onset of cooling.  In contrast, the millennial coolings at the start of Termination II deglaciation are synchronous with abrupt freshening events.  Thus, triggers of abrupt AMOC weakenings and recoveries may be diverse, and the sensitivity to different triggers may change with the evolution of climatic boundary conditions.  A longer set of stalagmite records provide evidence for more dynamic variations of Northern Hemisphere ice sheets and a succession of positive and negative feedbacks on ice melting rate during deglaciations. 

The recognition of past variation in atmospheric CO₂ in ice core archives, first characterized by Hans Oeschger, catalyzed efforts to estimate past atmospheric CO₂ from indirect proxies in the deeper past, to better estimate the earth system and cryosphere sensitivity to atmospheric CO₂ in warmer than preindustrial periods.  Over the last decades, the challenging path of deriving proxy CO₂ records has led to numerous paradoxes in the relationship between climate variables and estimated CO_2.  From the marine alkenone-based CO₂ proxy, a better representation of the biological processes imprinted on the proxy has improved the relationship between CO₂ and climate trends since the mid Miocene.  Such a representation allows more robust estimates of the scope of changes in atmospheric CO₂ across climate transitions, although estimates of absolute CO₂ still feature high uncertainty.  As orbital resolution CO₂ proxy records emerge for past warmer time periods, we are presented with new questions about the relationship between ice growth on Antarctica and CO₂, and the carbon cycle processes shifting carbon into and out of the atmosphere on orbital timescales.