An international team(1) modeled the coupling between the extent of sea ice and marine ice shelves, and the temperature of the waters near the North Atlantic surface. This model explains the steep temperature changes in Greenland and the North Atlantic during the last ice age, between 130,000 and 15,000 years ago. It also reproduces the phase shift between the temperatures of the two hemispheres during this period, as estimated from measurements in ice cores in Greenland and Antarctica. This work should help assess the risk of such abrupt changes in the near future.
The last glacial interval was marked by abrupt climatic changes called Dansgaard-Oeschger (DO) events. These events, characterized by significant temperature increases over Greenland up to 15°C in a few decades and a return to glacial conditions over several centuries, have been repeated many times during the last glacial cycle. However, the cause of these transitions and their out-of-phase relationship with corresponding events in Antarctica remains unclear. Indeed, a satisfactory theory of the Dansgaard-Oeschger cycles was still missing.
The team built a dynamic model to explain these DO events in Greenland, but also their counterparts observed in Antarctica. This model focuses on the interactions between ice sheets from the northern hemisphere ice caps (and more specifically Greenland), sea ice and ocean currents. It demonstrates that the repetitive nature and speed of the warming phase of DO events is based on the rapid retreat and slower regeneration of thick marine ice platforms, several hundred meters, and the much thiner sea ice, only a few meters around the caps of the northern hemisphere.
Variability of the last glacial interval as expressed by changes in oxygen isotope ratio (18O) obtained from two ice cores: blue in Greenland (NGRIP-North GReenland ice core project) and orange in Antarctica (WAIS ice core – West Antarctic ice sheet divide project). Age in thousands of years before the year 2000.
The proposed model successfully replicates the observed characteristics of changes in the oxygen isotope ratio, such as the sawtooth form of DO cycles (with sudden warming and slower cooling to glacier conditions), the intervals between successive DOs during the last 130 000 years, and also the phase shift of the climate signal observed in the ice cores of Greenland and Antarctica: when Greenland warmed, Antarctica cooled, and conversely, the abrupt warmings observed in Greenland not having their counterparts in Antarctica.
In addition, this study provides an explanation for abrupt climate changes and could thus help to more accurately assess the risk of abrupt climate transitions in the near future.
(1) Laboratories and institutes involved: Laboratoire de météorologie dynamique (LMD/IPSL, CNRS / ENS / École Polytechnique / Sorbonne Université), Potsdam institute for climate impact research (Germany), Grantham institute (Imperial college, UK), University of California (USA) and Columbia University (USA)