In the future's warmer climate, large, abrupt and frequent changes in ocean ventilation may be more likely than currently assumed, according to a new study. While current climate assessments recognize the severe impact that disruption of North Atlantic Deep Water (NADW) ventilation would have on global climate, disruption to this ocean phenomenon is generally considered to be a low probability tipping point. Now, results from this study suggest a need to reevaluate the long-held notion of warm-climate NADW stability and to consider rapid changes as a possible - if not intrinsic - feature of variability during warm climates. Atlantic meridional overturning circulation (AMOC) and NADW ventilation play a crucial role in mediating the distribution and storage of atmospheric carbon dioxide and heat, as well as in mediating Atlantic-spanning climate patterns. Previous modeling studies and proxy reconstructions of deep ocean circulation during the last several glaciations have suggested relative stability during the warmer interglacial periods. Eirik Galaasen and colleagues present a new high-resolution ocean sediment record recovered from the ocean bottom off the southern tip of Greenland, which provided a 500,000-year record of deep ocean circulation. The high sedimentation rate at the core site allowed the authors to evaluate AMOC and NADW variation at a sub-centennial resolution. Galaasen et al. discovered that disruptions to NADW stability have been relatively frequent during warm climate interglacials. Importantly, the results indicate that large perturbations in ocean circulation are more easily triggered than previously thought and have occurred in past climate conditions similar to those projected for the future. According to Thomas Stocker in a related Perspective, a comprehensive assessment concerning these tipping points and their risks is lacking. "To provide robust and actionable information to decision-makers and people, this should also be a priority for the 7th assessment cycle of the IPCC," Stocker writes.
