Exploring new glacial perspectives on tenets of abrupt climate change: North Atlantic stadials and the bi-polar seesaw

The recognition by 19^th Century science that glaciers not only move but were, at various times in the past, big enough to submerge continents gave rise to Ice Age Theory and revolutionised our understanding of the Earth system, demonstrating that climate can – and does – change. In the intervening years, however, glacial geology and the physical record of cryospheric growth and decay have been largely relegated to playing a supporting role to higher-resolution, better-dated palaeoclimate proxies that today dominate conversation around ice age cycles, the causes and impacts of abrupt climate change, and the nature of climate tipping points. For instance, ice cores revealed the dramatic shifts in atmospheric conditions during Dansgaard-Oeschger and Heinrich Stadial events, while marine geochemistry tells us of the ocean’s role as a dynamic CO₂ reservoir and global heat capacitor. Two key concepts to have emerged from palaeoclimatology include (1) that of North Atlantic ‘stadial’ events as periods of intense regional cooling, typically with fast onset and rapid termination, and (2) the existence of a bipolar seesaw, in which cooling (warming) in one hemisphere drives relative warming (cooling) in the other. Both are deeply rooted in modern conceptual models of abrupt climate change and incorporated in numerical model projections of future climate. Here, we draw from recent refinements in cosmogenic nuclide geochronology and a growing database of well-dated Late Pleistocene moraine records to explore how well these two seminal concepts stand up to scrutiny from a reinvigorated glacial perspective. Exploiting the sensitive yet innately straightforward relationship between melt season (summer) temperature and glacier mass balance, this emerging glacial record paints a fascinating picture of ‘stadial’ climate that contrasts with the traditional view of these severe perturbations as year-round cold anomalies driven by AMOC. We highlight strong similarities between our North Atlantic glacier records and those from other regions globally and propose that anomalous thermal seasonality in the North Atlantic is a regional by-product of overall global warming. The ramifications of this hypothesis extend far beyond the North Atlantic: the interhemispheric bi-polar seesaw hypothesis rests on the coincidence of apparent Northern Hemisphere stadial cooling and Southern Hemisphere warming. Should the pattern of globally uniform glacier (and thus climate) behaviour invoked here prove an accurate representation of atmospheric conditions during abrupt climate shifts, the physical basis for a bipolar seesaw mechanism is undermined.