Andrea Dutton

How fast future sea level rises will depend on the Antarctic Ice Sheet (AIS) response to warming. AIS projections are shaped by the assumption that sea-level peaks during past interglacials occurred after the North American ice sheet complex (NAIS) disappeared. We synthesize evidence from paleoceanography and allied disciplines to argue that NAIS persisted into some of the warmest interglacials of the last million years. We show that overlooking NAIS persistence may lead to underestimation of AIS sensitivity to warming, and propose that this paradigm shift opens research avenues that can increase confidence in the accuracy of climate and sea-level projections.

Rapid, millennial-scale changes in sea level have been proposed for the beginning, middle, and/or end of the Last Interglacial (LIG) [~129 to 116 thousand years ago (ka)]. Each of these scenarios has different implications for polar ice sheet behavior in a warming world. Here, we present a suite of ²³⁰ Th ages for fossil corals in the Seychelles within a detailed sedimentary and stratigraphic context to evaluate the evolution of sea level during this past warm period. The rise to peak sea level at ~122 to 123 ka was punctuated by two abrupt stratigraphic discontinuities, defining three distinct generations of reef growth. We attribute the evidence of episodic reef growth and ephemeral sea-level fall to the competing influence of Northern Hemisphere ice melt and Antarctic ice regrowth. Asynchronous ice sheet contributions would mask the full extent of retreat for individual ice sheets during the LIG and imply greater temperature sensitivity of ice sheets than previously inferred.

Mass loss from ice sheets in Greenland and Antarctica has quadrupled since the 1990s and now represents the dominant source of global mean sea-level rise from the cryosphere. This has raised concerns about their future stability and focussed attention on the global mean temperature thresholds that might trigger more rapid retreat or even collapse, with renewed calls to meet the more ambitious target of the Paris Climate Agreement and limit warming to +1.5 °C above pre-industrial. Here we synthesise multiple lines of evidence to show that +1.5 °C is too high and that even current climate forcing (+1.2 °C), if sustained, is likely to generate several metres of sea-level rise over the coming centuries, causing extensive loss and damage to coastal populations and challenging the implementation of adaptation measures. To avoid this requires a global mean temperature that is cooler than present and which we hypothesise to be closer to +1 °C above pre-industrial, possibly even lower, but further work is urgently required to more precisely determine a ‘safe limit’ for ice sheets.

The connection between abrupt high-latitude warming during the last glacial period and rapid temperature/hydrologic changes at lower latitudes has revealed strong inter-hemispheric teleconnections in the ocean–atmosphere system. How this connection influenced climate in the midcontinent of North America is unclear because climate archives with sufficient time resolution are scarce. This study investigates paleoclimate changes ~70–45 thousand years ago (ka) across multiple timescales (sub-annual to millennial) from speleothems in southern Wisconsin and Minnesota (USA). To do this we use fluorescent imaging (confocal laser microscopy) and a secondary ion mass spectrometer (SIMS) to collect high-resolution δ18O measurements (5-10µm) across individual stalagmite growth bands. Annual growth bands are 20-80 µm in width, allowing for subannual sampling resolution. Abrupt climate changes that were previously identified in a stalagmite from this region (Batchelor et al. 2023) are recorded as several large (2.0 to 3.0‰), negative δ18O excursions that occurred between 61–55 ka, coinciding with Dansgaard–Oeschger (DO) events 17–14. Here we extend the record from our prior work by 10 ka from a higher accumulation rate speleothem. Our high-resolution δ18O SIMS data show similar large (1 to 1.5‰) δ18O shifts during DO events 13 and 12 that occur in less than 30 years. When comparing the intra-annual δ18O gradient measured in different fluorescent bands, we observe a weaker gradient during DO interstadials, and a stronger gradient during DO stadials suggesting that seasonal changes in the hydrological system are also occurring during these events. Our results demonstrate large, abrupt temperature and hydrological responses to DO events during the last glacial period in midcontinent North America and indicate a strong seasonal climate response to DO warming and cooling. At the meeting we will present our published and unpublished measurements and related modeling simulations.