DO MODERN RATES OF SEA LEVEL CHANGE FOREWARN IMPENDING ICE SHEET COLLAPSE? (Invited Presentation)

As the rate of sea-level rise continues to accelerate, questions abound as to whether we are nearing or have crossed a critical tipping point for polar ice sheets. Yet, the rate and magnitude of polar ice sheet decay remains one of the most challenging aspects to model in projections of future sea-level rise. One means of addressing this question is to draw upon observations of previous warm periods such as the Last Interglacial to understand rates, magnitudes, and potential tipping points for polar ice sheet retreat.

Here we compare modern rates of sea-level change to rates of sea-level rise derived from fossil reefs in the Seychelles. These corals grew during the Last Interglacial period, when sea level is estimated to have peaked ~6-9 m higher than present as the result of partial retreat of polar ice sheets in both hemispheres.

We present 86 new U-Th ages from fossil corals, which reveal that global mean sea level was established at nearly +6 m at 128.7 ± 0.5 thousand years before present (ka BP). Based on coral ages and elevations alone, sea level appears to steadily rise to a peak at 122.8 ± 0.5 ka BP at a rate that is an order of a magnitude slower than that of the last few decades. However, our detailed sedimentary and stratigraphic analysis reveals two abrupt interruptions to vertical reef accretion, which define three distinct episodes of coral reef growth.

Our chronology reveals that the two discontinuities between reef units are coincident with step-wise ice sheet retreat, which can be temporally linked to abrupt paleoceanographic and climate changes at high latitudes. Both of these discontinuities are associated with evidence for higher rates of sea-level change, which punctuate the overall rising sea-level trend between ~129 and 122 ka. These observations reveal the capacity for step-wise retreat of polar ice in a warming world, resulting in abrupt changes in coral reef assemblages.