Oceanic forcing of penultimate deglacial and last interglacial sea-level rise

Abstract

Sea-level histories during the two most recent deglacial-interglacial intervals experienced significant differences1-3 despite both periods having similar changes in global mean temperature4,5 and forcing from greenhouse gases6. Although the last interglaciation (LIG) experienced stronger boreal summer insolation forcing than during the present interglaciation7, understanding why LIG global mean sea level may have been 6-9 m higher than present has proven particularly challenging2. During glacial as well as interglacial periods, extensive areas of polar ice sheets were grounded below sea level, with grounding lines and fringing ice shelves extending onto continental shelves8, suggesting that oceanic forcing by subsurface warming may also have contributed to ice-sheet loss9-12 analogous to ongoing changes by the Antarctic13,14 and Greenland15 ice sheets. Such forcing would have been especially effective during glacial periods when the Atlantic Meridional Overturning Circulation (AMOC) experienced large variations on millennial timescales16, with a reduction of the AMOC causing subsurface warming throughout much of the Atlantic basin9,12,17. Here we show that greater subsurface warming induced by the longer duration of reduced AMOC during the penultimate deglaciation can explain the more-rapid sea-level rise than during the last deglaciation. This greater forcing also contributed to excess loss from the Greenland and Antarctic ice sheets during the LIG, causing global mean sea level to rise at least 4 m above modern. When accounting for the combined influences of penultimate and last-interglacial deglaciation on glacial isostatic adjustment, this excess loss of polar ice during the LIG can explain much of the relative sea level recorded by fossil coral reefs and speleothems at intermediate- and far-field sites.