How did that happen when CO2 was "safe"?
There is no evidence to suggest that sea level changes and ice sheet collapse during the current interglacial are unprecedented, unusual, or unnatural in comparison to prior interglacials.
South Greenland ice-sheet collapse during Marine Isotope Stage 11
Varying levels of boreal summer insolation and associated Earth system feedbacks led to differing climate and ice-sheet states during late-Quaternary interglaciations. In particular, Marine Isotope Stage (MIS) 11 was an exceptionally long interglaciation and potentially had a global mean sea level 6 to 13 metres [20-43 feet] above the present level around 410,000 to 400,000 years ago1, 2, implying substantial mass loss from the Greenland ice sheet (GIS). There are, however, no model simulations and only limited proxy data3, 4 to constrain the magnitude of the GIS response to climate change during this ‘super interglacial’5, thus confounding efforts to assess climate/ice-sheet threshold behaviour6, 7 and associated sea-level rise1, 2. Here we show that the south GIS was drastically smaller during MIS 11 than it is now, with only a small residual ice dome over southernmost Greenland. We use the strontium–neodymium–lead isotopic composition of proglacial sediment discharged from south Greenland to constrain the provenance of terrigenous silt deposited on the Eirik Drift, a sedimentary deposit off the south Greenland margin. We identify a major reduction in sediment input derived from south Greenland’s Precambrian bedrock terranes, probably reflecting the cessation of subglacial erosion and sediment transport8 as a result of near-complete deglaciation of south Greenland. Comparison with ice-sheet configurations from numerical models7,9, 10, 11, 12 suggests that the GIS lost about 4.5 to 6 metres of sea-level-equivalent volume during MIS 11. This is evidence for late-Quaternary GIS collapse after it crossed a climate/ice-sheet stability threshold that may have been no more than several degrees above pre-industrial temperatures6, 7.