Reconstructing the evolution of sea level during past warmer epochs such as the Pliocene provides insight into the response of sea level and ice sheets to prolonged warming. Although estimates of global mean sea level (GMSL) during this time do exist, they vary by several tens of metres, hindering the assessment of past and future ice-sheet stability. Here we show that during the mid-Piacenzian Warm Period, which was on average 2-3 degrees Celsius warmer than the pre-industrial period, the GMSL (sea-level equivalent changes in global ice volume) was about 16.2 metres higher than today. During the even warmer Pliocene Climatic Optimum (about 4 degrees Celsius warmer than pre-industrial), our results show that GMSL was 23.5 metres above the present level (m.a.p.s.l.). We present six GMSL data points, ranging from 4.39 to 3.27 million years ago, that are based on overgrowths on phreatic (from water-filled caves) speleothems from the western Mediterranean Sea, near Mallorca, Spain. This record is unique owing to its clear relationship to sea level, its reliable U-Pb ages and its long timespan, which allows us to quantify uncertainties on potential uplift. Our data indicate that ice sheets are very sensitive to warming and provide important calibration targets for future ice-sheet models.

Supplement to: Dumitru, Oana-Alexandra; Austermann, Jacqueline; Polyak, Victor J; Fornós, Joan J; Asmerom, Yemane; Ginés, Joaquín; Ginés, Angel; Onac, Bogdan P (2019): Constraints on global mean sea level during Pliocene warmth. Nature