The mid-Pliocene warm period (MPWP, 3.3-2.9 Ma), with reconstructed atmospheric pCO(2) of 350-450 ppm, represents a potential analogue for climate change in the near future. Current highly cited estimates place MPWP maximum global mean sea level (GMSL) at 21 +/- 10 m above modern, requiring total loss of the Greenland and marine West Antarctic Ice Sheets and a substantial loss of the East Antarctic Ice Sheet, with only a concurrent 2-3 degrees C rise in global temperature. Many estimates of Pliocene GMSL are based on the partitioning of oxygen isotope records from benthic foraminifera (delta O-18(b)) into changes in deep-sea temperatures and terrestrial ice sheets. These isotopic budgets are underpinned by the assumption that the delta O-18 of Antarctic ice ((delta O-18(i)) was the same in the Pliocene as it is today, and while the sensitivity of delta O-18(b) to changing meltwater delta O-18 has been previously considered, these analyses neglect conservation of O-18/O-16 in the ocean-ice system. Using well-calibrated delta O-18-temperature relationships for Antarctic precipitation along with estimates of Pliocene Antarctic surface temperatures, we argue that the delta O-18(i) of the Pliocene Antarctic ice sheet was at minimum 1%-4% higher than present. Assuming conservation of O-18/O-16 in the ocean-ice system, this requires lower Pliocene seawater delta O-18 without a corresponding change in ice sheet mass. This effect alone accounts for 5%-20% of the delta O-18(b) difference between the MPWP interglacials and the modern. With this amended isotope budget, we present a new Pliocene GMSL estimate of 9-13.5 m above modern, which suggests that the East Antarctic Ice Sheet is less sensitive to radiative forcing than previously inferred from the geologic record.
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