Closing the sea level budget at the Last Glacial Maximum.

Establishing the volume of excess ice contained in the global ice sheets during the Last Glacial Maximum (LGM; ∼26,000–19,000 y ago) remains a longstanding problem in Ice Age climate dynamics. Expressed as the equivalent lowering of global mean sea level (GMSL), estimates of this value have varied from 105 (1) to 163 m (2), with many estimates suggesting ∼120 m (3). This wide range introduces substantial uncertainties in LGM global boundary conditions (ice sheet height and extent and continental shelf exposure) that strongly influence climate through their impacts on atmospheric and ocean circulation and global temperature. More recently, this uncertainty has influenced our understanding of present sea level change, whereby the ongoing glacial isostatic adjustment (GIA) of the solid Earth to the redistribution of mass since the LGM must be accounted for in satellite measurements of sea level and of current mass changes from shrinkage of glaciers and ice sheets. In PNAS, Lambeck et al. (4) present a comprehensive analysis of nearly 1,000 paleo-sea level markers to reconstruct GMSL change over the last 35,000 y, with their estimate for the LGM GMSL of 134–140 m being significantly larger than most published estimates but similar to a recent reassessment of the Barbados sea level record (130 m) that had been the basis for many estimates of ∼120 m (5). If it stands, this reconstruction will reduce uncertainties in our understanding of Ice Age climate and modern sea level change. At the same time, however, it raises a significant challenge: is it possible to explain this total sea level change from our best estimates of the contributions from individual LGM glaciers and ice sheets? There are two common approaches to deriving information on LGM sea level. One is to use ice sheet models to reconstruct LGM glaciers and ice sheets, but thus far, there has been only one systematic effort at reconstructing all LGM ice sheets and glaciers in this way (2). Most glaciological models to date, however, ignore far-field relative sea level (RSL) records that best represent GMSL changes. Therefore, there is no guarantee that a sum of individual ice sheet reconstructions will match far-field inferences. The other approach, taken by Lambeck et al. (4), is to invert far-field RSL records to infer GMSL changes and then distribute the equivalent mass among the various ice sheets. If the sum of the individual reconstructions from ice sheet models equals the GMSL change derived from far-field sites, the sea level budget is closed. If the budget is not closed, then the individual ice sheet reconstructions or the geophysical analysis and far-field data used to reconstruct GMSL must be reassessed. Lambeck et al. (4) use a standard iterative approach to reconstruct GMSL from far-field sites that involves estimating an initial global ice volume from those sites, distributing that mass among the different global ice sheets using geological and glaciological constraints, inverting the far-field records for information that improves the estimate of global ice volume, and assessing the ice sheet reconstructions from near-field sea level data and redistributing the mass as needed. This iterative approach is repeated until convergence is reached. There is one important caveat to their particular implementation, however, whereby the difference between the farfield–derived global value and the sum of the glaciers and Northern Hemisphere ice sheets is simply distributed within the Antarctic ice sheet and is not assessed with any near-field data. A necessary outcome of this approach is that it derives the earth rheology parameters (lithosphere thickness and mantle viscosity) that determine the GIA response. Despite there being a long history of inverting RSLs for GIA from the last Ice Age for this purpose (6), the parameters continue to be debated, particularly for the lower mantle viscosity (7, 8). Interestingly, Lambeck et al. (4) find two equally possible solutions for this parameter, reflecting a low-viscosity (LMlv) vs. a highviscosity (LMhv) lower mantle, that bracket the debated values, which leads to the reported range of LGM GMSL. Lambeck et al. (4) remain largely neutral on this result, other than to favor the LMhv value Fig. 1. (A) Published reconstructions of the ice equivalent sea level in ice sheets and glaciers at the LGM. In each case, the red squares are the values inferred from Lambeck et al. The horizontal red bar at 134–140 m is the value of GMSL lowering at the LGM derived by Lambeck et al. (4). GL, glaciers. (B) Reconstructions of the ice equivalent sea level in the AIS published since 1994. The red squares are the two solutions derived by Lambeck et al. (4). Glaciological reconstructions generally do not account for changing ocean area in their stated equivalent sea level contributions, thus inducing slightly smaller values than those given by most GIA-based reconstructions.