Comparison of East Antarctic ice core and Mount Moulton δ 18 O records, relative to the early‐Holocene peak value at ∼10 ka. The δ 18 O anomalies at Mount Moulton during the last interglacial period (green shading: MIS 5e interval) are significantly lower than in the East Antarctic records. Purple line: Mount Moulton δ 18 O. Black line: average δ 18 O anomaly of Talos Dome, EDML, Vostok, Dome Fuji, and EDC records from East Antarctica. Dark gray shading: ±1 standard deviation ( σ ) about the East Antarctic mean. Light gray shading: ±2 σ . The ice core records are shown individually in Figure S1 in the supporting information.

Other changes in climate boundary conditions that might be associated with WAIS collapse could influence our results. We assumed that the albedo does not change where the WAIS has been removed; in reality the land ice and ice shelves might be replaced by ocean water that could be free of sea ice in summer. On the other hand, Otto‐Bliesner et al. [ 2013 ] conducted a fully coupled experiment similar to ours (with simpler topography) using CCSM3, in which they replaced WAIS land ice and ice shelves with ocean and used 130 ka boundary conditions. In spite of these differences, their results show the same pattern and magnitude of temperature and winds changes (Figure S4 in the supporting information ). We conclude that cooling in the mountains of Marie Byrd Land, relative to the background climate state, and advection of warm anomalies toward adjacent areas of East Antarctica, are fundamental features of the climate response to a collapse of the WAIS.

The more variable response of the models over the Southern Ocean and most of East Antarctica suggests that the direct effects of WAIS collapse on climate over these regions are small, relative to other processes and feedbacks. In the aquaplanet experiments, large areas over the Southern Ocean cool, while in the fully coupled model results, warming occurs over nearly the entire Southern Ocean. The latter can be attributed to dynamical changes in the ocean due to wind forcing, as has also been observed in a recent WAIS collapse experiment with a coarse‐resolution fully coupled model [ Justino et al. , 2015 ], though we note that there is also warming over much of the Southern Ocean in the slab‐ocean experiment with ECHAM4.6. Meltwater fluxes—not considered in our experiments—may play a role in the ocean circulation response to a real WAIS collapse [ Holden et al. , 2010 ], but this would not alter the fundamental atmospheric response.

Adiabatic warming would be expected as a direct response to the thicker atmosphere where the topography is lowered. If warming were strictly adiabatic, there would be no change in potential temperature; hence, a change in potential temperature implies advection. This is clear in the potential temperature fields (Figure 3 ), indicating that atmospheric circulation changes account for the warming observed over the Ronne‐Filchner Ice Shelf and the Atlantic sector of the Southern Ocean, as well as areas of the ice sheet where the topography has not changed, including coastal Dronning Maud Land and in the vicinity of Hercules Dome. Cooling over the Ross sea and coastal areas of Marie Byrd Land reflects advection of cold‐air anomalies from the East Antarctic plateau.

The large‐scale wind and temperature changes over West Antarctica can be understood as consequences of relatively simple atmospheric dynamics. The cyclonic circulation anomaly over West Antarctica that occurs when the topography is lowered would be expected from potential vorticity conservation, in that stretching of the air column requires that it spin up in the same direction as the planetary vorticity, i.e., cyclonically. James [ 1988 ] showed that linear, barotropic dynamics causes an anticyclonic anomaly over topography when an idealized Antarctic‐like continent is added in a barotropic model; thus, removal of the topography causes a cyclonic anomaly. Watterson and James [ 1992 ] found further support for this purely dynamical mechanism in a primitive equation model. Parish and Cassano [ 2003 ] show that there is little sensitivity of the winds to strong longwave cooling of the continent, implying that the katabatic component of the observed large‐scale winds is weak. These considerations explain why all four models show such similar responses to the removal of topography, despite quite different model details.

4.2 Ice Core Interpretation

Our results have important implications for the interpretation of ice core paleotemperature records. Popp [2008] presaged our results by suggesting that the comparatively low δ18O values for MIS 5e (Figure 4) could be explained by altered atmospheric circulation owing to lowered elevations over the WAIS. Our results clearly support this idea. Given a characteristic scaling of δ18O with temperature of ∼0.8‰/∘C [e.g., Steig et al., 2013], the relatively small δ18O anomaly at Mount Moulton, 1–2‰ less than in the East Antarctic ice cores, is in good agreement with the temperature and δ18O anomalies from the GCM experiments (Figure S3 in the supporting information). The distinctive character of the Mount Moulton isotope record thus supports the idea that WAIS collapsed during MIS 5e. The implication is that the WAIS had already significantly lowered in elevation by ∼130 ka, when the Mount Moulton and East Antarctic isotope records begin to diverge (Figure 4).

There are some important caveats. Direct comparison of the model results for surface temperature with ice core δ18O implicitly assumes a fixed linear relationship. Masson‐Delmotte et al. [2011] made corrections for moisture source (i.e., sea surface) conditions for the East Antarctic records using deuterium excess measurements and found that the temperature difference between the Holocene and the MIS 5e based on δ18O alone is overestimated at some locations and underestimated at others. Deuterium excess data are not available for Mount Moulton, and we do not apply such corrections. While our simulations with ECHAM4.6 (Figure S3 in the supporting information) support the use of δ18O as a temperature proxy, the sensitivity to a variety of potential confounding factors will need to be examined in more detail with additional δ18O‐enabled GCM experiments. Another caveat is that the Mount Moulton record has not only relatively low δ18O values during MIS 5e but also has relatively high δ18O during the coldest part of the glacial period (∼20–40 ka) compared with other records (Figure S1 in the supporting information). Elevated δ18O at Mount Moulton might be expected if the WAIS were thicker than present, but evidence for a substantially thicker WAIS is equivocal [Steig et al., 2001]. Furthermore, other changes associated with MIS 5e climate that are not considered here, including more modest changes in WAIS short of a full “collapse,” may perhaps be sufficient to give rise to the anomalies observed.

Sime et al. [2009] suggested that the varying magnitude of the MIS 5e δ18O peak in East Antarctic ice cores could be explained by different δ18O/temperature sensitivities, owing to higher MIS 5e temperatures. However, they did not examine the possibility that atmospheric circulation changes associated with reduced topography could explain those differences. In our results with ECHAM4.6, the δ18O response (Figure S3 in the supporting information) is stronger at Dome C than at Vostok, consistent with the observations, even though the elevation at neither site changes. This is owing to the reduced topography in the major ice drainages in East Antarctica that accompany WAIS collapse (Figure 1). Such results are sensitive to the ice sheet configuration, but it is clear that topography‐induced atmospheric circulation changes must be accounted for in the interpretation of ice core records, even where the local topography does not change.

Our results provide guidance on the selection of new ice cores that could be used to obtain more definitive evidence for or against WAIS collapse. One interesting site is Fletcher Promontory, which extends from the WAIS into the Ronne‐Filchner Ice Shelf; an ice core record obtained there by the British Antarctic Survey may contain MIS 5e ice. An especially promising location is Hercules Dome, located between the WAIS and the South Pole. Hercules Dome is not likely to have significantly changed in elevation in response to WAIS collapse [Jacobel et al., 2005], but its proximity to the WAIS means that warmer temperatures and elevated δ18O anomalies at that location may be diagnostic of the changed atmospheric circulation associated with WAIS collapse.