The phase space of last glacial inception for the Northern Hemisphere from coupled ice and climate modelling

Abstract

Abstract. We present an ensemble of Last Glacial Inception (LGI) simulations for the Northern Hemisphere that largely captures inferred ice volume changes within proxy uncertainties. This ensemble was performed with LCice 1.0, a coupled ice sheet and climate model, varying parameters of both climate and ice sheet components, as well as the coupling between them. Certain characteristics of the spatio-temporal pattern of ice growth and subsequent retreat in both North America (NA) and Eurasia (EA) are sensitive to parameter changes, especially with respect to regional rates of ice growth and retreat. We find that the initial inception of ice over NA and EA is best characterized by the nucleation of ice at high latitude and high elevation sites. Subsequent spreading and merger along with large-scale conversion of snow fields dominate in different sectors. The latter plays an important role in the merging of eastern and western ice regions in NA. The inception peak ice volume in the ensemble occurs approximately at 111 ka and therefore lags the summer 60° N insolation minimum by more than 3 kyr. Ice volumes consistently peak earlier over EA than NA. The inception peak in North America is characterized by a merged Laurentide and Cordilleran ice sheet, with Davis Strait covered in ice in 80 % of simulations. Ice also bridges Greenland and Iceland in all runs by 114 ka and therefore blocks Denmark Strait. This latter feature would thereby divert the East Greenland Current and Denmark Strait overflow and thereby potentially have a significant impact on ocean circulation. The Eurasian ice sheet at its inception peak varies across ensemble runs between a continuous ice sheet to multiple smaller ice caps. In both continents, the colder high latitudes (Ellsmere and Svalbard) tend to grow ice through the entire simulation (to 102 ka), while lower latitudes lose ice after 110 ka. We find temperature decreases over the initial phases of the inception lead to the expansion of NA ice sheet area, and that subsequent precipitation increases contribute to its thickening. EA ice sheet area also expands with decreasing temperatures, but sea ice limits any increases in precipitation, leading to an earlier retreat away from the EA maximum ice sheet volume. We also examine the extent to which the capture of both LGI ice growth and retreat constrains the coupled ice/climate model sensitivity to changing atmospheric pCO2. For a standard transient climate response experiment (1 % increase in pCO2 until doubled), warming ranges between 0.6–2.0 °C for our initial set of 500 simulations without LGI constraint. The warming is reduced to 0.7–1.4 °C for the 55 member ensemble that captures both LGI ice growth and retreat. This therefore underlines the potential value of fully coupled ice/climate modelling of last glacial inception to constrain future climate change.