The relative contribution of orbital forcing and greenhouse gases to the North American deglaciation

Abstract

Understanding what drove Northern Hemisphere ice sheet melt during the last deglaciation (21–7 ka) can help constrain how sensitive contemporary ice sheets are to greenhouse gas (GHGs) changes. The roles of orbital forcing and GHGs in the deglaciation have previously been modeled but not yet quantified. Here for the first time we calculate the relative effect of these forcings on the North American deglaciation by driving a dynamical ice sheet model (GLIMMER‐CISM) with a set of unaccelerated transient deglacial simulations with a full primitive equation‐based ocean‐atmosphere general circulation model (FAMOUS). We find that by 9 ka, orbital forcing has caused 50% of the deglaciation, GHG 30%, and the interaction between the two 20%. Orbital forcing starts affecting the ice volume at 19 ka, 2000 years before CO2 starts increasing in our experiments, a delay which partly controls their relative effect.

Holocene lowering of the Laurentide ice sheet affects North Atlantic gyre circulation and climate

“…In previous studies, the evolution of ice sheet topography has been introduced as step changes (e.g. Gregoire et al 2015;Liu et al 2009), but as we show here, ice sheet changes that occur on a 500 year time scale can cause significant climate changes in regions remote from the ice sheet. Therefore, prescribing such ice sheet evolution as step changes at intervals of 500-1000 years or more may introduce artificial abrupt adjustments of the atmosphere and ocean in the North Atlantic sector.…”

Deglaciation of the Laurentide Ice Sheet from the Last Glacial Maximum

“…Bryson et al, 1969;Denton and Hughes, 1981;Boulton et al, 1995;Dyke and Prest, 1987;Dyke, 2004), augmented by numerical modelling, which has seen rapid developments over the last two decades (Marshall et al, 1996;Marshall et al, 2000;Tarasov et al, 2012;Peltier et al, 2015, Gregoire et al, 2015b. This body of work shows a clear asymmetry in retreat whereby the western and southern margins retreated back towards the major dispersal centres over Foxe Basin-Baffin Island and QuebecLabrador.…”

“…These ages dated the initial retreat of the ice margin from the LLGM moraines to 23.0 ± 0.6 ka, which they noted was synchronous with several other locations along the southern margin and coincided with the initial increase in summer insolation around 24-23 ka. They also pointed out that an acceleration in retreat after around 20.5 ka was likely driven by an acceleration in boreal summer insolation and that this occurred before any increase in atmospheric CO 2 , supporting an orbital forcing as the trigger for initial deglaciation Gregoire et al, 2015b). This response of the ice sheet to atmospheric forcing also implies a higher sensitivity of land-terminating margins to small changes in climate forcing than had hitherto been recognised, although it should be noted that overall recession of the ice sheet was minimal (see Section 3.1).…”

“…A further explanation for the asymmetric retreat may also relate to the release and routing of meltwater during deglaciation and its subsequent impact on ocean circulation (Carlson et al, 2009b;Hoffman et al, 2012;Jennings et al, 2015;Gregoire et al, 2015b). Climate modelling indicates that meltwater discharge events routed into the Labrador Sea (e.g.…”

“…involving albedo, CO 2 and oceanic feedbacks), including a delayed crustal rebound, which keeps the ice sheet elevation relatively low and increases ablation (Abe-Ouchi et al, 2013). Recent modelling by Gregoire et al (2015b) has attempted to partition the influence of increasing greenhouse gases (GHGs, e.g. CO 2 ) and orbital forcing on North American deglaciation.…”