Western Greenland ice sheet retreat history reveals elevated precipitation during the Holocene thermal maximum

Abstract

Abstract. We investigate changing precipitation patterns in the Kangerlussuaq region of western central Greenland during the Holocene thermal maximum (HTM), using a new chronology of ice sheet terminus position through the Holocene and a novel inverse modeling approach based on the unscented transform (UT). The UT is applied to estimate changes in annual precipitation in order to reduce the misfit between modeled and observed terminus positions. We demonstrate the effectiveness of the UT for time-dependent data assimilation, highlighting its low computational cost and trivial parallel implementation. Our results indicate that Holocene warming coincided with elevated precipitation, without which modeled retreat in the Kangerlussuaq region is more rapid than suggested by observations. Less conclusive is whether high temperatures during the HTM were specifically associated with a transient increase in precipitation, as the results depend on the assumed temperature history. Our results highlight the important role that changing precipitation patterns had in controlling ice sheet extent during the Holocene.

Cosmogenic isotope measurements from recently deglaciated bedrock as a new tool to decipher changes in Greenland Ice Sheet size

“…17). The higher precipitation scenario used in run 7 is broadly supported by proxy evidence of enhanced wintertime snowfall in southwestern Greenland and inferred precipitation using an ice-sheet flowline model (Thomas et al, 2018;Downs et al, 2020). Despite early Holocene warming, limited evidence suggests that this warmth, and its effect on ice-sheet mass balance, may be offset to some degree by increased precipitation.…”

“…The better model-data fit in the Kangerlussuaq region compared to the KNS region is likely the result of distinctly different ice-margin environments. The Kangerlussuaq region hosts what is primarily a land-terminating sector of the GrIS where ice-sheet behavior is dominated by surface mass balance (Cuzzone et al, 2019;Downs et al, 2020). Thus, ice-sheet behavior in the Kangerlussuaq region is almost entirely dictated by the climate forcings used in the model runs (Briner et al, 2020).…”

Millennial-scale migration of the frozen/melted basal boundary, western Greenland ice sheet

“…As a result, it is unknown to what extent the ice sheet may have retreated beyond its modern position during this time period, and the predictions from the inverse model are limited. Since our approach here is to closely honor the climate and terminus records from Downs' and others (2020) study, this limitation is carried through into this work.…”

“…assumed to be a constant fraction of ice overburden pressure P 0 , taken to be 85% of overburden based on observational constraints of basal water pressure (Wright and others, 2016). Effective pressure is then given by β 2 and was found by minimizing the misfit between modeled and observed surface velocities (Downs and others, 2020)…”

“…Because the outer flanks of western GrIS contain numerous deep and steep-sided subglacial troughs, we focus our investigation on a single flowline transect so that we can employ high time/space resolution in our model. Our transient model includes higher order stresses, a well-informed paleoclimate (Buizert and others, 2018;Downs and others, 2020), thermally active bedrock, and honors previously published records of terminus retreat (Briner and others, 2016;Lesnek and others, 2020;Young and others, 2020). We track the constitutive components of the basal heat budget during the transient simulation to isolate the energy sources and sinks associated with the migration of frozen/melted bed conditions.…”