Abstract
Glaciers and ice sheets are experiencing dramatic changes in response to recent climate change. This is true in both mountain and polar regions, where the extreme sensitivity of the cryosphere to warming temperatures may be exacerbated by amplification of global climate change. For glaciers and ice sheets, this sensitivity is due to a number of non-linear and threshold processes within glacier mass balance and glacier dynamics. Some of this is simply tied to the freezing point of water; snow and ice are no longer viable above 0°C, so a gradual warming that crosses this threshold triggers the onset of melting or gives rise to an abrupt regime shift between snowfall and rainfall. Other non-linear, temperature-dependent processes are more subtle, such as the evolution from polythermal to temperate ice, which supports faster ice flow, a shift from meltwater retention to runoff in temperate or ice-rich (i.e., heavily melt-affected) firn, and transitions from sublimation to melting under warmer and more humid atmospheric conditions. As melt seasons lengthen, there is also a longer snow-free season and an expansion of glacier ablation area, with the increased exposure of low-albedo ice non-linearly increasing melt rates and meltwater runoff. This can be accentuated by increased concentration of particulate matter associated with algal activity, dust loading from adjacent deglaciated terrain, and deposition of impurities from industrial and wildfire activity. The loss of ice and darkening of glaciers represent an effective transition from white to grey in the world's mountain regions. This article discusses these transitions and regime shifts in the context of challenges to model and project glacier and ice sheet response to climate change.
Highlights
The IPCC (2019) special report on oceans and the cryosphere in a changing climate draws attention to a number of non-linear changes to the cryosphere that are under way as the climate warms (Collins et al, 2019)
That is not to say that the Greenland Ice Sheet was insensitive to this warming; Yau et al (2016) estimate that the ice sheet lost about 70% of its current volume, the equivalent of ∼5 m of global sea level, before the ice sheet fully re-established itself during the early stages of the last glaciation
Extending the results reported by Ochwat et al (2021), Figure 2 plots simulated firn temperature evolution on upper Kaskawulsh Glacier for the period 1950–2020, using the coupled thermodynamic and hydrological model described in section Methods for the upper 35 m of firn
Summary
The IPCC (2019) special report on oceans and the cryosphere in a changing climate draws attention to a number of non-linear changes to the cryosphere that are under way as the climate warms (Collins et al, 2019). Sea ice loss and melting of the Greenland Ice Sheet have sometimes been described as “tipping elements,” vulnerable to irreversible decline beyond a particular climate threshold (e.g., Lenton et al, 2008; Ridley et al, 2010; King et al, 2020). Different sectors of the Greenland Ice Sheet have different climate sensitivities, so there can be large-scale decline of the ice sheet without a complete loss of ice. The survival of the central dome of the ice sheet in the last interglacial period is testament to this, as the core of the ice sheet proved resilient to temperatures 6 ± 2◦C warmer than present, sustained for several millennia (NEEM Community Members, 2013; Yau et al, 2016). The Greenland Ice Sheet was greatly impacted by the Eemian warming, but ice sheet retreat was neither inexorable nor irreversible
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