Cool no More! Mountain Glaciers Recouple to Atmospheric Warming over the Twenty-First Century

Warm atmospheric conditions promote the rise of local microclimates over mountain glaciers and the generation of cool katabatic winds. These “Glacier winds” act to reinforce a shallow boundary layer above melting snow and ice in the summer months, mediating their response to temperature fluctuations in the wider mountain domain. Recent observational studies have highlighted the magnitude to which these temperature fluctuations over glaciers are decoupled from broader temperature changes, promoting cooling that can slow down melting. Nevertheless, they have largely drawn upon individual glacier case studies with little broader quantification of its scale and relevance for glaciers response to climate change.

 

We compiled meteorological observations from > 350 on-glacier automatic weather station records from >60 glaciers around the world over the past three decades. We contrast the air temperature conditions on-glacier with local, off-glacier weather station records to find that above-ice air temperatures warms ~0.73°C on average for every 1°C change in the surrounding mountain atmosphere. Using a combination of available meteorological and topographical data for each glacier in our dataset, we build a statistical model to estimate the magnitude of cooling over mountain glaciers globally. A global estimate of near-surface cooling over mountain glaciers based upon ERA5-Land climatologies for 2000-2022 reveals stark regional variability in how glaciers ‘feel’ temperature warming in the mountains. We demonstrate that larger glaciers in warmer and more humid environments maintain stronger and more persistent glacier winds that promote greater localised cooling. In contrast, on small, fragmenting glaciers in drier environments, especially those where exposure to synoptic winds or increased debris cover is common, glacier winds and localised cooling are limited.  

 

We leverage ensemble climate estimates from socio-economic pathway SSP2-4.5 and SSP5-8.5 scenarios of sixth phase of the Coupled Model Intercomparison Project (CMIP6) as well as published estimates of glacier volume loss until 2100 to provide a first estimate of the expected future glacier cooling under a warmer climate. While glacier winds are expected to increase near-surface cooling under a warmer climate of the 2030’s and 2040’s in several glacier regions of the world, widespread glacier retreat will limit the development of these glacier winds and, ‘recouple’ temperature variability over glaciers to their surroundings and thus enhance the sensitivity of glaciers to broader climatic warming in the latter half of the 21st century. In the European Alps, it is estimated that the period of maximised glacier katabatic wind development has already passed, signalling the expected demise of cooler meteorological conditions there.