Abstract
Abstract. Around the world, small ice caps and glaciers have been losing mass and retreating since the start of the industrial era. Estimates are that this has contributed approximately 30 % of the observed sea-level rise over the same period. It is important to understand the relative importance of natural and anthropogenic components of this mass loss. One recent study concluded that the best estimate of the magnitude of the anthropogenic mass loss over the industrial era was only 25 % of the total, implying a predominantly natural cause. Here we show that the anthropogenic fraction of the total mass loss of a given glacier depends only on the magnitudes and rates of the natural and anthropogenic components of climate change and on the glacier's response time. We consider climate change over the past millennium using synthetic scenarios, palaeoclimate reconstructions, numerical climate simulations, and instrumental observations. We use these climate histories to drive a glacier model that can represent a wide range of glacier response times, and we evaluate the magnitude of the anthropogenic mass loss relative to the observed mass loss. The slow cooling over the preceding millennium followed by the rapid anthropogenic warming of the industrial era means that, over the full range of response times for small ice caps and glaciers, the central estimate of the magnitude of the anthropogenic mass loss is essentially 100 % of the observed mass loss. The anthropogenic magnitude may exceed 100 % in the event that, without anthropogenic climate forcing, glaciers would otherwise have been gaining mass. Our results bring assessments of the attribution of glacier mass loss into alignment with assessments of others aspects of climate change, such as global-mean temperature. Furthermore, these results reinforce the scientific and public understanding of centennial-scale glacier retreat as an unambiguous consequence of human activity.
Highlights
Over the past 150 years, mountain glaciers and ice caps around the world have retreated dramatically, and associated with this retreat has been a loss of ice mass
Images of glacier retreat are an iconic part of the public communication of anthropogenic climate change; and the loss of ice mass has contributed to global sea-level rise (Zemp et al, 2019), hazards in deglaciating alpine environments (Stuart-Smith et al, 2021), and downstream impacts on river flow and water security (Milner et al, 2017)
Many numerical simulations of climate change are initialized at the onset of significant anthropogenic emissions, meaning the middle to late nineteenth century. This leads to uncertainty about whether prior climate history may have left glaciers in a state of substantial disequilibrium at the start of the industrial era, which would affect the subsequent evolution of glacier length and mass balance
Summary
Over the past 150 years, mountain glaciers and ice caps around the world have retreated dramatically, and associated with this retreat has been a loss of ice mass. For the purpose of assessing the anthropogenic contribution to sea-level rise, one must try to estimate human influence on glaciers in the global aggregate To this end, Marzeion et al (2014) use output from pairs of climate model ensembles – one ensemble with all known climate forcings and one with just natural forcing. Many numerical simulations of climate change are initialized at the onset of significant anthropogenic emissions, meaning the middle to late nineteenth century This leads to uncertainty about whether prior climate history may have left glaciers in a state of substantial disequilibrium at the start of the industrial era, which would affect the subsequent evolution of glacier length and mass balance. For all climate scenarios considered, and for glaciers with response times of up to several centuries, we conclude that anthropogenic forcing has been the predominant driver of glacier mass loss since 1850
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