On the attribution of industrial-era glacier mass loss to anthropogenic climate change

Abstract

Abstract. Around the world, small ice caps and glaciers have been\nlosing mass and retreating since the start of the industrial era. Estimates are that\nthis has contributed approximately 30\u2009% of the observed sea-level rise\nover the same period. It is important to understand the relative importance\nof natural and anthropogenic components of this mass loss. One recent study\nconcluded that the best estimate of the magnitude of the anthropogenic mass\nloss over the industrial era was only 25\u2009% of the total, implying a\npredominantly natural cause. Here we show that the\nanthropogenic fraction of the total mass loss of a given glacier depends\nonly on the magnitudes and rates of the natural and anthropogenic components\nof climate change and on the glacier s response time. We consider climate\nchange over the past millennium using synthetic scenarios, palaeoclimate\nreconstructions, numerical climate simulations, and instrumental\nobservations. We use these climate histories to drive a glacier model that\ncan represent a wide range of glacier response times, and we evaluate the\nmagnitude of the anthropogenic mass loss relative to the observed mass loss.\nThe slow cooling over the preceding millennium followed by the rapid\nanthropogenic warming of the industrial era means that, over the full range\nof response times for small ice caps and glaciers, the central estimate of\nthe magnitude of the anthropogenic mass loss is essentially 100\u2009% of the\nobserved mass loss. The anthropogenic magnitude may exceed 100\u2009% in the\nevent that, without anthropogenic climate forcing, glaciers would otherwise\nhave been gaining mass. Our results bring assessments of the attribution of\nglacier mass loss into alignment with assessments of others aspects of\nclimate change, such as global-mean temperature. Furthermore, these results\nreinforce the scientific and public understanding of centennial-scale\nglacier retreat as an unambiguous consequence of human activity.