The Intergovernmental Panel on Climate Change (IPCC) estimates that the sum of all contributions to sea‐level rise for the period 1961–2004 was 1.1 ± 0.5 mm a−1, leaving 0.7 ± 0.7 of the 1.8 ± 0.5 mm a−1 observed sea‐level rise unexplained. Here, we compute the global surface mass balance of all mountain glaciers and ice caps (MG&IC), and find that part of this much‐discussed gap can be attributed to a larger contribution than previously assumed from mass loss of MG&IC, especially those around the Antarctic Peninsula. We estimate global surface mass loss of all MG&IC as 0.79 ± 0.34 mm a−1 sea‐level equivalent (SLE) compared to IPCC's 0.50 ± 0.18 mm a−1. The Antarctic MG&IC contributed 28% of the global estimate due to exceptional warming around the Antarctic Peninsula and high sensitivities to temperature similar to those we find in Iceland, Patagonia and Alaska.
Future climate warming is predicted to be more pronounced in the Arctic where approximately two-thirds of all small glaciers on Earth are located. A simple mass-balance model was applied to 42 glaciers and ice caps north of 608 N to estimate mass-balance sensitivities to a hypothetical climate perturbation. The model is based on daily temperature and precipitation data from climate stations in the vicinity of each glacier and ice cap. A regression analysis was made using a degree-day approach where the annual sum of positive daily air temperatures was correlated to measured summer mass balance, and the total annual snow precipitation was correlated to measured winter mass balance. The net mass-balance sensitivity to a hypothetical temperature increase of +1 K ranged from -0.2 to -2.0 m a -1 , and an assumed increase in precipitation of +10% changed the mass balance by < +0.1 to +0.4 m a -1 , thus on average offsetting the effect of a temperature increase by approximately 20%. Maritime glaciers showed considerably higher mass-balance sensitivities than continental glaciers, in agreement with similar previous studies. The highest sensitivities were found in Iceland, exceeding those reported in previous studies. Extrapolating our results, glaciers and ice caps north of 608 N are estimated to contribute $0.6 mm a -1 K -1 to global sea-level rise. Our results highlight the value of long-term massbalance records and meteorological records in remote areas.
Estimates of glacier contributions to future sea-level rise are often computed from massbalance sensitivities derived for a set of representative glaciers. Our purpose is to investigate how mass-balance projections and sensitivities vary when using different approaches to compute the glacier mass balance. We choose Storglaciären, Sweden, as a test site and apply five different models including temperature-index and energy-balance approaches further varying in spatial discretization. The models are calibrated using daily European Centre for Medium-Range Weather Forecasts re-analysis (ERA-40) data. We compute static mass-balance sensitivities and cumulative mass balances until 2100 based on daily temperatures predicted by a regional climate model. Net mass-balance sensitivities to a +1 K perturbation and a 10% increase in precipitation spanned from -0.41 to -0.61 and from 0.19 to 0.22 m a -1 , respectively. The cumulative mass balance for the period 2002-2100 in response to the climate-model predicted temperature changes varied between -81 and -92 m for four models, but was -121 m for the fully distributed detailed energy-balance model. This indicates that mass losses may be underestimated if temperature-index methods are used instead of detailed energy-balance approaches that account for the effects of temperature changes on all energy-balance components individually. Our results suggest that future glacier predictions are sensitive to the choice of the mass-balance model broadening the spectrum in uncertainties.
Abstract:A mass balance-runoff model is applied to Hofsjökull, an 880 km 2 ice cap in Iceland, in order to assess the importance of the firn layer for glacial runoff. The model is forced by daily temperature and precipitation data from a nearby meteorological station. Water is routed through the glacier using a linear reservoir model assuming different storage constants for firn, snow and ice. The model is calibrated and validated using mass balance data and satellite-derived snow facies maps. Simulated mass balances and snowline retreats are generally in good agreement with observations. Modelled cumulative mass balance for the entire ice cap over the period 1987/1988 to 2003/2004 is 7Ð3 m, with uninterrupted negative mass balances since 1993/1994. Perturbing the model with a uniform temperature (C1 K) and precipitation (C10%) increase yields static mass balance sensitivities of 0Ð95 m a 1 and C0Ð23 m a 1 respectively. Removing the firn layer under otherwise likewise conditions results in almost unchanged total runoff volumes, but yields a redistribution of discharge within the year. Early summer discharge (June to mid August) is amplified by roughly 5-10%, whereas late-summer/autumn discharge (mid August to November) is reduced by 15-20% as a result of accelerated water flow through the glacial hydrological system. In comparison, applying a climate-model-based temperature and precipitation scenario for Iceland until 2050 results in higher runoff throughout the year, increasing total runoff by roughly one-third. The results emphasize the role of the firn layer in delaying water flow through glaciers, and the influence on discharge seasonality when firn areas shrink in response to climate-change-induced glacier wastage.
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