Abstract
Abstract. Changes in glacier length reflect the integrated response to local fluctuations in temperature and precipitation resulting from both external forcing (e.g., volcanic eruptions or anthropogenic CO2) and internal climate variability. In order to interpret the climate history reflected in the glacier moraine record, the influence of both sources of climate variability must therefore be considered. Here we study the last millennium of glacier-length variability across the globe using a simple dynamic glacier model, which we force with temperature and precipitation time series from a 13-member ensemble of simulations from a global climate model. The ensemble allows us to quantify the contributions to glacier-length variability from external forcing (given by the ensemble mean) and internal variability (given by the ensemble spread). Within this framework, we find that internal variability is the predominant source of length fluctuations for glaciers with a shorter response time (less than a few decades). However, for glaciers with longer response timescales (more than a few decades) external forcing has a greater influence than internal variability. We further find that external forcing also dominates when the response of glaciers from widely separated regions is averaged. Single-forcing simulations indicate that, for this climate model, most of the forced response over the last millennium, pre-anthropogenic warming, has been driven by global-scale temperature change associated with volcanic aerosols.
Highlights
The length of a glacier reflects both its topographic and climate setting and arises from a balance between accumulation and ablation, mediated by the catchment geometry and the dynamics of ice flow
Changes in climate can arise from three factors: first, natural climate forcings, such as solar luminosity, variations in the Earth’s orbit, and volcanic eruptions that alter the fluxes of energy entering or leaving the climate system; second, anthropogenic forcing, such as emissions of CO2 and industrial aerosols, that affect Earth’s energy budget; and third, the internal climate variability that would occur in the atmosphere–ocean–cryosphere system even under constant external conditions
Comparing the simulated trends in bs over the 20th century with those derived from observed temperatures (Fig. 1b; colored vs. black lines), we find good agreement at South Cascade and Martial Este but not at Silvretta, where the decrease in bs in the Last Millennium Ensemble (LME) ensemble mean is too low by 0.07 m per decade, reflecting a local warming trend that is too low by 0.063 K per decade (Eq 2, with λ = 1.11 m/K)
Summary
The length of a glacier reflects both its topographic and climate setting and arises from a balance between accumulation (mass gain) and ablation (mass loss), mediated by the catchment geometry and the dynamics of ice flow. Baker in Washington state, for example, Roe and O’Neal (2009) found that internal variability alone can produce kilometer-scale excursions in glacier length on multi-decadal and centennial timescales This result highlights the importance of considering internal variability in addition to forced climate change as a potential cause of past variations in glacier length. No systematic assessment of the relative importance of forced versus internal variability has been conducted Both the local magnitude and the spatial coherence of glacier-length variability have been studied for specific regions using existing observations. The analyses highlight the importance of glacier response time: shortresponse-time (i.e., less than a few decades) glaciers have uncorrelated length fluctuations driven predominantly by regional, internal climate variability (a low SNR), whereas longer-response-time glaciers respond coherently to external changes in climate forcing, predominantly associated with volcanic forcing (a high SNR). We extend our analyses to a global network of 76 well-observed glaciers to assess the coherence of glacier response among, and within, individual glacierized regions
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