Valentina Radić

The multimillennial sea-level commitment of global warming

Global mean sea level has been steadily rising over the last century, is projected to increase by the end of this century, and will continue to rise beyond the year 2100 unless the current global mean temperature trend is reversed. Inertia in the climate and global carbon system, however, causes the global mean temperature to decline slowly even after greenhouse gas emissions have ceased, raising the question of how much sea-level commitment is expected for different levels of global mean temperature increase above preindustrial levels. Although sea-level rise over the last century has been dominated by ocean warming and loss of glaciers, the sensitivity suggested from records of past sea levels indicates important contributions should also be expected from the Greenland and Antarctic Ice Sheets. Uncertainties in the paleo-reconstructions, however, necessitate additional strategies to better constrain the sea-level commitment. Here we combine paleo-evidence with simulations from physical models to estimate the future sea-level commitment on a multimillennial time scale and compute associated regional sea-level patterns. Oceanic thermal expansion and the Antarctic Ice Sheet contribute quasi-linearly, with 0.4 m °C−1 and 1.2 m °C−1 of warming, respectively. The saturation of the contribution from glaciers is overcompensated by the nonlinear response of the Greenland Ice Sheet. As a consequence we are committed to a sea-level rise of approximately 2.3 m °C−1 within the next 2,000 y. Considering the lifetime of anthropogenic greenhouse gases, this imposes the need for fundamental adaptation strategies on multicentennial time scales.

Mountain glaciers and ice caps around Antarctica make a large sea‐level rise contribution

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 ± ...

Global response of glacier runoff to twenty‐first century climate change

The hydrology of many important river systems in the world is influenced by the presence of glaciers in their upper reaches. We assess the global-scale response of glacier runoff to climate change, where glacier runoff is defined as all melt and rain water that runs off the glacierized area without refreezing. With an elevation-dependent glacier mass balance model, we project monthly glacier runoff for all mountain glaciers and ice caps outside Antarctica until 2100 using temperature and precipitation scenarios from 14 global climate models. We aggregate results for 18 glacierized regions. Despite continuous glacier net mass loss in all regions, trends in annual glacier runoff differ significantly among regions depending on the balance between increased glacier melt and reduction in glacier storage as glaciers shrink. While most regions show significant negative runoff trends, some regions exhibit steady increases in runoff (Canadian and Russian Arctic), or increases followed by decreases (Svalbard and Iceland). Annual glacier runoff is dominated by melt in most regions, but rain is a major contributor in the monsoon-affected regions of Asia and maritime regions such as New Zealand and Iceland. Annual net glacier mass loss dominates total glacier melt especially in some high-latitude regions, while seasonal melt is dominant in wetter climate regimes. Our results highlight the variety of glacier runoff responses to climate change and the need to include glacier net mass loss in assessments of future hydrological change.

Modeling future glacier mass balance and volume changes using ERA‐40 reanalysis and climate models: A sensitivity study at Storglaciären, Sweden

(1) Modeling the response of glaciers to future climate change is important for predicting changes in global sea level rise and local water resources. We compute until the year 2100 the mass balance and volume evolution of Storglaciaren, a small valley glacier in Sweden, using a temperature index mass balance model. We focus on the sensitivity of results to the choice of climate model and variants of adjusting ERA-40 temperatures to local conditions. ERA-40 temperature and precipitation series from 1961 to 2001 are validated and used both as input to the mass balance model and for statistical downscaling of one regional and six global climate models (GCMs). Future volume projections are computed using area-volume scaling and constant glacier area. ERA-40 data correlate well with observations and capture observed interannual variability of temperature and precipitation. The mass balance model driven by several variants of ERA-40 input performs similarly well regardless of temporal resolution of the input series (daily or monthly). The model explains � 70% of variance of measured mass balance when the input temperatures are reduced by the lapse rate that maximizes model performance. Fitting ERA-40 temperatures to observations close to the glacier does not improve the performance of the model, leading us to conclude that ERA-40 can be used for mass balance modeling independent of meteorological observations. Projected future volume series show a loss of 50-90% of the initial volume by 2100. The differences in volume projections vary by 40% of the initial volume for six different GCMs input to mass balance model, while each volume projection varies by 20% depending on whether volume-area scaling or constant area is used and by 10% depending on details in the mass balance model used. The correction of biases in the seasonal temperature cycle of the GCMs with respect to the ERA-40 data is crucial for deriving realistic volume evolution. Static mass balance sensitivities to temperature and precipitation change in the 21st century are � 0.48 m yr � 1 K � 1 and 0.025 m yr � 1 per % increase, respectively.

Analysis of scaling methods in deriving future volume evolutions of valley glaciers

Volume–area scaling is a common tool for deriving future volume evolutions of valley glaciers and their contribution to sea-level rise. We analyze the performance of scaling relationships for deriving volume projections in comparison to projections from a one-dimensional ice-flow model. The model is calibrated for six glaciers (Nigardsbreen, Rhonegletscher, South Cascade Glacier, Sofiyskiy glacier, midre Lovenbreen and Abramov glacier). Volume evolutions forced by different hypothetical mass-balance perturbations are compared with those obtained from volume–area (V-A), volume–length (V-L) and volume–area–length (V-A-L) scaling. Results show that the scaling methods mostly underestimate the volume losses predicted by the ice-flow model, up to 47% for V-A scaling and up to 18% for V-L scaling by the end of the 100 year simulation period. In general, V-L scaling produces closer simulations of volume evolutions derived from the ice-flow model, suggesting that V-L scaling may be a better approach for deriving volume projections than V-A scaling. Sensitivity experiments show that the initial volumes and volume evolutions are highly sensitive to the choice of the scaling constants, yielding both over- and underestimates. However, when normalized by initial volume, volume evolutions are relatively insensitive to the choice of scaling constants, especially in the V-L scaling. The 100 year volume projections differ within 10% of initial volume when the V-A scaling exponent commonly assumed, γ = 1.375, is varied by −30% to +45% ( γ = [0.95, 2.00]) and the V-L scaling exponent, q = 2.2, is varied by −30% to +45% ( q = [1.52, 3.20]). This is encouraging for the use of scaling methods in glacier volume projections, particularly since scaling exponents may vary between glaciers and the scaling constants are generally unknown.

Glaciers in the Earth’s Hydrological Cycle: Assessments of Glacier Mass and Runoff Changes on Global and Regional Scales

Changes in mass contained by mountain glaciers and ice caps can modify the Earth’s hydrological cycle on multiple scales. On a global scale, the mass loss from glaciers contributes to sea-level rise. On regional and local scales, glacier meltwater is an important contributor to and modulator of river flow. In light of strongly accelerated worldwide glacier retreat, the associated glacier mass losses raise concerns over the sustainability of water supplies in many parts of the world. Here, we review recent attempts to quantify glacier mass changes and their effect on river runoff on regional and global scales. We find that glacier runoff is defined ambiguously in the literature, hampering direct comparison of findings on the importance of glacier contribution to runoff. Despite consensus on the hydrological implications to be expected from projected future warming, there is a pressing need for quantifying the associated regional-scale changes in glacier runoff and responses in different climate regimes.

Volume-area scaling vs flowline modelling in glacier volume projections

Abstract Volume–area scaling provides a practical alternative to ice-flow modelling to account for glacier size changes when modelling the future evolution of glaciers; however, uncertainties remain as to the validity of this approach under non-steady conditions. We address these uncertainties by deriving scaling exponents in the volume–area relationship from one-dimensional ice-flow modelling. We generate a set of 37 synthetic steady-state glaciers of different sizes, and then model their volume evolution due to climate warming and cooling as prescribed by negative and positive mass-balance perturbations, respectively, on a century timescale. The scaling exponent derived for the steady-state glaciers (γ = 1.56) differs from the exponents derived for the glaciers in transient (non-steady) state by up to 86%. Nevertheless, volume projections employing volume–area scaling are relatively insensitive to these differences in scaling exponents. Volume–area scaling agrees well with the results from ice-flow modelling. In addition, the scaling method is able to simulate the approach of a glacier to a new steady state, if mass-balance elevation feedback is approximated by removing or adding elevation bands at the lowest part of the glacier as the glacier retreats or advances. If area changes are approximated in the mass-balance computations in this way, our results indicate that volume–area scaling is a powerful tool for glacier volume projections on multi-century timescales.

Ice Volume and Subglacial Topography for Western Canadian Glaciers from Mass Balance Fields, Thinning Rates, and a Bed Stress Model

AbstractA method is described to estimate the thickness of glacier ice using information derived from the measured ice extent, surface topography, surface mass balance, and rate of thinning or thickening of the ice column. Shear stress beneath an ice column is assumed to be simply related to ice thickness and surface slope, as for an inclined slab, but this calculation is cast as a linear optimization problem so that a smoothness regularization can be applied. Assignment of bed stress is based on the flow law for ice and a mass balance calculation but must be preceded by delineation of the ice flow drainage basin. Validation of the method is accomplished by comparing thickness estimates to the known thickness generated by a numerical ice dynamics model. Once validated, the method is used to estimate the subglacial topography for all glaciers in western Canada that lie south of 60°N. Adding the present ice volume of each glacier gives the estimated total volume as 2320 km3, equivalent to 5.8 mm of sea leve...

Climate sensitivity of Storglaciaren, Sweden: an intercomparison of mass-balance models using ERA-40 re-analysis and regional climate model data

Abstract Estimates of glacier contributions to future sea-level rise are often computed from mass-balance 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 0.22ma–1, respectively. The cumulative mass balance for the period 2002–2100 in response to the climate-model predicted temperature changes varied between –81 and –92m for four models, but was –121m 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.

Future changes in autumn atmospheric river events in British Columbia, Canada, as projected by CMIP5 global climate models

Atmospheric rivers (ARs) often trigger extreme precipitation events in British Columbia (BC), Canada. Here we analyze how well the autumn AR events with the highest probability for extreme precipitation over BC, henceforth called AR-extreme events, are simulated in five Coupled Model Intercomparison Project Phase 5 (CMIP5) global climate models (GCMs) and how these AR-extreme events are projected to change by the end of the century. We examine the daily synoptic patterns of integrated water vapor transport (IVT) over the Pacific Ocean that favor the formation of AR-extreme events. Our analysis and comparison with AR-extreme events in four reanalysis products for the period 1979–2010 reveal that the GCMs more successfully resolve their seasonality and interannual variability than their frequencies and amount of precipitation brought to BC. For the CMIP5 scenarios Representative Concentration Pathway (RCP) 4.5 and RCP8.5, the frequency of AR-extreme events will increase for the period 2070–2100 with the largest increase in December. All models project an increase in total precipitation over BC, due to the increase in frequency and intensity of the AR-extreme events; however, the dominant factor is the increase in frequency, especially of those events with precipitation exceeding 20 mmd−1. The path of the ARs during the AR-extreme events is projected to move northward, bringing stronger IVT and more precipitation to the north coast of BC, while the south coast may become drier than at the present day. The shift in the ARs is driven by the northward shift in the Aleutian Low pressure system, especially in RCP8.5.