ALBERT

Description: The Columbia Climate Center, in partnership with World Wildlife Fund, Woods Hole Research Center, and Arctic 21, held a workshop titled A 5 C Arctic in a 2 C World on July 20 and 21, 2016. The workshop was co-sponsored by the International Arctic Research Center (University of Alaska Fairbanks), the Arctic Institute of North America (Canada), the MEOPAR Network (Marine Environmental Observation, Prediction, and Response), and the Future Ocean Excellence Cluster. The goal of the workshop was to advance thinking on the science and policy implications of the temperature change in the context of the 1.5 to 2 C warming expected for the globe, as dis- cussed during the 21st session of the Conference of the Parties of the United Nations Framework Convention on Climate Change at Paris in 2015. For the Arctic, such an increase means an antic- ipated increase of roughly 3.5 to 5 C. An international group of 41 experts shared perspectives on the regional and global impacts of an up to +5 C Arctic, examined the feasibility of actively lowering Arctic temperatures, and considered realistic timescales associated with such interventions. The group also discussed the science and the political and governance actions required for alternative Arctic futures.

Description: The largest uncertainty in projections of future sea-level change still results from the potentially changing dynamical ice discharge from Antarctica. While ice discharge can alter through a number of processes, basal ice-shelf melting induced by a warming ocean has been identified as a major if not the major cause for possible additional ice flow across the grounding line. Here we derive dynamic ice-sheet response functions for basal ice-shelf melting using experiments carried out within the Sea-level Response to Ice Sheet Evolution (SeaRISE) intercomparison project with five different Antarctic ice-sheet models. As used here these response functions provide separate contributions for four different Antarctic drainage regions. Under the assumptions of linear-response theory we project future ice-discharge for each model, each region and each of the four Representative Concentration Pathways (RCP) using oceanic temperatures from 19 comprehensive climate models of the Coupled Model Intercomparison Project, CMIP-5, and two ocean models from the EU-project Ice2Sea. Uncertainty in the climatic forcing, the oceanic response and the ice-model differences is combined into an uncertainty range of future Antarctic ice-discharge induced from basal ice-shelf melt. The additional ice-loss (Table 6) is clearly scenario-dependent and results in a median of 0.07 m (66%-range: 0.04–0.10 m; 90%-range: −0.01–0.26 m) of global sea-level equivalent for the low-emission RCP-2.6 scenario and yields 0.1 m (66%-range: 0.06–0.14 m; 90%-range: −0.01–0.45 m) for the strongest RCP-8.5. If only models with an explicit representation of ice-shelves are taken into account the scenario dependence remains and the values change to: 0.05 m (66%-range: 0.03–0.08 m) for RCP-2.6 and 0.07 m (66%-range: 0.04–0.11 m) for RCP-8.5. These results were obtained using a time delay between the surface warming signal and the subsurface oceanic warming as observed in the CMIP-5 models. Without this time delay the ranges for all ice-models changes to 0.10 m (66%-range: 0.07–0.12 m; 90%-range: 0.01–0.28 m) for RCP-2.6 and 0.15 m (66%-range: 0.10–0.21 m; 90%-range: 0.02–0.53 m) for RCP-8.5. All probability distributions as provided in Fig. 10 are highly skewed towards high values.

Description: The largest uncertainty in projections of future sea-level change results from the potentially changing dynamical ice discharge from Antarctica. Basal ice-shelf melting induced by a warming ocean has been identified as a major cause for additional ice flow across the grounding line. Here we attempt to estimate the uncertainty range of future ice discharge from Antarctica by combining uncertainty in the climatic forcing, the oceanic response and the ice-sheet model response. The uncertainty in the global mean temperature increase is obtained from historically constrained emulations with the MAGICC-6.0 (Model for the Assessment of Greenhouse gas Induced Climate Change) model. The oceanic forcing is derived from scaling of the subsurface with the atmospheric warming from 19 comprehensive climate models of the Coupled Model Intercomparison Project (CMIP-5) and two ocean models from the EU-project Ice2Sea. The dynamic ice-sheet response is derived from linear response functions for basal ice-shelf melting for four different Antarctic drainage regions using experiments from the Sea-level Response to Ice Sheet Evolution (SeaRISE) intercomparison project with five different Antarctic ice-sheet models. The resulting uncertainty range for the historic Antarctic contribution to global sea-level rise from 1992 to 2011 agrees with the observed contribution for this period if we use the three ice-sheet models with an explicit representation of ice-shelf dynamics and account for the time-delayed warming of the oceanic subsurface compared to the surface air temperature. The median of the additional ice loss for the 21st century is computed to 0.07 m (66% range: 0.02–0.14 m; 90% range: 0.0–0.23 m) of global sea-level equivalent for the low-emission RCP-2.6 (Representative Concentration Pathway) scenario and 0.09 m (66% range: 0.04–0.21 m; 90% range: 0.01–0.37 m) for the strongest RCP-8.5. Assuming no time delay between the atmospheric warming and the oceanic subsurface, these values increase to 0.09 m (66% range: 0.04–0.17 m; 90% range: 0.02–0.25 m) for RCP-2.6 and 0.15 m (66% range: 0.07–0.28 m; 90% range: 0.04–0.43 m) for RCP-8.5. All probability distributions are highly skewed towards high values. The applied ice-sheet models are coarse resolution with limitations in the representation of grounding-line motion. Within the constraints of the applied methods, the uncertainty induced from different ice-sheet models is smaller than that induced by the external forcing to the ice sheets.

Description: Ice-shelf pinning points such as ice rises or ice rumples can have an important role in regulating the ice discharge of marine outlet glaciers. For instance, the observed gradual ungrounding of the ice shelf of West Antarctica's Thwaites Glacier from its last pinning points diminishes the buttressing effect of the ice shelf and thus contributes to the destabilization of the outlet. Here we use an idealized experimental setting to simulate the response of an Antarctic-type marine outlet glacier to a successive ungrounding of its ice shelf from a pinning point. This is realized by perturbing steady states by a step-wise lowering of the pinning point, which induces a buttressing reduction. After the complete detachment of the ice shelf from the pinning point the perturbation is reversed, i.e., the pinnning point is incrementally elevated toward its initial elevation. First results show that the glacier retreat down the landward down-sloping (retrograde) bed, induced by the loss in buttressing, can be reversible in case of a relatively flat retrograde bed slope. For steeper slopes, glacier retreat and re-advance show a hysteretic behavior. Thus, if the bed depression is sufficiently deep, the glacier does not recover from its fully retreated state even for pinning-point elevations that are higher than the initial elevation.

Description: Boyd et al. (1) are correct to note that our paper did not factor in potential adaptation measures such as levees, as, indeed, we explicitly indicated (2). Because our analysis extended up to 2,000 years into the future, we excluded consideration of adaptive responses because of the great degree of...

Description: Assessing global impacts of unexpected meteorological events in an increasingly connected world economy is important for estimating the costs of climate change. We show that since the beginning of the 21st century, the structural evolution of the global supply network has been such as to foster an increase of climate-related production losses. We compute first- and higher-order losses from heat stress–induced reductions in productivity under changing economic and climatic conditions between 1991 and 2011. Since 2001, the economic connectivity has augmented in such a way as to facilitate the cascading of production loss. The influence of this structural change has dominated over the effect of the comparably weak climate warming during this decade. Thus, particularly under future warming, the intensification of international trade has the potential to amplify climate losses if no adaptation measures are taken.

Description: The Antarctic Ice Sheet stores water equivalent to 58 m in global sea-level rise. We show in simulations using the Parallel Ice Sheet Model that burning the currently attainable fossil fuel resources is sufficient to eliminate the ice sheet. With cumulative fossil fuel emissions of 10,000 gigatonnes of carbon (GtC), Antarctica is projected to become almost ice-free with an average contribution to sea-level rise exceeding 3 m per century during the first millennium. Consistent with recent observations and simulations, the West Antarctic Ice Sheet becomes unstable with 600 to 800 GtC of additional carbon emissions. Beyond this additional carbon release, the destabilization of ice basins in both West and East Antarctica results in a threshold increase in global sea level. Unabated carbon emissions thus threaten the Antarctic Ice Sheet in its entirety with associated sea-level rise that far exceeds that of all other possible sources.

Description: The future evolution of the Antarctic Ice Sheet represents the largest uncertainty in sea-level projections of this and upcoming centuries. Recently, satellite observations and high-resolution simulations have suggested the initiation of an ice-sheet instability in the Amundsen Sea sector of West Antarctica, caused by the last decades’ enhanced basal ice-shelf...

Description: 〈p〉There is evidence that a self-sustaining ice discharge from the West Antarctic Ice Sheet (WAIS) has started, potentially leading to its disintegration. The associated sea level rise of more than 3m would pose a serious challenge to highly populated areas including metropolises such as Calcutta, Shanghai, New York City, and Tokyo. Here, we show that the WAIS may be stabilized through mass deposition in coastal regions around Pine Island and Thwaites glaciers. In our numerical simulations, a minimum of 7400 Gt of additional snowfall stabilizes the flow if applied over a short period of 10 years onto the region (–2 mm year〈sup〉–1〈/sup〉 sea level equivalent). Mass deposition at a lower rate increases the intervention time and the required total amount of snow. We find that the precise conditions of such an operation are crucial, and potential benefits need to be weighed against environmental hazards, future risks, and enormous technical challenges.〈/p〉