CGD 2008 Profiles in Science: Dr. David Bailey

Summary of achievements

In my position as CCSM Polar Climate Working Group (PCWG) and sea ice model liaison, I am responsible for supporting the scientific projects of interest to the PCWG, generating sea ice diagnostic plots for simulations for the general CCSM community, aiding new users of CCSM with experimental setup and design, and ongoing maintenance and development of the Community Ice CodE (CICE), the sea ice component of the CCSM. Some recent examples of PCWG projects that I've been involved in are: providing sea ice concentration from IPCC 21st century projections for use in polar bear habitat prediction models; performing ensemble experiments using the CCSM to investigate possible sea ice response under idealized greenhouse gas commitment scenarios and predictability of September sea ice extent. Earlier this year I had participated in a workshop sponsored by NSF Arctic System Science (ARCSS) with particular focus on the 2007 September ice minimum. I was an active participant in the working group on predictability of the sea ice, which eventually led to an open community September sea ice outlook. I have also been involved with 20th century hindcast runs using the CCSM as a basis for short-term prediction studies. I am currently testing and developing new parameterizations in the sea ice for melt ponds, shortwave radiation, and aerosol deposition on sea ice. I am also peripherally involved in the grand challenge project to run the CCSM at very high resolution.

Publications

Holland, M.M., C.M. Bitz, B. Tremblay, D.A. Bailey, 2008: The role of natural versus forced change in future rapid summer Arctic ice loss, AGU Monograph on "Arctic Sea Ice Decline", in press.

Abstract: Climate model simulations from the Community Climate System Model (CCSM3) suggest that Arctic sea ice could undergo rapid September ice retreat in the 21st century. A previous study indicated that this results from a thinning of sea ice to more vulnerable conditions, a "kick" in the form of pulse-like increases in ocean heat transport and positive feedbacks that accelerate the retreat. Here we further examine the factors affecting these events, including the role of natural versus forced change and the possibility of threshold-like behavior in the simulated sea ice cover. We find little indication that a critical sea ice state is reached that then leads to rapid ice loss. Instead our results suggest that the rapid ice loss events result from anthropogenic change reinforced by growing intrinsic variability. The natural variability in summer ice extent increases in the 21st century due to the thinning ice cover. As the ice thins, large regions can easily melt out resulting in considerable ice extent variations. The important role of natural variability in the simulated rapid ice loss is such that we find little capability for predicting these events based on a knowledge of prior ice and ocean conditions. This is supported by results from sensitivity simulations initialized several years prior to an event, which exhibit little predictive skill.

Figure caption: Timeseries of September ice extent for the (a) Run 1 ensemble member and (b-f) sensitivity runs listed in Table 2. The 5-year running mean smoothed ice extent from Run 1 is shown in red in all of the panels. Abrupt events are indicated by the grey shading.

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Durner, G.M., D.C. Douglas, R.M. Nielsen, S.C. Amstrup, T.L McDonald I. Stirling, M. Mauritzen, E.W. Born, O. Wiig, E. DeWeaver, M.C. Serreze S.E. Belikov, M.M. Holland, J. Maslanik, J. Aars, D.A. Bailey, and A.E. Derocher, 2008: Predicting 21st Century Polar Bear Habitat Distribution from Global Climate Models, Ecological Monographs, in press.

Abstract: Projections of polar bear (Ursus maritimus) sea ice habitat distribution in the polar basin during the 21st century were developed to understand the consequences of anticipated sea ice reductions on polar bear populations. We used location data from satellite-collared polar bears and environmental data (e.g., bathymetry, distance to coastlines, and sea ice) collected from 1985-1995 to build Resource Selection Functions (RSF). RSFs described habitats polar bears preferred in summer, autumn, winter and spring. When applied to independent data from 1996-2006, the RSFs consistently identified habitats most frequently used by polar bears. We applied the RSFs to monthly maps of 21st century sea ice concentration projected by 10 general circulation models (GCMs) used in the Intergovernmental Panel of Climate Change Fourth Assessment Report, under the A1B greenhouse-gas forcing scenario. Despite variation in their projections, all GCMs indicated habitat losses in the polar basin during the 21st century. Losses in the highest-valued RSF habitat (optimal habitat) were greatest in the southern seas of the polar basin, especially the Chukchi and Barents seas, and least along the Arctic Ocean shores of Banks Island to northern Greenland. Average loss of optimal polar bear habitat was greatest during summer; from an observed 1.0 million km² in 1985-1995 (baseline) to a projected multi-model average of 0.32 million km² in 2090-2099 (-68% change). Projected winter losses of polar bear habitat were less; from 1.7 million km² in 1985-1995 to 1.4 million km² in 2090-2099 (-17% change). Habitat losses based on GCM multi-model averages may be conservative; simulated rates of habitat loss during 1985-2006 from many GCMs were less than observed rates of loss. Although a reduction in the total amount of optimal habitat will likely reduce polar bear populations, exact relationships between habitat losses and population demographics remain unknown. Density and energetic effects may become important as polar bears make long distance annual migrations from traditional winter ranges to remnant high-latitude summer sea ice. These impacts will likely affect specific sex and age groups differently and may ultimately preclude bears from seasonally returning to their traditional ranges.

Figure caption: Projected changes (based on 10 IPCC AR-4 GCM models run with the SRES-A1B forcing scenario) in the spatial distribution and integrated annual area of optimal polar bear habitat. Base map shows the cumulative number of months per decade where optimal polar bear habitat was either lost (red) or gained (blue) from 2001-2010 to 2041-2050. Offshore gray shading denotes areas whre optimal habitat was absent in both periods. Insets show the average annual (∑ 12 months) cumulative area of optimal habitat (right y-axis, line plot) for four 10-year periods in the 21st century (x-axis midpoints), and their associated percent change in area (left y-axis, histograms) relative to the first decade (2001-2010).