A Bayesian Network Modeling Approach to Forecasting the 21st Century Worldwide Status of Polar Bears - Arctic Sea Ice Decline: Observations, Projections, Mechanisms, and Implications - page 207

A Bayesian Network Modeling Approach to Forecasting the
21st Century Worldwide Status of Polar Bears

Steven C. Amstrup

Alaska Science Center, U.S. Geological Survey, Anchorage, Alaska, USA

Bruce G. Marcot

Pacific Northwest Research Station, USDA Forest Service, Portland, Oregon, USA

David C. Douglas

Alaska Science Center, U.S. Geological Survey, Juneau, Alaska, USA

To inform the U.S. Fish and Wildlife Service decision, whether or not to list
polar bears as threatened under the Endangered Species Act (ESA), we projected
the status of the world's polar bears ( Ursus maritimus ) for decades centered on
future years 2025, 2050, 2075, and 2095. We defined four ecoregions based on
current and projected sea ice conditions: seasonal ice, Canadian Archipelago,
polar basin divergent, and polar basin convergent ecoregions. We incorporated
general circulation model projections of future sea ice into a Bayesian network
(BN) model structured around the factors considered in ESA decisions. This
first-generation BN model combined empirical data, interpretations of data, and
professional judgments of one polar bear expert into a probabilistic framework that
identifies causal links between environmental stressors and polar bear responses.
We provide guidance regarding steps necessary to refine the model, including
adding inputs from other experts. The BN model projected extirpation of polar
bears from the seasonal ice and polar basin divergent ecoregions, where ≈2/3 of the
world's polar bears currently occur, by mid century. Projections were less dire in
other ecoregions. Decline in ice habitat was the overriding factor driving the model
outcomes. Although this is a first-generation model, the dependence of polar bears
on sea ice is universally accepted, and the observed sea ice decline is faster than
models suggest. Therefore, incorporating judgments of multiple experts in a final
model is not expected to fundamentally alter the outlook for polar bears described
here.

1. INTRODUCTION

Polar bears depend upon sea ice for access to their prey
and for other aspects of their life history [ Stirling and Ørit-
sland , 1995; Stirling and Lunn , 1997; Amstrup , 2003]. Ob-
served declines in sea ice availability have been associated

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