‘Addressing sea ice loss will require global policy solutions …’
FRISCO — Loss of Arctic sea ice caused by increasing concentrations of greenhouse gases remains as the primary threat to polar bears, U.S. Geological Survey researchers said after updating their research models.
Even if greenhouse gas emissions drastically reduced, sea ice will continue to shrink for decades, leading to a significant loss of polar bear habitat in many parts of the Arctic. The Canadian Archipelago is a notable exception. That region may serve as a climate refuge for the bears and other ice-dependent species, the federal scientists said.
The latest modeing looked at polar bear populations in four Arctic ecoregions based on sea ice projections from the Intergovernmental Panel on Climate Change under two greenhouse gas emission scenarios — one with stabilization in climate warming by century’s end because of reduced GHG emissions, the other looked with unabated rates of GHG emissions, leading to increased warming by century’s end.
“Addressing sea ice loss will require global policy solutions to reduce greenhouse gas emissions and likely be years in the making,” said USGS research ecologist Mike Runge. “Because carbon emissions accumulate over time, there will be a lag, likely on the order of several decades, between mitigation of emissions and meaningful stabilization of sea ice loss.”
If greenhouse gas emissions aren’t reduced, polar bear populations in two of the four ecoregions are projected to shrink significantly 25 years sooner than under the stabilized scenario.
The models, updated from 2010, evaluated specific threats to polar bears such as sea ice loss, prey availability, hunting, and increased human activities, and incorporated new findings on regional variation in polar bear response to sea ice loss.
“Substantial sea ice loss and expected declines in the availability of marine prey that polar bears eat are the most important specific reasons for the increasingly worse outlook for polar bear populations,” said USGS biologist and lead author Todd Atwood.
“We found that other environmental stressors such as trans-Arctic shipping, oil and gas exploration, disease and contaminants, sustainable harvest and defense of life takes, had only negligible effects on polar bear populations—compared to the much larger effects of sea ice loss and associated declines in their ability to access prey,” Atwood said.
If the summer ice-free period lengthens beyond 4 months — as forecasted to occur during the last half of this century in the unabated scenario — the negative effects on polar bears will be more pronounced.
Polar bears rely on ice as the platform for hunting their primary prey – ice seals – and when sea ice completely melts in summer, the bears must retreat to land where their access to seals is limited. Other research this year has shown that terrestrial foods available to polar bears during these land-bound months are unlikely to help polar bear populations adapt to sea ice loss.
USGS scientists’ research found that managing threats other than greenhouse gas emissions could slow the progression of polar bear populations to an increasingly worse status. The most optimistic prognosis for polar bears would require immediate and aggressive reductions of greenhouse gas emissions that would limit global warming to less than 2 degrees Celsius above pre-industrial levels.
The U.S. Fish and Wildlife Service listed the polar bear as threatened under the Endangered Species Act in 2008 due to the threat posed by sea ice loss. The polar bear was the first species to be listed because of climate change. A plan to address recovery of the polar bear will be released into the Federal Register by the USFWS for public review on July 2, 2015.
The updated forecast for polar bears was developed by USGS as part of its Changing Arctic Ecosystems Initiative, together with collaborators from the U.S. Forest Service and Polar Bears International. The polar bear forecasting report is available online.