A study by polar bear researchers in Alaska concludes that thermal imaging used by oil exploration companies to detect polar bears in dens works less than half the time.

That doesn't mean polar bears are being disturbed, said researcher Tom Smith, an associate professor of wildlife science at Brigham Young University. Exploration companies use other tools to avoid operating within a mile of a den, and no industry disturbance, to his knowledge, has killed a polar bear cub, Smith said.

But exploration company should understand that finding polar bears through aerial infrared sensors can be thrown off by wind or moisture in the air, he said.

"We're just trying to encourage those people doing it to pay more attention to the sensitive nature of this tool," Smith said.

Pregnant polar bears starting in October dig dens in snow drifts for giving birth and nursing cubs. Females give birth in mid-winter. Females and cubs abandon dens by mid-April and head toward sea ice, where they hunt for seals.

The urgency to keep polar bears safe stems from declining numbers of bears and the expansion of drilling activity. The number of southern Beaufort Sea polar bears, one of two populations in Alaska, fell by about 40 percent from 2000 to 2010, according to the authors.

"Maximizing cub survival potential is essential for polar bears in this region to persist," the authors said.

Meanwhile, exploration in polar bear habitat continues to expand. Congress in 2017 approved President Donald Trump's Tax Cuts and Jobs Act, which requires a lease sale by 2021 on the coastal plain of the Arctic National Wildlife Refuge, a major denning location for southern Beaufort Sea polar bears.

Infrared technology mounted on airplanes has been a tool to detect thermal images from polar bear dens since 2004. Alaska oil companies don't look for every polar bear den but survey areas where they might install a drill pad, pipeline, winter ice road or other infrastructure.

Researchers looked at 33 known polar bear dens and compared them to dens detected by industry thermal imaging flights from 2004 to 2016. The flights had detected 15 dens, or 41 percent.

"A certain percentage, you're never going to see because they're just under too much snow," Smith said. If the ceiling of the den is more than a meter deep, the bear's body heat will not reach the surface, he said. Fresh snow or a wind cools the top of a den and prevents detection.

"We don't know how long it takes for that delicate heat signature to reestablish at the top of a snowdrift," he said.

Water crystals in the air play havoc with infrared light.

"That's like trying to shine a light through a room full of prisms," he said.

The study recommended that the infrared detection be used only when weather conditions are appropriate and that the industry develop new den-location technology.

Patrick Bergt, regulatory and legal affairs manager for the Alaska Oil and Gas Association, called the study an academic exercise that looked at a fraction of the best available data and did not account for all measures used by the industry. Aerial infrared is just one management tool for locating polar bear dens, he said in an email response to questions.

"All personnel working off pads in polar bear habitat are provided specialized training on how to identify signs of polar bear dens, such as tailings and other indications of excavations," he said.

Oilfield mobile equipment typically doesn’t travel over topographical features necessary for polar bears to create dens, he said.

"If it is steep enough for polar bear dens, it is too steep for much of our equipment," he said.

There has been no evidence for more than 40 years of maternal dens destroyed by industry activity, he said, a record that can be attributed to management tools and operator experience, Bergt said.

The study was published last week in the journal PLOS ONE.

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