American researchers spent two summers studying polar bears on the Arctic sea ice. They found that sea ice, where polar bears spend much of their lives, is melting faster than experts predicted it would. The researchers say this change in the animals' home territory is threatening their survival.

The scientists published a report on their study in the journal Science. The report says polar bears face a difficult future unless countries reduce air pollution.

Polar bears live on the ice. It is where they hunt, mate and raise their young. But a team of researchers has found that the animals' ice habitat is warming up, and shrinking.

The team was from the University of Wyoming. Its members worked in the Arctic between 2008 and 2010. Merav Ben David was among the researchers. She says they learned that polar bears often have trouble finding food. She says the lack of food can affect their mental health.

"So if shortening of the spring hunting season, lengthening of the summer season where they are food-deprived and still experiencing difficulties in getting food in the winter, polar bears are stressed physiologically."

Ms. Ben David says if the animals do not eat enough food in the spring, they are in poor health when winter arrives.

The researchers traveled on helicopters and ships. They captured more than 20 polar bears, and tested their blood. Before releasing the animals, the researchers equipped them with devices to measure temperature. They then followed the movement of the polar bears on ice and on land.

Other studies had suggested that the bears could deal with a lack of food in the summer by reducing their activity -- like the animals do when they hibernate in the winter. But Ms. Ben David says the researchers discovered that did not happen.

"We found that polar bears -- like their nearest relatives the brown bears -- are incapable of reducing their metabolic rate to the level that we see during winter hibernation. So there are limits on how long they can go without feeding in the summer."

Ms. Ben David says 20,000 to 25,000 polar bears live in the Arctic. She says the world should take action to protect them.

"If we want to be responsible citizens of this planet, we have to do everything in our power to stop, reverse the trend of sea ice loss."

She says countries must work to limit the greenhouse gases that studies have linked to rising temperatures. She says if that does not happen, polar bears could one day disappear from our planet.

I'm Christopher Jones-Cruise.

VOA Science Correspondent Rosanne Skirble reported this story from Washington. Christopher Jones-Cruise adapted it for VOA Learning English. George Grow was the editor.

habitat – n. the environment where a plant or animal normally lives

deprived – adj. to not have something

stressed – adj. related to strong feelings or concerns

physiologically – adv. of or relating to the body

hibernate – v. to spend the winter sleeping or resting

incapable – adj. not able to do something

metabolic rate – n. the rate at which a living thing (animal, people) turns food into energy

reverse – v. to cause something to stop or return to an earlier state

trend – n. a general direction of change; a way of behaving

greenhouse gases – n. gases blamed for the greenhouse effect, which is defined as the warming of the atmosphere by air pollution

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