Even Slight Warming Harms Some Species, Experts Say

Stefan Lovgren
for National Geographic News

April 20, 2005
It seems like warmer temperatures should come as welcome relief to the caribou that roam the harsh lands of Alaska.

But global warming may be having an adverse effect on the hardened animals. The number of caribou in one of Alaska’s four herds has dropped from 187,000 in 1989 to 120,000 today.

The decline, scientists suggest, could be due to warmer weather. The caribou now struggle with everything from more mosquitoes to more snow, brought on by the changing climate.

“The caribou can cope with cold temperatures,” said Steve Arthur, a biologist with the Alaska Department of Fish and Game in Fairbanks. “But hot weather and insect activity interfere with their ability to feed, reducing their energy and decreasing their ability to get through the winter.”

The caribou are hardly alone. Around the world many animals are struggling to cope with rising temperatures.

A National Geographic four-part TV series, Strange Days on Planet Earth, which begins tonight on PBS, links seemingly disparate phenomena—such as Alaska’s caribou decline and the disappearance of species from California tide pools—to climate change.

Many scientists are questioning whether organisms can evolve rapidly enough to keep pace with climate change. Some researchers suggest that even the tiniest temperature rises could mean the difference between life and death for many animals.

Shifting Seasons

There is little doubt that the planet is warming. Over the last century the average temperature has climbed about 1 degree Fahrenheit (0.6 of a degree Celsius) around the world.

Most scientists believe the warming is due primarily to an atmospheric increase in carbon dioxide—a greenhouse gas—caused by the burning of fossil fuels such as coal and petroleum.

A United Nations panel projects that average global temperatures will rise an additional 3 to 10 degrees Fahrenheit (1.6 to 5.5 degrees Celsius) by 2099.

Alaska is at the front lines of global climate change. One report showed that temperatures have increased 4 to 7 degrees Fahrenheit (2.2 to 3.9 degrees Celsius) in some parts of Alaska in the past 50 years.

The overall caribou population in Alaska is 700,000 to one million. The Porcupine herd, which Arthur has been monitoring, spans the Alaska-Canada border. The herd’s population peaked in 1989. Since then its numbers have declined by 3 percent per year.

Warmer weather may actually have led to greater reproductive success for the caribou. The caribou calving season is in early June, immediately after the snow disappears, a time when nutrients in the vegetation are most concentrated. Rising temperatures mean the date of initial “green-up” of vegetation has been earlier each year, which is helping the newborn calves.

However, the earlier green-up also means the vegetations’ nutrient levels eventually also decline earlier. If caribou don’t get enough nutrient-rich food, they may not be able to build up enough energy to survive the winter.

“The total amount of food doesn’t change,” Arthur said. “There’s only a shift in time [when the food is available], and that doesn’t help the caribou in the long run.”

Warmer weather can also lead to longer breeding periods for mosquitoes. To escape growing swarms of mosquitoes, the caribou herds may move up on rocky hillsides, where the grazing is poor, expanding precious energy and taking in less energy by way of feeding.

Rising temperatures may also bring more precipitation. More snow not only buries food deeper but also makes avoiding predators like wolves much more difficult. Arthur warns that the caribou are becoming more vulnerable to big storms.

“It’s possible that we could be looking at a threshold, where the caribou could no longer recover [from natural events like big storms],” Arthur said.

Temperature Swings

Other species may already be living close to the edge. Take, for example, the crabs, snails, and mussels found in the rocky tidal pools along the California coast.

These creatures experience extreme temperature swings, especially those animals that are exposed to air. Their body temperatures can change from lows of 55 to 57 degrees Fahrenheit (13 to 14 degrees Celsius) to highs above 86 degrees Fahrenheit (30 degrees Celsius).

To find out how rising temperatures could affect these animals, scientists tested the thermal tolerance of porcelain crabs by hooking them up to a heart-rate monitor, immersing them in water, and gradually increasing the temperature.

“We found that crabs that are experiencing the highest temperatures up on the rocks are already living close to their limit,” said George Somero, a marine science professor at Stanford University’s Hopkins Marine Station in Pacific Grove, California.

“Their hearts stop beating in water only slightly warmer than what they currently experience in the wild,” he said.

Testing has shown the same thing in mussels and snails.

Somero says the animals that occur highest on the rock have already evolved a higher degree of heat tolerance than those found underwater. With global warming increasing, the animals may be edging closer to their absolute limits.

“They don’t have a reserve capacity to cope with higher temperatures,” he said. “They’re already pushing the envelope in terms of what they can do about their physiology.”