Brad Griffith had caribou calves on his mind when he headed into his Fairbanks office one Sunday evening. The Alaskan caribou biologist has planned to study some satellite images to see what they could tell him about the calving grounds of the Porcupine Caribou Herd. But another question had been nagging him.

Griffith had ten years worth of data showing that the rate of calf survival in the herd varied significantly from year to year. The first month of life is a critical period for the calves. Each year half of all caribou deaths are from calves that die in June.

"I had been scratching my head over what had been going on with calf survival. I was doing the typical wildlife biologist stuff, trying to see if the bears were eating them. The typical stuff. We already knew there was a warming trend in the North, so what else was left? But I had this hunch that something else was going on," says Griffith.

Griffith's hunch was that maybe vegetation on the calving grounds had something to do with whether calves lived or died in the month of June. The extensive series of satellite images proved his theory right. They showed that spring vegetation had been greening up earlier and earlier on the calving grounds over a ten-year period, providing food for the caribou at a particularly important time.

The caribou have been travelling for about two months by the time they reach their calving grounds on the Arctic Coast. The cows need fresh green growth to feed themselves and produce milk for their young. At the calving grounds, the cows crowd together in areas with the most forage.

Sections of tundra where there is plenty to eat will have ten times as many cows on them as more peripheral ones. Griffith found that calves born in these more productive areas have an even higher overall survival rate than those born on the fringes.

"I pounded the daylight out of the data to make sure that I was not getting a phony answer, and talked to all of the geniuses who work with satellite imagery," he says. "Who would ever have thought that we could predict caribou calf survival from a satellite picture?"

Griffith's work shows a clear link between climate change and the Porcupine Caribou Herd. Warmer temperatures lead to an earlier green-up and quicker growth of vegetation, and those factors increase the survival rate of calves.

Over the last two decades, summer temperatures in the North have already increased by about 2°C. Scientists predict that arctic regions will be more strongly affected by global warming than more southerly regions. The herd has been increasing in size since the late 1970s, and numbers about 130,000 animals today, but it is much too early to say how global warming could affect the herd in the future.

While an early green-up of vegetation could be one bonus of climate change for the caribou, there could definitely be negative effects as well. Griffith wants to check satellite images to see whether vegetation on the tundra might be drying up earlier in the summer.

Caribou need to fatten up for the winter. A lack of forage in late summer could affect the health of the cows and decrease their chances of giving birth the next year.

Some scientists think that global warming eventually could increase snowfall in the North. Warmer summer temperatures also increase insect activity. If there is deeper snow, caribou spend more time digging and less time feeding. Harassment by insects also cuts down on feeding time.

Biologists are also trying to understand how the interplay of these different factors could affect the size of the herd. Working together, the Canadian Wildlife Service and the Institute of Arctic Biology at the University of Alaska in Fairbanks have developed a computer model that helps them look into the future. The program predicts how changes in certain variables could affect pregnancy rates and calf survival rates.

This model shows that an increase in snow depth and insect harassment would cause the herd to shrink in size. The impact of the earlier green-up of vegetation will also be incorporated into the computer program.

A huge amount of data is available for this effort as the Porcupine Caribou herd is the most well-studied group of caribou in the world. Controversy over proposed resource development on the herd's calving grounds on the Arctic Coast in Alaska has sparked much of the research effort.

These animals are extremely loyal to their calving grounds, and people such as the Gwitchin, whose way of life depends on the herd, fear the worst if this critical habitat is developed. The increasing concerns about climate change raise more questions about the long-term health of the herd.

When asked whether caribou are capable of adapting to a warmer world, Griffith points out that "they made it through the Pleistocene extinction." Nearly all of the other mammals living in the North during the Ice Ages went extinct after the climate warmed up, but caribou survived the change and prospered. They are now found across the circumpolar North.

Griffith compares caribou to another member of the deer family. He says that mule deer will stay put and starve to death if their environment changes, but caribou are used to travelling huge distances. The average cow migrates about 2700 kilometres per year.

"They can find the good foraging places every year. They are pretty adaptive animals," he says.

Griffith is with the Fish and Wildlife Research Unit of the U.S. Geological Survey in Alaska. For more information on the Porcupine Caribou Herd, contact the Canadian Wildlife Service in Whitehorse. A simplified version of the computer model looking at the impacts of climate change on caribou is available here on Taiga Net.