This past spring the Porcupine Caribou Herd did something that they rarely do. Instead of migrating all the way to the north coast of Yukon and Alaska, the cows were slow to leave their wintering areas and gave birth well south and east of their normal calving grounds.

Barrenground caribou are extremely loyal to their calving grounds and will push hard to reach them. But this year snow stopped the caribou in their tracks. While it could be nothing but a chance event -- another weather anomaly -- it still highlights concerns that some biologists already have about snow and caribou and spring conditions.

Caribou are superbly adapted to life in the North, and every year they migrate for hundreds of kilometres over ice, snow and frozen ground. But they still go out of their way to avoid deep snow.

Snow saps their strength when they have to move through it, and forces them to spend more time digging for lichens and other food hidden beneath it.

Models of global warming predict that more snow could fall in the future, and some biologists are starting to wonder what such a change could mean for the herd. They are also wondering how much of an impact climate change is already having on the caribou.

Between 1972 and 1989 the Porcupine Caribou Herd steadily increased in number, at a rate of about three percent every year, until it reached a peak of 178,000 animals. But since then the herd has declined at a similar rate. The last census in 1998, found 129,000 animals in the herd.

The problem does not appear to be with the number of calves being born, or the number surviving the critical first month of life. These numbers have been up in recent years.

It is predicted that climate change will heat up northwestern North America more than the rest of the continent. If temperatures rise during the winter and spring, more snow would fall as warm air can hold more moisture than cold air.

Summer temperatures are on the rise in the North, but less is known about winter and spring conditions. Don Russell with the Canadian Wildlife Service decided to look more closely at weather records for these two seasons to see if they could help explain the herd's decline.

Russell analyzed precipitation and temperature records for the years 1970 to 1999 for four northern stations -- Shingle Point and Old Crow in the Yukon, and Fort MacPherson and Aklavik in the NWT.

He found little evidence for a warming trend in the winter months over the past 30 years, even though there has been a slight increase in precipitation during this time. The major warming has occurred in the spring, and Russell wonders how this trend is affecting the spring snowpack.

Increased temperatures could be forming what is sometimes called sugar snow, a bottomless mush that will not support any weight. Even caribou with their large hooves flounder in this sort of snow, exhausting their already depleted reserves of energy.

Another possibility is that the caribou are climbing up onto the ridges to avoid the deeper snow. The travelling there would be easier, but it would also make it easier for predators such as wolves to overtake them.

Russell says that more work needs to be done to see what changes might be taking place in the spring. "The indicators that we are monitoring, such as the birth rate and the number of calves surviving to March, are all indicating a healthy herd, so it seems like something is happening between March and the calving season," he says.

I would like to look at spring conditions, and see what habitat choices they are making, and how much energy they are using to go through deeper snow."

Oddly enough, the high calf survival rate is also be linked to climate change. The warm spring temperatures have been melting the snow earlier, allowing vegetation on the calving grounds to green up earlier, and supplying the caribou with a sorely needed energy and protein boost when they really need it. One study has shown that this early green-up increases the percentage of calves that survive their first month of life.

The spring of 2000 does not fit this scenario. It is thought that the caribou stopped delayed migrating because the snow melted later than normal. However, even this variation on a trend fits in with climate change predictions.

Climate researchers tell us that global warming could cause more wild swings in the weather, and that seems to be the case this year in the northern Yukon.

"One of the characteristics of a warming trend is that there is greater annual variation, so even if the spring temperatures are continuing to get warmer, you can still get a cold one as well," says Russell.

It remains to be seen how many calves were born this spring, and how many will survive long enough to begin the fall migration. Whatever the news, the future could show that spring is indeed the cruelest season for the Porcupine Caribou Herd.

For more information on the Porcupine Caribou Herd, contact the Canadian Wildlife Service at 393-6700. Information on northern caribou can also be found at www.taiga.net/top/caribou.html.