By Helen Briggs BBC News Online science reporter

Rain falling on snow is creating ice that restricts their food supply, says a US team.

Rainfall in the northern latitudes where the animals live has been increasing in recent years.

You have an ice layer at the surface several centimetres thick that even a person couldn't get through without tools Jaakko Putkonen

According to a climate change model put together by researchers at the University of Washington, things can only get worse.

The problem is caused by rain falling on snow in the far northern latitudes during winter.

The water seeps into the soil and freezes, producing a coating of ice.

The ice layer stops hoofed animals such as reindeer, caribou and musk ox from getting to the lichens and mosses they eat.

Ice layer

According to study author, Jaakko Putkonen, it is a growing problem in far north places such as the Norwegian island of Spitsbergen, midway between Norway and the North Pole.

Herders may have to bring out hay

"You have an ice layer at the surface several centimetres thick that even a person couldn't get through without tools," he says.

"I have seen soil temperatures remain at the freezing point for as long as two months because of the slowly freezing water below the thick snow pack.

"Even when ice layers are not impenetrable, the warmer soil surface temperatures promote the growth of fungi and toxic moulds among the lichens, so the animals avoid those areas."

Dr Putkonen's team constructed a model of snow and soil heat generation to study the effects of climate change in areas such as northern Alaska and Canada, Greenland, northern Scandinavia, and Russia.

Future threat

The model predicts that by 2089, the land mass affected by rain falling on snow will have risen by 40%.

"The bottom line is that the rain will penetrate farther into the interiors of the continents, where most of the reindeer are," says Dr Putkonen.

Santa's presents will arrive on time this year Lynn Rosentrater, WWF International Arctic Programme

It is not clear what the effect will be on reindeer. In some areas, herders may have to supply hay in winter for them to feed on.

Another concern is that they may damage their hooves pawing at the ice.

Lynn Rosentrater, climate scientist in the WWF International Arctic Programme, says reindeer will end up starving or will have to migrate in search of other food sources.

"This will be the future environment for reindeer - it's a very frightening prospect," she told BBC News Online.

The WWF says it is very concerned about the reindeer's future with respect to climate change and other threats. But Dr Rosentrater says reindeer populations in the Arctic are stable at the moment.

"I don't want to scare anyone - Santa's presents will arrive on time this year," she joked.

The University of Washington team is calling for a more in-depth study of how hoofed animals in northern areas are affected by global climate change.

Their research will be published in the journal Geophysical Research Letters.

Main image: Jaakko Putkonen.