Weather is caused by disturbances within the atmosphere, but climate is controlled by external influences, such as the distribution of oceans and continents, the extent of ice and snow-cover, variations in the amount of energy from the sun, and changes in the chemical composition of the atmosphere.

BBC: Time to spend more money preparing for colder winters?

Last month was the fourth-warmest January going back to 1895 and had the third-lowest snow cover going back to the '60s, government data show.

WSJ: Mild Winter Could Help Home Depot Earnings

Added to the general atmospheric warming in the region, the researchers also describe an amplification process whereby reduced snow cover on the surrounding tundra and less sea-ice in the Arctic Ocean push up temperatures still further.

BBC: Canadian glaciers face 'big losses'

"It means there is more light penetrating through the ice - that depends to a high degree on the snow cover but once it has melted the light can get through, " Dr Gerland said.

BBC: Arctic ice melting at 'amazing' speed, scientists find

With less snow cover and a longer growing season, these non-migratory reindeer have taken advantage of the increased plant abundance, with the result that reindeer populations and their ability to reproduce are up, while mortality is down.

CNN: The changing face of the Arctic

Also show are agricultural land use and land surfaces with little or no plant cover - such as bare ground, urban areas and permanent snow and ice.

BBC: How Earth's land is used

To cover a slope with enough snow to ski on, guns must be stationed about 15-30 metres apart, so hundreds are needed at the average resort.

ECONOMIST: Snowmaking