Snow cover and mostly barren tundra reflect large portions of the sun's rays. When darker shrubs and trees spring up in arctic areas they absorb more energy, heating up their surroundings and the earth's atmosphere.
Researchers in Finland have now carried out a comparison between an area in Norway where reindeer are not allowed to graze in the summer, and a similar area in Finland where grazing reindeer have kept shrubs and tree from growing.
They have found that the heat radiated by the overgrown area in Norway is at a much higher level.
"The heat difference between what happens there and in the Finnish area during three spring months, March, April and May, would be enough to melt a cubic kilometre of ice. That is no small matter,” explains Professor Lauri Oksanen.
Oksanen points out that snow cover best reflects the rays of the sun back into space. Dense forestlands reflect the least. The light-coloured lichens, also known as reindeer moss, that are an important part of reindeer diet, are also much more reflective than shrubs or trees. So, too many reindeer over-grazing on lichens could help add to, rather than help reduce climate change.
One major problem that Professor Oksanen is concerned about in the north is the improvements to forest growth brought about by land management, such as the draining of wetlands.
"If wetlands and poorly growing forests could be brought back so that the forests were left sparse and the wetlands returned to a natural state, it would significantly cool the atmosphere," Lauri Oksanen says.