Scientists say the expansion of forests in the Arctic circle may be contributing to climate change.
As the planet warms, trees have been able to spread further north into the arctic tundra where it was previously too cold for trees to grow.
It had been thought this might slow down climate change by trapping more carbon dioxide.
But scientists at the University of Exeter in the United Kingdom have discovered new arctic forests in Sweden stimulate the decomposition of organic matter trapped in the soil.
The researchers say this releases more carbon dioxide than the trees are able to capture from the atmosphere.