I have to reproduce a large part of this, because of Nature's silly way of putting stories under a paywall after a short time:
Forestry experts have again warned that climate change could transform forests from sinks to sources of carbon. The carbon storing capacity of global forests could be lost entirely if the earth heats up 2.5° Celsius above pre-industrial levels, according to a new report...
In a warmer world, subtropical and southern temperate forests such as those in the western United States, northern China, southern Europe, the Mediterranean and Australia will experience more intense and frequent droughts, increasing the incidence of fire and pests. This would lead to more carbon being released — a recent report in Science2 found that a 2005 drought in the Amazon basin released about 1.2 billion–1.6 billion tonnes of carbon (See 'Climate change crisis for rainforests').
The coniferous forests of Canada, Finland, Russia and Sweden that make up the boreal region are expected to experience more warming than forests in the equatorial zone. Although warmer temperatures could initially fuel a northward expansion of the forest, the short-term positive impacts would be cancelled out by damage from increased insect invasions, fires and storms.
The shift from sink to source is already happening. The mountain pine beetle has devastated the forests of western Canada. The outbreak currently covers 14 million hectares — roughly 3.5 times the size of Switzerland, says Allan Carroll, an insect ecologist with the Canadian Forest Service in Victoria, British Columbia. By 2020, the projected end of the outbreak, about 270 megatonnes of carbon will have been emitted to the atmosphere3. "That's the equivalent of five years of emissions from the entire transportation sector in Canada," says Carroll.
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