Forests could emit more carbon than they store if temperatures rise.
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 report1.
The analysis by the International Union of Forest Research Organizations (IUFRO) is a synthesis of existing information. "This is the first time it has been put together on a global scale," says Alexander Buck, IUFRO deputy executive director, in Vienna. "It is the most thorough assessment of the negative and positive effects of climate change on the world's forests."
The report examines how four different forest types — boreal, temperate, tropical and sub-tropical — would be affected under four climate-change scenarios: unavoidable, stable, growth and fast growth. Growth and fast growth correspond to business-as-usual emissions, with fast growth representing the pattern of high emissions observed since 2000. Under these two scenarios, forests will have difficulty adapting to climate change.
Droughts, insect invasions, fires and storms would cause widespread forest destruction. "The impacts of these fires and pest infestations will lead to an additional release of carbon into the atmosphere, which again exacerbates climate change," says Buck.
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.
The report stresses that sustainable forest-management practices urgently need to be put in place around the globe to reduce the vulnerability of forests to climate change. This could, for example, include changing forest management practices to allow controlled burning of woodland. To date, fire-prevention policies in regions such as western North America have sought to suppress forest fires altogether — leaving forests more susceptible to large-scale fires and insect attack.
"So few forests are being managed sustainably, particularly in tropical regions, and there is little being done to monitor the impacts of climate change on forests," says John Innes, a forestry expert at the University of British Columbia in Vancouver, and one of the report's authors.
The report will be presented at the United Nations Forum on Forests, which meets in New York from 20 April to 1 May. Buck says the report could have an impact on future climate negotiations, including the Copenhagen climate conference in December. "The climate convention is focused on reducing emissions from deforestation, but it has not adequately considered the problem of adaptation. Policy-makers have indicated that they need more information for their negotiations," he adds.
Carroll thinks that some governments are now ready to listen. "The mountain pine beetle outbreak and the climate signal associated with it is the canary in the coal mine about future disturbances. It's caused jurisdictions to perk up and take notice," says Carroll.
Seppälä, R., Buck, A. & Katila, P. (eds) Adaptation of Forests and People to Climate Change — A Global Assessment Report. IUFRO World Series Volume 22 (IUFRO, 2009).
Philips, O. L. et al. Science 323, 1344–1347 (2009).
Kurz, W. A. et al. Nature 452, 987–990 (2008).