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Forests affect the climate
Forests play an important role in regulating the earth’s temperature and weather patterns by storing large quantities of carbon and water. This regulatory function also has a profound effect on the local climate. Trees provide shade, which in turn lowers summer temperatures and prevents the soil from drying out. They reduce heat loss from the ground in winter and prevent storm damage by providing shelter from wind. Global warming, which on a geological timescale is occurring in the equivalent of a split second, is significantly upsetting the balance of carbon and water stored in forest ecosystems. Disruption to this intricate and poorly understood web of interactions that governs both global carbon fluxes and the very structure and composition of forest ecosystems is likely to be devastating. Furthermore, by regulating the global carbon cycle, forests also have a profound effect on the global climate.

Climate change affects forests
Global warming is believed to negatively affect a substantial fraction of existing forests and a third of today’s forests are likely to change their species composition. A temperature increase of 3°C by 2100 would result in forest ecosystems having to move 500 km towards the poles or 500m in elevation in order to find the same climatic conditions. Such distances are far beyond the average rate of dispersal for individual tree species, let alone entire forest ecosystems.

Early warnings about the consequences of the impacts of climate change on forests have been documented in, among others, The Carbon Bomb: Climate change and the fate of the northern Boreal forests - a 1994 Greenpeace report which states:

"Studies on the global carbon cycle suggest that boreal forests are not absorbing as much carbon as they did before 1976. As a result, the atmosphere already appears to contain 10-15 billion tones of carbon more than it would have if forests had continued to absorb carbon at the pre-1976 rate. If boreal forests continue to decline, estimates suggest that burning and rotting of boreal forests could contribute to the release of up to 225 billion tones of extra carbon into the atmosphere, increasing current levels by a third. This would accelerate the rate of climate change."
The Carbon Bomb, p. 2.

While it is possible that the boreal forest could expand into the frozen tundra as temperatures increase, such an expansion would likely be delayed by slow tree migration rates. Even in the long-term, the boreal forest is unlikely to move northward fast enough to compensate for the breakdown of boreal forests at the southern part into open woodlands and grassland, which in turn will result in a lowered biological diversity and a reduced ability of these ecosystems to store carbon and water.

Other forest ecosystems are faced with a similar fate; according to the Intergovernmental Panel on Climate Change (IPCC), a UN panel of climate scientists, it is likely that many tree species will not be able to change their geographic distribution fast enough to keep up with projected shifts in suitable climate and extinctions are expected to occur.