Climate scientists could be about to give oak, ash and maple a bad name. They warn today that expanding forests in the temperate zones of Europe, the US and Asia could add to global warming.
Johannes Feddema of the University of Kansas and six colleagues from the US National Centre for Atmospheric Research report in Science journal that they looked at changes in land use - the growth of cities, clearing of forests for agriculture, and draining of marshes - and their impact on climate change in the next 100 years. They confirmed something environmentalists have predicted for decades - the destruction of the Amazon forest would make the local climate 2C (4F) warmer because trees soak up carbon dioxide from the atmosphere, and burning them releases it. But then the scientists looked at temperate zones and found the opposite.
Simulations predicted the conversion of north American and European forests and grassland to agriculture would cool the region and counteract the effects of global warming by 25%-50%. This is because ripening corn and other staples would reflect more sunlight back into space, and release more moisture into the air, while dark forests would absorb sunlight and send thermometers soaring. Ken Caldeira and a Carnegie Institution team backed the finding in Geophysical Research Letters. "We were hoping to find that growing forests in the US would help slow global warming. But if we are not careful, growing forests could make global warming even worse."
In July, a Newcastle University team argued that forests soaked up water and evaporated it into the atmosphere twice as fast as grassland or crops. In September, European researchers showed that in the hot summer of 2003, most of the carbon stored in forests in the previous four years was released back into the atmosphere, to accelerate global warming and trigger yet more heatwaves. Such findings could exasperate US utility companies that have planted forests to compensate for fossil fuel use, and infuriate ecologists and conservationists who wish to protect forests. The Kansas team called its study a "first step" and Prof Caldeira said: "I like forests. They provide good habitats for plants and animals, and tropical forests are good for climate, so we should be particularly careful to preserve them. But in terms of climate change, we should focus our efforts on things that can really make a difference, like improving efficiency ..."