Forest fires cool northern climates

Forest fires cool northern climates

16 November 2006

published by www.newscientist.com

Forest fires in northern countries may cause regional cooling and not warming,as was previously thought. Researchers say their new findings could mean that ona global scale, forest fires will not affect climate change one way or the other.

Unusually large fires have blazed across Canada, Alaska, Russia, Norway,Sweden and other northern countries over the past decade. Researchers have saidthat warmer climates, longer summers and generally drier conditions may beincreasing their frequency.

It was thought releasing the large amounts of carbon dioxide stored in treeswould mean that forest fires would contribute to the greenhouse effect and starta fiery feedback loop. Not so, says a team of 17 US and Australian researchersin Science.

The team took an overall look at the effects of a forest fire that burnedabout 6.7 hectares (16.5 acres) of the Donnely Flats in central Alaska in June1999. Some researchers looked changes in how the area reflected radiation fromthe sun, while others looked at greenhouse gas emissions and changes invegetation.

They then plugged all their data into a computer model and projected it 80years forward to see how a single fire would affect climate in the short andmedium term.

They found that while temperatures did warm for about one year after the fire,this was reversed within 10 to 15 years. Averaged over 80 years, the overalleffect was a cooling of temperatures.

More snow

One of the biggest reasons for the cooling effect is that while the darkspruce trees absorb the warmth of the Sun, the snow reflects it back. After theforest fire had removed the trees, only snow was left on the ground.

However, black soot resting on the snow immediately after the fires darkensthe ground, which accounts for the initial warming. The soot disappeared withthe snow melt in the spring and the following winter pure snow covered the area,and started the cooling trend.

“This cooling effect cancels the impact of the greenhouse gases, so thenet effect of fire is close to neutral when averaged globally, and in northernregions may lead to slightly colder temperatures,” explains James Randersonof the University of California at Irvine, who led the study.

Eventually, new grasses, shrubs and trees grow back. But researchers at theUniversity of Florida who participated in the study found that the first treesto replace the burnt conifers were aspen, birch and other deciduous trees.

Their large, light-green leaves reflected more of the Sun’s energy thantheir pine-needled predecessors, further contributing to the cooling trend. Andthe new trees are deciduous not evergreen, meaning they lost their leaves in thewinter exposing the reflective snow.

Burn interval

It will take decades for the black spruce to grow back at Donnely Flats, saythe researchers. In northern countries, forest fires typically recur in the samearea every 80 to 150 years.

Climate researchers predict this interval will shorten as global temperaturesrise due to greenhouse gas emissions. This means that returning conifers wouldsoon be burned again, once more causing a long-term drop in Northerntemperatures.

But the study does not suggest that cutting down trees is a good option totackle global warming in northern latitudes, cautions Randerson: “There aremany, many reasons for maintaining northern forest ecosystems. They havetremendous value from the perspective of water resources, wildlife, timber, andrecreation.”

Journal reference: Science (vol 314, p 1130)