It is an article of faith that preserving trees is critical to cooling our warming planet.
Millions of hectares of forest are wiped out every year, cut down to make room for agriculture and, increasingly, urban growth, mostly in developing nations. Such deforestation is a major contributor of carbon dioxide to the atmosphere, responsible for 17 percent of the heat-trapping greenhouse gases that are warming the planet, according to the U.S. Environmental Protection Agency.
Governments and environmental groups worldwide have worked hard to reduce that total and provide incentives to plant more trees.
But now two Dartmouth College researchers are suggesting that in some limited circumstances, the cooling value of an open, snowy field may be greater than the climate benefits that a stand of trees can provide — and that it is possible to calculate the places where that might be the case.
In the short term, they said, such knowledge would be useful in forestry management. In the longer term, nations with vast expanses of snowfields that reflect heat back into space might seek to be credited for that contribution to reducing global warming during international climate negotiations.
"In some cases, the cooling influence of albedo can equal and surpass the climatic benefits of carbon sequestration from forest growth," postdoctoral fellow David Lutz and professor of environmental studies Richard Howarth wrote in their paper. Funded by the National Science Foundation, it was presented at the fall meeting of the American Geophysical Union last week.
Albedo is the reflectivity of the Earth's surface, the amount of solar energy it sends back into space. White surfaces, such as snow, send back more of this shortwave radiation than dark surfaces such as green, leafy forests.
What that means, in a practical sense, is that when managers are trying to determine when to harvest trees, they might want to account for the benefits of the open field they will leave behind.
In places where snow remains on the ground for considerable lengths of time and trees grow slowly, limiting the amount of carbon they can take in, it might make sense to take down trees sooner rather than later and leave fields unplanted longer.
"In spruce-fir stands, very short rotation periods of just 25 years become economically optimal when albedo is considered," Lutz and Howarth wrote. They noted that there is a body of research that shows that cutting down boreal forests can produce "net climatic benefits."
"This is an important publication," said Jonathan Thompson, a senior ecologist at the Harvard Forest at Harvard University, who was not associated with the research. "It starts to quantify the trade-offs between those two (benefits)."
One place where that may be the case is the White Mountain National Forest in New Hampshire, where Lutz and Howarth conducted their research, using models to assign values to albedo and carbon capture. High on those mountains, the snowfall is heavy and remains on the ground well into the spring, while tree coverage can be sparse and slow-growing.
"This should guide policy into saying, 'Well, maybe we shouldn't pay people to grow forests in northern Maine,' " Lutz said. "You may be doing the opposite of what you want."
But the pair stressed that their work should not be construed as support for heavy cutting of trees or deforestation.
The researchers also considered the value of the timber itself, noting that a forest management decision could come down to whether the wood is used, for example, for furniture or pellets to run a power plant. Only a small portion of the world's trees are used to produce durable goods, Thompson said.
"In areas where the albedo effect can roughly cancel out the carbon effect, (then) what you should really concentrate on is the value of the wood products," Howarth said.
Climate change may change the equation. The warming planet will have less snow and shorter winters, reducing the albedo that could help cool it, the researchers noted, saying more research needs to be conducted in this area.
"One of the next steps you'd need to think about is how climate change will actually occur and how that will change the snow pattern and whether the albedo is going to change," Thompson said. "The snowpack is always changing."