Where to plant forests to fight climate change?
21 Nov 2009
I was surprised to see this headline in New Scientist: “Trees in far north provide biggest climate benefit”.
That goes against everything I’d read before on forests and climate change, and would seem to shoot down the idea of covering the Sahara and Australia with forests as a way of fighting climate change. (And there are far more details over on Oliver Morton’s blog.)
If what the New Scientist story said were true, it would seem to be the death knell for the Sahara forest idea, and it would seem people should plant more forests in Canada and Russia instead.
Here’s the deal: Tree trunks are also mostly carbon, made from CO2 in the air. So if you plant a new forest, as it grows it pulls CO2 out of the air, counteracting some of people’s CO2 emissions. That would cool the planet, compared with what the temperature would have been otherwise.
But forests are also dark, so they absorb a lot of sunlight, turning it into heat—just as black seats in a car will burn your butt on a hot day, but light-colored seats aren’t so bad. If you replace light-colored land—say, deserts, or icy tundra—with forests, then the trees absorb more heat than what was there before. In that way, forests put in the wrong places could actually heat the planet.
What I’d read before said that planting forests up north would actually warm the planet. That’s why when scientist Ken Caldeira wrote an opinion piece on this for the New York Times, it was called “When Being Green Raises the Heat”. That’s why I was surprised by the New Scientist story.
To try to sort this out, I got a hold of Leonard Ornstein, the biologist behind the Sahara forest idea. He says, “The problem is due to confusion between the rate of bio-sequestration per unit area, and the total bio-sequestration per unit area.” (By bio-sequestration, he means the carbon that living things pull from the air and lock away in tree trunks, soils, and so on.)
“The rate [of carbon storage] in the tropics is MUCH greater than anywhere else,” Ornstein says. That is, in the warm, moist belt around the equator, trees grow much faster, storing up far more carbon in their trunks each year.
On the other hand, forests farther from the equator—say in the U.S. or Canada or Russia—win out on total amount of carbon stored per acre. That’s because they lock a lot of carbon away in the soils, which builds up over a long time. In the tropics, bugs, microbes and other creepy crawlies eat up the fallen leaves and other bits of stuff, releasing its carbon back into the atmosphere again, so there’s less carbon stored in tropical forests’ soils, acre for acre.
“So, in the ‘short’ term, afforestation of large deserts yields both the most rapid and largest draw down for a few centuries,” Ornstein says. “But over millennia, total sequestration from temperate and boreal reforestation could be greater—if those reforested areas are large enough.”
Unfortunately we don’t have millennia to stop temperatures from rising disastrously. We have decades. In this case, slow and steady does not win the race; it’s a sprint.