Adding vast forests to cut climate change could boomerang, study says

Dec 13, 2011 By Peter Reuell
“There are lots of reasons why planting trees is a good thing,” said Abigail Swann, Giorgio Ruffolo Post-Doctoral Fellow in Sustainability Science. “There are many local benefits. But when you think about it as a global-scale climate mitigation strategy … we need to think about what some of the side effects would be." Credit: Meghan Dhaliwal/Harvard Staff Photographer

Planting a tree is always a good thing, right? After all, trees provide natural beauty and wildlife habitat, and are good for the environment.

But what if, instead of one tree, you’re planting a million ? Or 10 million? Or 100 million?

Such massive expansion of forest cover in North America and Eurasia — proposed by some analysts as a way to combat climate change — could have the surprising effect of altering the environment by increasing temperatures in some parts of the world, resulting in changes in rainfall patterns across the globe, Harvard researchers say.

The research, described in the Proceedings of the National Academy of Sciences on Dec. 5, found that “we can’t plant our way out of the (climate) problem,” said Abigail Swann, Giorgio Ruffolo Post-Doctoral Fellow in Sustainability Science, and the paper’s lead author.

“There are lots of reasons why planting trees is a good thing,” Swann said. “There are many local benefits. But when you think about it as a global-scale climate mitigation strategy … we need to think about what some of the side effects would be.

“The trees would soak up carbon, but the changes in rainfall patterns would cause the Amazon forest to be less productive,” Swann added. “The surest way to mitigate is to reduce emissions directly, rather than trying to plant trees.”

Studying worldwide climate changes, however, is easier said than done.

To do so, Swann turned to a computer and the Community Climate Model, maintained by the National Center for Atmospheric Research in Boulder, Colo. Using the model, she explored the effects of replanting all the agricultural fields and grasslands in North America and Eurasia with forests. The results were not encouraging.

Swann demonstrated that, while new forests would soak up carbon from the atmosphere, the addition of millions of acres of new forest would actually increase temperatures where there is not sufficient soil moisture to support the tree demand, leading to changes in atmospheric circulation and in rainfall patterns.

As unlikely as the idea sounds, Swann said the possibility of replanting forests across vast swaths of North America is more likely than many may think.

“Most of the corn produced in the U.S. goes into producing ethanol,” Swann said. “If there is a switch from corn-based to cellulosic ethanol, there could be significant amounts of agricultural land that we might consider replanting as forest.

“That being said, I think the central finding of this paper is that there are many complications with using forests to plant your way out of the problem,” Swann continued.

“You don’t get much benefit in terms of lowering global temperatures, and you have a number of remote side effects from increasing forest cover,” Swann said. “This paper is more of a warning that we need to think this problem through more carefully, if we are going to go ahead with something like this as a mitigation strategy, and reducing emissions directly is a much better way to remove carbon from the atmosphere.”


This story is published courtesy of the Harvard Gazette, Harvard University’s official newspaper. For additional university news, visit Harvard.edu.

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Waterdog
5 / 5 (2) Dec 13, 2011
long on hyperbole, short on details and facts. How does planting trees hurt global warming? Are we talking monoculture planting or planting that mimics nature? Many questions -few answers!
Nanobanano
2.8 / 5 (5) Dec 13, 2011
long on hyperbole, short on details and facts. How does planting trees hurt global warming? Are we talking monoculture planting or planting that mimics nature? Many questions -few answers!


It's complicated.

Trees store water, which changes the atmosphere.

Trees break up the soil at deeper layers than grasses or shrubs, and this brings carbon to the surface.

Leaves fall and decompose, which releases much of the carbon right back into the atmosphere anyway.

It probably takes 30 years or more for most trees to grow to a TON of mass in the trunk and branches, and only maybe half of that is Carbon.

Then you have to figure, if you have more forests you'll have more forest fires, especially in N. America, which totally off-sets much of the sequestration of the wood...

If you planeted a million acres with 1000 trees each it would take 30 years for them to sequester a half ton of carbon each, which is only a half billion tons in 30 years....continued..
Nanobanano
2.6 / 5 (5) Dec 13, 2011
Since carbon represents 12/44 of a CO2 molecule by mass, then a half billion tons of Carbon only equals about one THIRD of one part per million CO2.

This means it would take 30 years for a million acres of forest to sequester 1/6th of our CURRENT annual surplus of ATMOSPHERIC CO2, IF there are no forest fires.

In order to absorb the entire excess CO2 we make, most of which is presently sequestered in the oceans, we would need 18 million acres of ADDITIONAL trees, AND we would need them to never experience a forest fire...and of course, that would take 30 years to offset just ONE year worth of our CO2 production.

In order to offset all of the CO2 production in 30 years, we'd need 30 years worth of trees, which would be 540 million acres of 1000 trees each, and again, they can never have a forest fire.

540 million acres is close to 2% of all land area on the planet, or half the area of the United States.
GSwift7
2 / 5 (4) Dec 13, 2011
This paper is more of a warning that we need to think this problem through more carefully, if we are going to go ahead with something like this as a mitigation strategy, and reducing emissions directly is a much better way to remove carbon from the atmosphere


The US East Coast has already been re-forested to a large extent, so it should be possible to test her hypothesis to some degree, at least on a regional scale.

However, if she is correct, and planting a bunch of trees causes a temperature increase, then it is worth mentioning that there are also advantages to warmer temperatures. It is not a one sided topic any way you slice it.

Since I love forests, hiking, camping, etc I say forests are good, regardless of the co2/temperature thing.
RealityCheck
2.6 / 5 (5) Dec 13, 2011
Unfortunately, the logic, conclusions and the 'observations' are flawed in this 'work'. It ignore that:-

1. The transpiration from trees spreads over the year, so moisture does not escape into the atmosphere 'all at once' after rainfall as in desert/grass plains, and the shade also conserves local moisture;

2. Deep root systems create fissures and pores etc which create deep storage of rainwater, hence increased habitat potential for other biomass systems like fungi, worms, rodents, lizards, insects and birds in that 'food chain' so created by forest;

3. Erosion is controlled and soils improved for later food production activities harvesting compatible 'understory' plantings;

4. Tree-transpired moisture creates moist microclimates & local rain/dew patterns, so 'cycling' moisture locally and creating streams etc.

5. HARNESSING sunlight FIXES atmospheric carbon using sunlight that OTHERWISE would heat surface/atmosph, so TWO 'bangs for the buck'!

Rethink it methinks!

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