Trees’ footprints smaller than steel’s

U. WASHINGTON (US) — By regularly harvesting trees and using wood in place of steel and concrete, the amount of carbon dioxide taken out of the atmosphere by a forest could be quadrupled in 100 years.

“Every time you see a wood building, it’s a storehouse of carbon from the forest. When you see steel or concrete, you’re seeing the emissions of carbon dioxide that had to go into the atmosphere for those structures to go up,” says Bruce Lippke, professor of forest resources at the University of Washington.

New research, published in the journal Carbon Management, identifies many opportunities to use wood that will displace products that cause a one-way flow of carbon dioxide from fossil fuel emissions to the atmosphere, contributing to the risk of global warming.

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Sustainably managed forests are essentially carbon neutral as they provide an equal, two-way flow of carbon dioxide: the gas that trees absorb while growing eventually goes back to the atmosphere when, for example, a tree falls in the forest and decays, trees burn in a wildfire, or a wood cabinet goes to a landfill and rots.

The best approach for reducing carbon emissions involves growing wood as fast as possible, harvesting before tree growth begins to taper off, and using the wood in place of products that are most fossil-fuel intensive, or even using woody biomass to produce biofuels for use in place of fossil fuels.

The study doesn’t advocate that all forests be harvested in this way, just the ones that are particularly needed to use to help counter the buildup of carbon dioxide in the atmosphere. Older forests provide many needed ecological values although their ability to absorb carbon dioxide slows down.

“While the carbon in the wood stored in forests is substantial, like any garden, forests have limited capacity to absorb carbon from the atmosphere as they age,” Lippke says. “And there’s always a chance a fire will sweep through a mature forest, immediately releasing the carbon dioxide in the trees back to the atmosphere.

“However, like harvesting a garden sustainably, we can use the wood grown in our forests for products and biofuels to displace the use of fossil-intensive products and fuels like steel, concrete, coal, and oil.”

Tradeoffs are best revealed through life cycle analysis—sometimes called a cradle-to-grave analysis, Loppke says, that assesses environmental impacts for all stages of a product including materials extraction, energy for processing and manufacturing, product use, and ultimate disposal.

Some of the longest-lived wood products are those used for housing and light industrial buildings, estimated to have a useful life of at least 80 years. For every use of wood there are alternatives, for example, wood studs can be replaced by steel studs, wood floors by concrete slab floors, and woody biofuels by fossil fuel.

Using life cycle analysis the researchers compared replacing steel floor joists with engineered wood joists, to reduce the carbon footprint by almost 10 tons of carbon dioxide for every ton of wood used. In another example, wood flooring instead of concrete slab flooring was found to reduce the carbon footprint by approximately 3.5 tons of carbon dioxide for every ton of wood used.

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