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Study: Thinning Forests for Bioenergy Can Worsen Climate

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A new study out of the Geos Institute in Ashland, Oregon concludes that selectively logging or “thinning” forests for bioenergy can increase the amount of carbon dioxide in the atmosphere and exacerbate climate change.

The study, “Thinning Combined With Biomass Energy Production May Increase, Rather Than Reduce, Greenhouse Gas Emissions,” by D.A. DellaSala and M. Koopman, challenges bioenergy and timber industry assertions that logging forests will aid in the fight against climate change.

DellaSala and Koopman also refute assumptions that wildfires are bigger or more severe than in the past, citing multiple studies showing that the occurrence of wildfire has actually “changed little from historical (early European settlement) times.”

The Western Governor’s Association has stated that 10.6 million acres of western forests are available for “hazardous fuel reduction.” Yet, instead of instead of the build up of “fuel” (aka small trees and understory plants) being the main driver of large wildfire, the study authors blame climate, namely drought and high temperatures, explaining that, “during severe weather events, even thinned sites will burn.”

Instead of preventing large wildfires, the study argues that thinning can increase the chance of severe fire by opening the forest canopy which can dry out the forest, leaving flammable slash piles on the ground, and allowing winds to penetrate the previously sheltered stands, potentially spreading wildfire. Post-fire “salvage” logging is also thought to increase the risk of a re-burn.  

Carbon emissions from wildfire have long been an argument to log forests, in an effort to harness energy from trees that may burn at some point anyway. Yet findings show that after a fire the majority of the carbon remains in dead trees, with severe fires that kill most trees in the area emitting 5-30% of stored carbon. Severe fires account for 12-14% of the area burned in large fires. 

Even in the cases where thinning would be effective at stopping wildfire--typically small fires of limited threat to public safety--the study cites computer simulations estimating a 5-8% chance of a thinned parcel experiencing fire within the first twenty years, when fuels are lowest. The chance of encountering severe fire is 2%.

DellaSala and Koopman also urge an accurate carbon accounting of forest bioenergy, cautioning that the amount of carbon dioxide released from burning woody biomass is “often comparable to coal and much larger than that of oil and natural gas due to inefficiencies in burning wood for fuel compared to more energy- dense fossil fuels.”

In the rare cases in which a thinned forest is allowed to grow back without repeated logging, the several decades over which forests could reabsorb carbon “conflicts with current policy imperatives requiring drastic cuts in emissions over the near term.”

The study warns about “large-scale clearing of forests” at a time when natural forests are needed to buffer the planet against runaway climate change.  

"Woody biomass," said DellaSalla in a December 17 phone interview, "almost never pencils out as an efficient renewable energy source."