Q: Why did the Northern Forest Center commission this study?


A: People have questions about the climate impact of wood energy and we felt there was a need for unbiased science-based information on the specific impact of using the region’s wood pellets for heat. Previous studies and media have reported a wide range of results regarding greenhouse gas without clearly distinguishing between the impacts of using wood for heat versus electricity or assessing the full life-cycle impact of the fuel.

Q: Who did this study and what makes them credible?


A: The study was conducted by John Gunn, Ph.D. and Thomas Buchholz, Ph.D. of the Spatial Informatics Group – Natural Assets Laboratory, a non-profit scientific research organization “with a mission to develop and apply the scientific foundation needed to link economic and environmental interests by accounting for the full value of natural assets.” They have broad experience in forest ecology, forestry, carbon accounting, and related topics.

Q: What does this study assess?


A: Our study looked specifically at a life-cycle analysis of the greenhouse gas impacts of using wood pellets from local mills for heat in the Northern Forest. “Local mills” include 9 of the 10 mills serving the region (one mill did not respond to our survey).

Q: What makes this study unique?

A: This study is based on real data from local mills and actual forest harvesting data; it is specific to using these pellets for heat.  No other study is this specific. The research scientists found that from day one, using wood pellets for heat reduces greenhouse gas emissions by 54% compared to oil and 59% compared to natural gas.

Q: Since trees grow back, isn’t wood heat carbon neutral?

A: We describe wood heat as “carbon better.” On a macro scale, wood heat can come very close to carbon neutral over the long term, if the overall stocks of forest in the region remain constant or increase. Making and transporting the pellets generates greenhouse gas emissions; this impact, which was calculated from real data provided by the mills, is incorporated into the life-cycle analysis. (Extraction and transportation of fossil fuels was considered in the analysis as well.) Bottom line, wood pellets produce far less greenhouse gas than oil and natural gas when used as a heating fuel in the Northern Forest region, when you consider the entire life cycle of the fuels.

Q: What’s the difference between “greenhouse gas” and “carbon” emissions?

A: Strictly speaking, “carbon” is an element and “greenhouse gases” are a group of gases that trap heat in the atmosphere. Carbon dioxide makes up the vast majority of greenhouse gas in the atmosphere. Greenhouse gas impact is typically measured in carbon dioxide equivalents (CO2e). The terms greenhouse gas emissions and carbon emissions are often used interchangeably.

Q: I heard conflicting information about the carbon or greenhouse gas impacts of using wood. What should I believe?

A: Results depend on many factors, including whether the wood is generating heat or electricity, whether the results report emissions from combustion only or assess the full life-cycle impacts of the fuel, and specifics related to how the fuel is sourced, processed and delivered. In short, context matters.

Our study assesses the greenhouse gas impact of using wood pellets produced by local pellet mills for heat in the Northern Forest. In that context:
  • From day one, using wood pellets for heat reduces greenhouse gas emissions by 54% compared to oil and 59% compared to natural gas.
  • In 50 years, GHG emissions from pellets drop to 62% less than oil, 67% less than natural gas, and 56% less than propane.

Some reporting of past studies failed to specify the type of energy being produced and has resulted in incorrect generalizations. For instance, the actual results of a study conducted by the Manomet Center for Conservation Sciences (Biomass Sustainability and Carbon Policy Study) found that when it comes to heat, replacing heating oil with efficient wood pellet heat cut greenhouse gas emissions by 25% by 2050. Press coverage of the Manomet study, however, resulted in a range of misleading headlines. 

Q: What about air source heat pumps as a low-carbon heat source?

A: Heat pumps can be a low-carbon heating choice but that’s not always the case. If they’re connected to the electric grid, their greenhouse gas impact depends on the electric fuel mix (i.e. the source of the grid electricity – natural gas, nuclear, etc.). Solar-powered heat pumps are preferable from a carbon standpoint. Even then, heat pumps are rarely a whole-house heating solution and are usually backed up by fossil fuel heating systems. Also, heat pumps aren’t suitable for all houses – they work best in very energy efficient, open-concept homes, not so well in the older, less open and less well insulated houses common in this region.

Q: Does the source of wood used in the pellets make a difference?

A: Yes. In general, the more sawdust (i.e. residue from a sawmill) that goes into a pellet, the lower the greenhouse gas impact.

Q: What about the potential for overharvesting the region’s forests?

The Northern Forest has ample wood supply. In 2015 (most recent data available) the standing live inventory is 937 million cords of wood. Annual net growth is 21.6 million cords. Annual harvest is 12.6 million cords. Markets for low-grade wood (the type that is used in pellets) have been declining. Maine lost 36% of its low-grade market since 2013 through mill closures. This market change means that more low-grade wood is available for producing wood pellets without increasing the harvest rate.

Q: The study is based on Forest Inventory data for Maine – what makes it relevant for the rest of the region?

A: The forest composition across the Northern Forest is consistent that the researchers felt comfortable using the Maine data from the Forest Inventory and Analysis National Program as a proxy for the entire region.

Q: Where did the pellet mill data come from?

A: Nine of the ten mills in the Northern Forest completed a survey that asked, among other topics, about:
  • Feedstock amount (green tons)
  • % hardwood versus softwood species
  • Source of feedstock (forest operations, primary wood processing facility, etc.)
  • Type of feedstock from the forest (harvest residue, small diameter trees, etc.)
  • Average transportation distance from feedstock source to the facility
  • Total pellet production capacity and actual pellet production (most recent year and five year average)
  • Electricity source (% grid, waste wood, solar, wind, other)
  • Process heat source (% natural gas, wood waste, oil, etc.)
  • Typical transport distance to the end user

Q: Wouldn’t the greenhouse gas impact be lower if we didn’t cut trees at all?

A: If all trees could be left growing indefinitely, the greenhouse gas impacts would be lower. However, the forest does not exist in a vacuum. About 85% of the Northern Forest is privately owned, and landowners will decide how to use the trees on their land. Without markets for trees, many landowners would look for other economic opportunities, which could include clearing the land and eliminating the carbon sink permanently. In addition, forests are dynamic systems and are subject to destruction from invasive species, weather and fire.

The Center is not advocating that we increase harvesting to supply energy markets. Rather, we believe that using some of the harvested wood (much of it residuals from other purposes) for wood pellet heat to replace fossil fuels will cut greenhouse gas impacts by more than 50%, starting immediately.

Q: Isn’t natural gas better than oil from a climate perspective?

A: When you account for the full life cycle – including methane leakage associated with natural gas distribution – natural gas has higher greenhouse gas emissions than heating oil.  Methane is a greenhouse gas 30 times more carbon intensive than carbon dioxide, so even a small amount of methane emissions has a major effect. This “distribution loss” (Alvarez et al., 2012) is often excluded from discussions of the climate impact of natural gas.

Relevant Studies

2006 IPCC Guidelines for National Greenhouse Gas Inventories

ForGATE - A Forest Sector Greenhouse Gas Assessment Tool for Maine: Calibration and Overview

Proceedings of the National Academy of Sciences of the United States of America - “Greater focus needed on methane leakage from natural gas infrastructure”