Is burning biomass contributing to global warming? Should burning wood or other biomass be considered clean, green renewable energy?

Is Burning Wood Carbon Neutral or Does it Contribute to Climate Change? Proponents of harvesting trees for fuel will say that burning wood is carbon neutral as the carbon released by burning the wood will be sequestered in new trees that grow to replace the ones cut down for fuel. Climate scientists will say that burning wood is not really renewable as it takes decades or even centuries for the new trees to grow to the size of the ones cut down, and the tree loses the ability to grow and absorb or sequester carbon after it gets cut down. Climate scientists have been rejecting the “burning wood is carbon neutral” assumption for more than 20 years.

Burning wood produces carbon dioxide (CO2), a greenhouse gas, just like burning coal or natural gas. The question is how much CO2 is released to generate a given amount of electricity? In terms of pounds of CO2 emitted per Megawatt hour of electricity generated, burning biomass, including wood, is worse than coal and twice as bad as natural gas! Sounds terrible and might suggest that burning wood is neither clean or green, but as we will see below, that is not the whole story.

Consider the Fuel Source and Alternatives for Disposal

In judging the climate or health impact of any biomass project, one must consider the alternatives for disposing of that given biomass, especially agricultural waste products. What would happen to that biomass stream if the plants that burn it now were shut down? And what are the climate change and air quality impacts of other disposal alternatives? For agricultural waste as well as the slash associated with forest thinning and timber harvesting activities, one alternative is to let those materials decay naturally on the ground, which will not only release the carbon in the biomass to the atmosphere in short period of time, but much of that carbon will be released as methane, a gas that is more than 100 times more warming than carbon dioxide during its time in the atmosphere. Same problem arrises with chipping wood and letting it decay on the forest floor.

One other alternative for biomass disposal is open pile burning, which also releases the carbon to the atmosphere as carbon dioxide, carbon monoxide, AND as carbon black, soot or particulates. The climate change and air quality/health impacts of open pile burning are also serious.

From both a greenhouse gas emissions and air quality/health perspective, it is far better to burn biomass in a biomass plant that makes electricity, than to let it decay naturally or burn in an open pile. As long as we continue to eat the associated foods, we will need to somehow dispose of the associated agricultural waste. By making electricity, one not only derives some economic value from the biomass, AND be cause of the electricity generated, less electricity needs to be derived from coal or natural gas.

Forest Biomass Can Come From Either Dead and Down or Living Trees 

When it comes to forest biomass, if one is using already dead or down trees, or trees or slash that come from sustainable forest thinning operations, then again it is better to derive some economic value via generating electricity that the alternatives discussed above, natural decay or open pile burning. Burning dead and down wood is carbon neutral as that carbon would be released anyway in the near term and you are not cutting down living trees that are actively sequestering carbon.

The global warming problem associated with burning wood arrises when you are cutting down healthy trees for the sole purpose of creating fuel, e.g. making wood pellets to fuel a biomass facility as the Climate Central article describes. To assure sustainable operations, it is critical to size the biomass facility so that only sustainably harvested or dead and down wood is needed.

The sizing of the Mariposa biomass plant will be based on our fuel availability study and then the size will be such that we are assured that we will use no more than half the available sustainably harvested wood. Using sustainably harvested wood is also a requirement of the law that will allow us to sell the electricity to PG&E at a favorable rate. We will never need to, or be allowed to, cut down a tree because we need more fuel to keep the plant running at full capacity.

You Don’t Have to Burn the Wood to Make Electricity 

The above discussion assumes that the wood is burned at the biomass plant and all of the carbon in the wood is released to the atmosphere as CO2. That is how almost all biomass plants in California operate today. Our biomass plant will NOT burn the wood, but instead will char the wood using a process known as pyrolysis, or heating wood in the absence of oxygen. As explained in the FAQ, with pyrolysis, the hydrogen in the wood and some of the carbon is released as a synthetic gas that can be cleanly burned to make electricity. At the same time, much of the carbon in the wood is retained as form of charcoal, called biochar, rather than being released to the atmosphere. The fact that all the carbon is not released brings the pounds of CO2 released per Megawatt hour well below the levels for coal and petroleum and similar to natural gas.

The Carbon in the Wood can be Sequestered Reversing Climate Change 

The fact that the biochar can be used as a soil supplement that increase a soil’s fertility and water-retention ability, or as activated carbon for water filtration means that the carbon in the biochar will stay sequestered, and not released for hundreds or thousands of years. Actually using trees to absorb CO2 from the atmosphere, turning that carbon into biochar and burying it, is the only practical way known right now of sucking CO2 out of the atmosphere and reversing global warming. This is explained in this article and this one about research at Oxford University.   From a global warming/climate change point of view, biomass plants that make biochar and low carbon electricity are a win/win. And that is what we plan to do with the Mariposa Biomass Project. Hopefully others up and down the Sierra will do the same and there are a number of such plants in the planning stages right now.


So when you read about trees being cut down to make wood pellets for fuel for power plants in Europe and Asia, that is a problem, the practice does contribute to global warming and should be stopped. On the other hand, if one is careful to use already dead or sustainably harvested wood and then sequester much of the carbon in that wood via biochar, that is like undrilling oil or unmeaning coal, to quote Bill McKibben. It is reversing climate change while promoting forest health and fire safety, the other important objectives of the Mariposa Biomass Project.