In this article we examine the effect of wood burning on climate change and answer the wider question whether biomass burning is inherently carbon neutral. We also look at pollution caused by wood combustion. These are important matters both for climate change mitigation and for the issue of what constitutes renewable energy.
- Do Wildfires Affect Global Warming?
- Who Says Wood Has No Effect On Climate Change?
- Wood Burning is Carbon Neutral
- Wood Burning Is Not Carbon Neutral
- Effect of Wood Burning On CO2 Emissions
- Is Wood Better For Climate Than Coal Or Natural Gas?
- What’s the Effect of a Wood Burning Stove on Climate Change?
- What Are The Pollution Effects of Burning Wood?
Whether wood burning is good or bad for the climate system is equally important for both the developing and developed world.
In developing countries, wood used to be the principal source of energy for cooking food and for keeping warm. 1 Nowadays, there is less reliance on fuelwood, but charcoal is the main cooking fuel in sub-Saharan Africa, South Asia, Latin America and the Caribbean, especially in urban areas. 2 3 4
Unfortunately, the thermal energy yield of wood is low. On average, between 4 kg and 10 kg of wood is needed to produce 1 kg of charcoal. For example, in 2017, global charcoal production was 51.2 million tons. This would have required roughly 200-500 million tons of wood.
In developed countries – where, ironically, the priority is decarbonization, not carbonization – the focus is on renewable energy, and wood is now seen as a legitimate renewable fuel, on a par with wind and solar power. On April 23, 2018, the head of the U.S. Environmental Protection Agency (EPA) announced that the agency now defines wood as a “carbon-neutral” fuel. This announcement, coupled with other goverment PR campaigns extolling the benefits of renewable energy, and millions of dollars worth of grants available for producers of renewables, has led to a resurgence in demand on both sides of the Atlantic, for this new bioenergy. 5
Do Wildfires Affect Global Warming?
Yes. Whenever there’s a serious outbreak of wildfires in the world, governments, scientists, satellite-based atmosphere-monitoring services and the media, all talk about the climate threat posed by the fires’ emissions of carbon dioxide (CO2).
On November 30, 2018, for instance, U.S. Secretary of the Interior Ryan Zinke announced that according to estimates from the U.S. Geological Survey (USGS), recent Californian wildfires had released 68 million tons of CO2 into the atmosphere – the equivalent of the entire annual emissions of the state’s electricity power plants.
In 2019, according to NASA, Arctic fires in Siberia, Alaska, Greenland and Canada released a record 50 million tons of CO2 in the month of June – equivalent to the total annual emissions of Sweden – and 79 million tons in July. The Russian fires alone generated a cloud of smoke and soot larger in area than the European Union.
Greenpeace Russia’s wildland fire expert, Anton Beneslavsky, warned: “if we talk about mitigating climate change, we need to urgently pay attention to the wildfire issue.” 6
In 2019-2020, record-breaking Australian bushfires that devastated areas across NSW are estimated to have released twice the country’s entire annual greenhouse gas emissions from fossil-fuels, or roughly 830 million tons. 7 According to David Bowman, professor of pyrogeography and fire science at the University of Tasmania, so much damage has been caused that Australian forests may take more than 100 years to re-absorb the carbon that has been emitted in less than one fire season.
In January 2020, the British Meterological Office stated that Australia’s 2019-2020 wildfires were expected to contribute 2 percent to the increase in the atmospheric level of carbon dioxide, which is projected to reach 417 ppm – one of the largest annual increases in atmospheric CO2 on record.
If Wildfires Release CO2, Wood Burning Affects Climate Change, Too
Nobody says: “wildfires release greenhouse gases like carbon dioxide but don’t worry, it won’t affect global warming because wood is carbon-neutral.” They just say – “Look at all that nasty CO2 going up into the air and ramping up global warming.” In other words, if wildfires are bad for climate change, then surely burning wood in a power station is also bad for climate change.
In other words, wood-burning has no place in any rational climate action plan.
Carbon Emissions From Burning Woodchips & Wood Pellets, Compared to Fossil Fuels
|Woodchips (25% MC)||14||37%||7|
|Wood pellets (10% MC/from dry wood)||17||45%||15|
|Wood pellets (10% MC/from green)||17||45%||80|
|Grasses/straw (15% MC)||14.5||38%||5-15|
Sources: “Carbon and energy balances for a range of biofuels options.” Elsayed, MA, et al; 2003 PDF; “Greenhouse gas reporting: Conversion factors 2016.” UK Govt, Dept Trade & Industry Study (DTI URN 03/836).
Who Says Wood Has No Effect On Climate Change?
Let’s start with the US forest products industry. They have been pushing for ages to get the EPA to redefine wood as carbon neutral. Now that the EPA has obliged by designating wood as carbon neutral, it has made wood an attractive fuel for generating electricity, and the forest products industry is likely to make lots of money.
The EU is also a wood supporter. The bloc wishes to reduce its dependency on coal and other fossil fuels such as petroleum and natural gas. It has anointed wood as the new renewable, despite widespread objections from climate scientists, who warn that encouraging wood burning will boost carbon emissions, not curb them.
Critics also point to the fact that classifying wood as a renewable makes it it eligible for beneficial tax treatment, climate change grants and subsidies, and less onerous environmental regulation. As a result, they fear it could encourage deforestation in Southeast Asia as well as illegal logging in primary forests like the Congo Rainforest, and lead to more deforestation around the world, causing serious harm to biodiversity in the name of climate action. They believe that deforestation in the Amazon Rainforest is already causing enough loss of biodiversity without adding to it.
The poster child for this EU policy is the 4,000-megawatt Drax power plant in North Yorkshire, UK. Initially coal-fired, the plant has since converted two-thirds of its boilers from coal to wood combustion, spurred on by $700 million of government subsidies. Its biomass operation is currently fuelled by more than 6 million tons of wood pellets imported annually from the United States. By switching to wood, Drax saves 12 million tons of CO2 emissions a year, making it “the largest carbon-saving project in Europe,” according to its management. It’s a view shared by the EU and the U.K. 8
Many other industrialized nations – including the U.S., South Korea, and Japan – have designated biomass (including wood) as a carbon-neutral source of energy, and have taken steps to incorporate it as key part of their strategies to reduce their emission of greenhouse gases and decarbonize their economies. See also: 10 Reasons Why Plants are Important.
It’s worth noting that, under the terms of the UN Paris climate agreement – which nearly all nations signed up to – any loss of carbon from forests supplying wood to power plants must be declared as a change to the carbon sink capacity of the country concerned. This is an important conservation policy, adhered to by both the UNFCCC and its scientific advisory panel, the Intergovernmental Panel on Climate Change (IPCC). Thus by implication, tree conservation is a stated objective of both the world’s top two climate bodies. See also: Tree-Planting the Answer to Global Warming?
Wood Burning is Carbon Neutral
Those who favour the policy of burning wood for fuel, maintain that this activity is largely carbon neutral for three separate reasons.
First, the wood being burned has already absorbed a similar amount of CO2 during its lifetime. It cannot therefore be viewed as adding to our climate crisis.
Second, the wood burned is derived from ‘forestry residues’ (slash) – tree branches and other material that are left after commercial timber harvesting, as well as ‘sawmill residues’ like sawdust and chips. Because these materials are expected to decay over time, emitting CO2 in the process, burning them to generate energy emits no more carbon than if they were left to decompose.
Third, any carbon dioxide (CO2) emitted during combustion will be recaptured via photosynthesis by other newly-planted trees over the course of their lifetime. This natural absorption of CO2 means that, over the long term, burning wood is emission-free and cannot be compared to the combustion of fossil fuels like coal and gas.
Wood Burning Is Not Carbon Neutral
Critics counter these contentions, with the following arguments:
• To limit global warming, we need to reduce our CO2 emissions. To say that a 60-year old tree can be cut down (thus eliminating its ability to absorb CO2 via photosynthesis), and burned (thus releasing heat-trapping CO2 into the atmosphere) without adding to the greenhouse effect and raising temperatures – simply because of its earlier success in absorbing CO2 – is nonsense. Everyone knows that CO2 is a dangerous greenhouse gas and, if released into the air, it will add to global warming. 9
• The ‘forestry residues’ argument is equally weak. First off, it takes decades for trees’ tops and branches to decompose on the forest floor, and during that process, much of their carbon is absorbed by the soil and its organisms. By contrast, burning a tree trunk releases all its carbon instantaneously, which is bad news. We want our emissions to peak no later than 2050. Also, as we explain below, there isn’t enough ‘residue’ or ‘waste’ to go round, which is why wood suppliers are now using entire trees, sometimes extracted from first-growth forests. This completely undermines the case for wood being carbon neutral.
• Cutting down and burning a tree takes a matter of hours, or days. For a newly-planted sapling to absorb enough CO2 to compensate, takes at least 60 years. During this time, the CO2 released by the burnt tree will not only add to climate change, part of it (about 20 percent) will remain in the atmosphere for hundreds, even thousands, of years. 10
• In practice, most tree plantations are harvested at 20 year intervals – well before its trees are able to absorb enough CO2 to offset earlier losses. One recent study, for instance, estimates that it takes 40-100 years for a managed forest to sequester the same amount of carbon as a natural forest. 11
• Perhaps the biggest weakness of the “newly-planted trees absorbing CO2” argument, is that in most cases nobody can be certain that the new trees will ever be planted, or allowed to grow to full maturity. But unless forests are guaranteed to be planted and to be allowed to grow to carbon parity, the use of wood pellets for fuel is bound to result in more CO2 and more global warming.
• Cutting down trees – never mind the widening of the forest roads and staging areas – exposes the forest floor to more light and higher temperatures. This leads to more rapid microbial decomposition of organic debris in the soil, resulting in higher emissions of CO2. And soil typically stores up to twice as much carbon as trees do. See also: Why is Soil So Important to the Planet? According to a recent study, forest soils are already giving off more carbon as warming intensifies. 12
• As we have seen, the EU is already using wood as a “carbon neutral” energy source to meet renewable energy goals laid down by the Paris Accord. One of the major arguments used to justify the use of wood, is that only tree limbs, woodchips and sawdust would be incinerated. But the reality appears to be quite different.
The surge in demand for climate friendly wood pellets has triggered an increase in logging both in the southeastern United States and and across Europe. This includes illegal logging of old-growth forest in sensitive biomes, like the Carpathian Mountains of Romania and national reserves in Poland and Slovakia. 8
Forests in the U.S. are being devastated by logging to satisfy demand for wood pellets to fuel biomass energy plants in Europe and Japan, according to a report by the Natural Resources Defense Council, the Dogwood Alliance, and the Southern Environmental Law Center. According to the report, mature hardwood forests in North Carolina, Virginia, and along the Gulf Coast are being clear-cut, with whole trees being trucked to processing mills. 13 According to the United Nations, forested areas in the U.S. Southeast are suffering at four times the rate of those in the Amazon Basin.
Critics contend that these arguments prove that wood is not carbon neutral and should not be relied upon as a renewable or sustainable source of energy.
In 2011, the Scientific Committee of the European Environment Agency (EEA) published a statement, advising on carbon accounting in relation to bioenergy, in which it warned of a “serious error” in the baseline definition in European bioenergy policy. According to the Committee, counting biomass used for power generation as ‘carbon neutral’ or having ‘zero-emissions’ is incorrect, and will have “immense” negative consequences for the environment. (See also: The Carbon Cycle: How Does it Work?)
In 2012, a U.S. Environmental Protection Agency Science Advisory Board ruled that bioenergy (energy from biomass including wood) is not inherently “carbon neutral” in the near term – a view shared by more than 90 leading U.S. scientists.
Effect of Wood Burning On CO2 Emissions
When wood is burned, it generates higher CO2 emissions per unit of energy generated than fossil fuels. For example, as outlined in a recent study, chimney emissions from burning wood for heat can be 30 percent higher than those of coal and 2.5 times greater than those of natural gas, per unit of generated energy. 14 When incinerating wood for electricity, smokestack emissions can be 1.5 times those of coal (per MWh) and more than three times higher than those of natural gas.
In any event, it takes a long time before the CO2 captured by the planting and growth of additional trees offsets the increase in CO2 emissions caused by burning the original trees. This delay in achieving ‘carbon balance’ is referred to as carbon payback time. Carbon payback time varies according to the type of tree burned, the location and type of forest it came from, the type of energy generation facility in which it was burned, and the type of fossil fuel being replaced. If the trees come from first-growth forests, or from biomes like the Amazon Rainforest, or if the planted trees are not allowed to reach maturity there may never be a complete payback.
One study, for example, analyzed a broad mix of forests and harvesting regimes, and discovered that most options had a payback time of more than a century, with a minimum payback time (applying to only a small number of cases) of at least 30 years. 15
Unfortunately, as our climate crisis intensifies, we need to dramatically reduce greenhouse gas emissions over the next three decades and peak global emissions as soon as possible, if we are to limit global temperature projections to 2°C above pre-industrial levels. So now is not the time to encourage an increase in atmospheric CO2 levels over a 10-year or 100-year time scale.
Is Wood Better For Climate Than Coal Or Natural Gas?
A coal-fired power station emits on average 1,018 kilograms of CO2 per megawatt hour of electricity. Natural gas plants emit about 437 kilograms per megawatt hour. Whether a wood-powered plant can improve on these figures, depends on a range of factors. This is why the UK Department of Energy & Climate Change designed a special biomass calculator to assess the operations at the Drax power station.
According to this calculator, if Drax burns only wood residues like twigs or sawdust that would otherwise have been burnt as waste, then emissions will be under 100 kilograms per megawatt hour – that is, at least ten times lower than burning coal.
If Drax burns biomass from boreal forests in the Northern Hemisphere this produces higher emissions than natural gas, but still lower than coal.
However, a number of other scenarios show that burning wood is worse than burning coal – up to 5,174 kilograms per megawatt hour – five times higher than emissions from coal. This is partly due to the fact that the calorific value (heat) of wood chips and wood pellets is significantly lower than that of coal or natural gas, which means that more wood needs to be burned.
What’s the Effect of a Wood Burning Stove on Climate Change?
The effect of a wood burning stove on climate change varies with circumstances. A stove is bad for global warming in some respects, but good in others.
Whenever wood is burned, methane (CH4) is also produced – roughly 70 g of CO2 equivalent per kilogram of wood. In addition, wood stoves release carbon dioxide (CO2), and nitrous oxide (N2O), as well as carbon monoxide (CO) and certain precursors to ozone (O3), another greenhouse gas. All these greenhouse gases add to global warming.
Wood stoves also give off tiny aerosols of partially combusted material, called black carbon, which has both a climate warming and climate cooling effect. It warms by absorbing sunlight and by reducing the ice albedo when it falls to earth. It cools by blocking sunlight from reaching the surface of the Earth.
On balance, wood stoves increase climate change twice as much as they reduce it.
What Are The Pollution Effects of Burning Wood?
In addition to its effects on climate change, wood burning is also a major contributor to air pollution, through the release of several highly toxic air pollutants including: carbon monoxide, acrolein, benzene, formaldehyde, and polycyclic aromatic hydrocarbons (PAHs). Some of these react with ground level ozone in urban areas, in Asia, to boost levels of urban smog during the dry winter months. See also: Asian Brown Cloud: Toxic Haze.
However, the worst environmental effects of burning wood come from the release of very fine particles (particulate matter, or PM2.5) of partially burnt material, seen as smoke. These microscopic aerosols can damage the respiratory system, causing bronchitis and other lung problems. See, for example, the Health Effects of Air Pollution.
in the developing world, where as many as 3 billion people use indoor open fires for cooking and heating, wood and biomass are the most common fuels. This leads to high levels of indoor air pollution, which – according to WHO – accounts for 3.8 million premature deaths, annually.
Particulate matter makes asthma symptoms worse and can trigger asthma attacks. It can also cause heart attacks, stroke, and heart failure. If you have heart or lung disease, emphysema or asthma, it’s extremely important to limit your exposure to smoke. The more efficiently you burn wood, by using only dry, seasoned wood, and by burning it thoroughly, the less smoke and particulate matter is created.
The World Health Organization (WHO) states that airborne particles – notably “PM2.5”, that is microscopic particles less than 2.5 microns in diameter – accounts for more than 4.2 million premature deaths around the world. 16
- “Fuelwood and Charcoal in Developing Countries.” FAO.
- “What role will charcoal play in the coming decades? Insights from up-to-date findings and reviews.” Ghilardi, A., Mwampamba, T., and Dutt, G. (2013). Energy Sustain. Dev. 17, 73–74.
- “Charcoal, livelihoods, and poverty reduction: evidence from sub-Saharan Africa.” Zulu, L. C., and Richardson, R. B. (2013). Energy Sustain. Dev. 17, 127–137.
- “Dispelling misconceptions to improve attitudes and policy outlook on charcoal in developing countries.” Mwampamba, T. H., et al; (2013). Energy Sustain. Dev. 17, 75–85.
- “U.S. EPA says it will define wood as a ‘carbon-neutral’ fuel, reigniting debate.”
- “Massive Arctic wildfires emitted more CO2 in June than Sweden does in an entire year.” Aug 17, 2019
- “Australian bushfires to contribute to huge annual increase in global carbon dioxide.” The Guardian. Jan 24, 2020.
- “Carbon Loophole: Why Is Wood Burning Counted as Green Energy?” Fred Pearce.
- “Is woody bioenergy carbon neutral? A comparative assessment of emissions from consumption of woody bioenergy and fossil fuel.” Giuliana Zanchi, et al; 2011.
- See also: “Correcting a fundamental error in greenhouse gas accounting related to bioenergy.” Helmut Haberl, et al; Energy Policy. Volume 45, June 2012, Pages 18-23.
- “Are wood pellets a green fuel?” William H. Schlesinger. Science, 2018; 359 (6382): 1328.
- “Long-term pattern and magnitude of soil carbon feedback to the climate system in a warming world.” J. M. Melillo, et al; Science, 2017; 358 (6359)
- “Global Markets for Biomass Energy are Devastating U.S. Forests.” NRDC Report. 2019
- “Range and uncertainties in estimating delays in greenhouse gas mitigation potential of forest bioenergy sourced from Canadian forests.” Jerome Laganiere, et al; 2015.
- “Carbon debt and carbon sequestration parity in forest bioenergy production.” Stephen R. Mitchell et al; 2012.
- World Health Organization