Because concrete and steel used to construct buildings are a major source of global greenhouse gas emissions, there is growing interest in “mass timber” — a supposedly lower-carbon option — to replace them. New ways of gluing pieces of wood together to form strong beams and structural panels provide opportunities to use wood even in tall buildings. But new research finds that using wood in construction is likely to increase emissions for many decades, even relative to using concrete and steel. Truly “mass” timber would also require vast additional harvesting of the world’s forests.
In our report, The Global Land Squeeze: Managing Growing Competition for Land, we do find that using wood to replace concrete and steel could have lower emissions under certain conditions, but they’re challenging to achieve. And currently, the fast-growing plantations that could supply wood in these conditions are already needed to meet other growing demands for wood.
Here are five interrelated reasons that turning to more wood for buildings is not a climate- or environmentally friendly solution overall:
1) Most wood (and its stored carbon) is lost during production.
When trees grow, they transform carbon in the air into wood, removing it from the atmosphere and storing it. If a whole tree, roots and all, were somehow transferred into a building, this carbon would likely remain stored for decades, so none of the tree’s carbon would be emitted and warm the climate.
Unfortunately, only a small portion of a harvested tree typically makes it into a building. When a tree is harvested, a third or more of its wood is typically left behind in the forest as roots or small branches that then decompose. Bark comprises 10-15% of the wood removed from the forest and is generally burned. When milling logs into lumber, much of the wood becomes small chips or sawdust, some of which are also burned. Some chips and sawdust get turned into paper or wood panels for furniture, which store carbon longer, but rarely for decades. They are also burned or decompose when thrown out.
These processes all emit carbon that would otherwise remain stored if forests were kept standing.
2) Harvesting wood is not carbon-neutral.
Like any other physical emissions, the losses of carbon from decomposing and burning wood must be factored into the overall calculation of the climate impacts of using wood for construction.
Yet surveying more than 60 papers that attempt this kind of calculation, our report finds that nearly all studies claiming climate benefits from such uses of wood exclude these emissions from their accounting. Their theory is that use of wood is “carbon-neutral” so long as a forest is harvested “sustainably.” “Sustainably” typically means that the quantity of wood removed by a year’s harvest only matches the forest’s growth in that time. The reasoning seems to be that so long as the existing carbon stock in the forest remains the same, harvesting wood does not add carbon to the air.
But this approach is based on flawed assumptions. Logically, if forests would grow and absorb more carbon if some parts were not harvested for wood, then harvesting and only maintaining pre-existing carbon stocks decreases the carbon the forest stores relative to a no-harvest scenario. As an analogy, if you add money to your savings account each year and others take out only that amount, your savings account does not decline, but you are still poorer than if the others had left your account alone to let it grow. Harvesting trees makes the world poorer in stored carbon and increases carbon in the atmosphere even if forests do not shrink overall from year to year.
This forest growth is far from disposable if the world is to curb climate change. Scientists estimate that when people add a ton of carbon to the air, it spurs additional tree growth that removes one quarter of that ton from the atmosphere. To the extent this growth is reduced by harvesting wood, climate change is worse.
Also, even while some wealthier countries that cleared much of their forests in the past have reduced their agricultural land and are now regrowing forests, this regrowth helps to balance out carbon losses due to forest clearing in poorer countries. (Some tropical deforestation is even occurring due to the imports of food by wealthier countries.) Again, regardless of the reason for the forest growth, reducing it through harvesting increases warming.
3) Using wood in construction will most likely increase climate warming for decades.
We use the same model in our report as we used in a paper published recently in Nature to estimate how much of the carbon in a tree is lost to the atmosphere each year after a harvest. (See the related article here.) We also credit wood harvests for the fact that after harvest, newly regrowing forests can grow faster than older, unharvested forests. Over long enough time periods, regrowing forests can thereby pay off most of the so-called “carbon debt” from the carbon lost from previously harvested forests. The physical part of manufacturing wood panels and beams for construction also generally emits less carbon than manufacturing concrete and steel, and we factor in those relative emissions savings.
The precise results vary from one forest type to another, but when estimating how much wood can likely be incorporated into buildings, we find that using wood in construction will probably increase carbon in the air for decades relative to using concrete and steel.
In general, the world cannot afford to mitigate climate change by taking actions that increase warming for many decades even if they eventually reduce emissions many decades later. We can formally value that cost of increased warming for decades using a “discount rate” to express our preference for mitigating climate change by reducing emissions right away. This method results in an estimate of the equivalent level of permanent emissions in the year of harvest. Using that approach, the use of wood — relative to concrete and steel — also increases emissions in most circumstances.
4) Relying only on plantation forests in warm climates for mass timber might yield climate benefits from a specific hectare, but not when factoring in the growing needs for wood.
Our analysis looked at a range of different forest types and assumptions around how much wood is used for construction, and we estimated not just the total, but the percentage change in emissions when using wood for construction relative to using concrete and steel. Our findings were consistent with those of other papers that did not treat wood as inherently carbon-neutral. Like us, they found most uses would increase emissions for decades, but might reduce emissions in certain conditions. This finding is also consistent with the findings of European Commission’s “New Forest Strategy to 2030” (although revealed only in a single sentence on page 5 of the document).
For example, we found that if 70% of harvested wood from some natural forests, such as western U.S. forests, could replace concrete and steel, that would reduce emissions. But because of the production wastes noted above, we doubt that 70% is achievable today. Even if this target were achieved, the greenhouse gas reductions of using wood instead of concrete and steel would likely be less than 25% — an amount too small to justify major investments in this strategy nor worth the ecological effects on forests.
We did find large greenhouse gas reductions — sometimes approaching 100% — could be achieved if 70% of harvested wood was used for construction and that the wood came from fast-growing forest plantations in warm, wet parts of the world, such as the tropics. (In these fast-growing plantations, there were even some scenarios where there was a climate benefit when only 40% of the harvested wood was used for construction.) However, at present and for the foreseeable future, the wood from these fast-growing plantations is already needed to meet current and projected wood demand and so is not available to support additional demand from policies promoting mass timber.
5) Mass timber would have large adverse effects on the world’s forests.
Supplying large quantities of wood for construction would also require large increases in wood harvesting. Even without mass timber, we project wood used for timber, paper and all purposes other than fuel will be 90% higher in 2050 than in 2010. Roughly 800 million hectares of forests will be harvested for wood overall in that period, an area of land the size of the continental United States. Resulting annual emissions from wood harvests will range from 3.5-4.2 billion tons of emissions per year over these years, more than 10% of recent global CO2 emissions.
Any move toward mass timber would greatly increase these wood demands. To provide 50% of new construction with wood over this period would require that annual, industrial wood harvests not just average 40% more between 2010 and 2050, but double. There is no way to direct the necessary harvests just at “preferred” forest types, because if you divert wood from them for additional construction, other forests will have to be harvested to replace the wood these forests now supply for other uses.
There is also no free land that’s available today for developing new forest plantations when you consider climate costs. Although tropical plantations have limited biodiversity and use vast quantities of water, they generate prodigious quantities of wood per hectare and can help meet existing wood demands. Yet expanding them would require taking over agricultural lands —even as the world’s food requirements and agricultural land area are expanding. Using agricultural lands for forest plantations requires that agricultural lands expand somewhere else.
Some researchers have argued for converting wetter, poorly grazed pastures in Latin America to forest plantations, but this pasture is already needed for other purposes. Most must be improved just to meet rising demand for meat and milk without further deforestation. If enough such improvements can happen, some of this land should be converted to cropland to supply more soybeans and other crops that are otherwise also likely to expand into forests. Or these degraded areas could be converted back into natural forests to sequester carbon.
Another possibility would be to convert natural tropical forests, rather than agricultural land, into fast-growing plantations for mass timber. We find that this approach, too, might have climate benefits if no other ways of reducing emissions from concrete and steel emerge, but doing so would pose unacceptable costs to biodiversity and the environment.
Mitigating the Global Land Squeeze
Overall, our review finds that the broad interest in mass timber has been based on incomplete carbon accounting that treats wood as inherently carbon-neutral. Our analysis does not preclude the possibility that some wood from some forest plantations might pass a climate test. But such uses should only happen if and when the world has increased crop and pasture yields and reduced meat consumption in high-income countries enough to reduce agricultural land demand and make some current farmland available for forest plantations.
Even if that day happens, we should also hope that promising solutions for concrete and steel emerge so that the world can have both low-carbon construction and more natural forests. Because most concrete and steel are used for purposes other than construction, finding low-carbon techniques for producing them is critical for addressing climate change.
Our analysis is ultimately cautionary: Because the world already faces a global land squeeze as it tries to balance growing human demands for food, wood and urban development with nature and climate goals, it is dangerous to adopt policies that encourage yet more human demands for land and its outputs. The world has a fixed quantity of land; people should be working hard and creatively not to expand, but rather to reduce their footprint on it.
