Can Timber Construction Be a Significant Carbon Sink?

COP26, the United Nations climate change conference, begins Oct. 31 in Glasgow, Scotland. There will be much to discuss, most notably the findings recently released by the Intergovernmental Panel on Climate Change in its Sixth Assessment Report. The “most up-to-date physical understanding of the climate system and climate change” delivers the most defensible and incriminating evidence yet about humanity’s adverse influence on the planet, according to the IPCC. Nearly 4,000 pages of data reinforce what has become familiar bad news: global surface temperature has increased by 1.09°C, sea level rise has tripled, the last five years were the hottest on record, and heat waves are certain to increase. The need for rapid and consequential change is clear.

In several instances, IPCC AR6 addresses the influence of the built environment on climate change—and vice versa. Today, buildings’ responsibility for 40% of global emissions is a familiar data point. Additionally, the built environment exerts more pressure on the environment than any industrial sector: According to a 2016 assessment by engineering faculty at the University of Cambridge, buildings account for “50% of all extracted materials, 42% of final energy consumption, 35% of greenhouse gases (GHGs) emissions, and 32% of waste flows” in the European Union—and for similar percentages worldwide.

Although IPCC AR6 does not address in-depth building strategies for greenhouse gas mitigation, it does reference the April 2020 Nature Sustainability article “Buildings as a Global Carbon Sink.” Written by faculty at the Yale School of Forestry and Environmental Studies, the Potsdam Institute for Climate Impact Research, among other institutions, the study advances analysis of the use of timber in building construction as a carbon sequestration strategy on a planetary scale. Prior research has highlighted wood’s capacity to store carbon and, in most cases, outperform steel and concrete in terms of environmental performance.

The authors of this article broaden this consideration to address far-reaching hypothetical implications and ask, “Might it be possible to transform this potential threat to the global climate system into a powerful means to mitigate climate change?”

The researchers evaluate four timber-transition scenarios occurring over the next three decades, from 2020 to 2050. The first, “business as usual” scenario assumes no change in the current use rates for concrete, steel, and timber building structures and enclosures; timber’s current share is a mere 0.5%. The second, third, and fourth scenarios presume timber has a market share of 10%, 50%, and 90%, respectively.

The latter three scenarios are premised on the increasingly aggressive development of timber manufacturing infrastructure. The 10% target is theoretically possible for countries that currently manufacture mass-timber building products; 50% includes nations with the potential to build such infrastructure; and 90% includes less-developed nations that do not yet demonstrate this capacity.

The upshot of the study is a projected sequestration of between 0.01 and 0.68 gigatons of carbon (GtC) annually— depending on the scenario—or 0.25 to 20 GtC in 30 years. To put these numbers in perspective, the World Climate Research Program, in 2019, estimated the global carbon output to be 11.5 GtC per year minus the carbon sinks (ocean and land) of 5.7 GtC/yr, resulting in a net increase of 5.4 GtC/yr.

Thus, even the most dramatic impact of 0.68 GtC/yr, if timber assumes a 90% market share, would constitute a small, if measurable, dent in total CO₂ emissions. (This quantity is more similar to the sequestration capacity of the world’s forests, which is 1.1 GtC plus or minus 0.8 GtC per year, according to the “Buildings as a Global Carbon Sink” authors.) However, this estimate considers carbon storage alone; it does not include the reduction in CO₂ emissions due to the lack of new steel and concrete buildings. Based on the WCRP numbers, buildings were responsible for roughly 2.16 excess GtC/yr in 2019 (using the built environment’s carbon emissions contribution of 40%). Given the preponderance of concrete and steel structures and enclosures that contributed to this amount, a future 90% timber transition—as improbable as it might seem—would be significant in achieving global net zero.

But wait: Do enough forests exist for such a blue-sky goal? Would any trees be left standing? According to the “Buildings as a Global Carbon Sink” study, estimates indicate that “more wood can be harvested without compromising the sustainable regrowth of forest resources over the next thirty years.” However, the demand would require harvesting a combination of softwood and hardwood tree species as well as bamboo. The 90% target would also necessitate harvest from nonprotected forest regions and utilize all established tree plantations. Such significant resourcing would intensify the disruption of ecosystems, resulting in new CO₂ emissions. However, the authors point out that, in the 90% scenario, there also would be relatively less mineral, metal, and sand mining—and less CO₂ impact.

In the August 2020 essay “Buildings as a Global Carbon Sink? A Reality Check on Feasibility Limits,” in One Earth, researchers from the University of Cambridge, Edinburgh Napier University, and University of Colorado Boulder argue that the increased use of timber for building construction “threatens to intensify deforestation and illegal logging.” The authors add that the 2020–2050 timber demand estimated by the Nature Sustainability study authors “would exceed supply by ~3,900 [metric tons] with current figures for forest area, and total timber supply represents only ~36% of what would be required for building all projected new floor area out of timber.”

Based on this cautionary second opinion, more research is needed to assess the viability of any scheme that proposes a significant worldwide transition to timber. Nevertheless, the idea to use buildings as a planetary carbon sequestration strategy is laudable both for its boldness and its necessity.

The views and conclusions from this author are not necessarily those of ARCHITECT magazine or of The American Institute of Architects.