Focus on Arts and Ecology

It's so carbon negative that we should just build a billion of them.

By Lloyd Alter, June 22, 2021, Fact checked by Haley Mast

In an earlier post on this year's Serpentine Pavilion, a temporary building commissioned by the Serpentine Gallery exposing Londoners to international architects, there was a lot of discussion about its carbon footprint. A structural engineer working on the job tried to justify it, noting that "the pavilion overall is carbon negative by 9,000 Kg – largely due to the reused steel of the frame." We questioned that statement, suggesting that he was counting "avoided emissions" saved by not using new steel, but and wondered if this was legit carbon accounting.

Subsequent to this, structural engineering firm AECON, who worked on the pavilion, doubled down, no, they tripled down, claiming that the total cradle to grave embodied carbon emissions of the pavilion are -31,000 kg of CO2 equivalent." According to Dezeen,

"The construction of this year's  Serpentine Pavilion removed 31 tonnes of  carbon from the atmosphere, according to a report by construction consultant  AECOM. As a result, the structure can claim to be carbon negative, meaning that it will remove more CO2 equivalent from the atmosphere than it emits, up to the point it is dismantled."

According to the Life Cycle Assessment, which was not published, construction of the building "emits approximately 60 tonnes of carbon dioxide equivalent and absorbs around 91 tonnes via the timber and other biomaterials used in its construction, according to the life cycle assessment (LCA) prepared by AECOM." This nets out the 31 tonnes of carbon. While there is a lot of concrete and steel in the building, "all these emissions are outweighed by carbon sequestered in the wood, plywood and cork used to build the pavilion, according to AECOM."

"The sequestration of the timber and the cork more than compensates for the emissions," said AECOM sustainability director David Cheshire. This accounts for the 31 tonnes of "negative emissions."

This seemed... odd. As one wag on Twitter noted, we should just keep building Serpentine pavilions until our carbon problems disappear. We have to reduce our emissions by about 32 billion tonnes per year, and with the Serpentine negative by 31 tonnes, we just need to build a billion of them every year and our problems are solved!

The question of how much carbon is stored, or sequestered, by using wood is complex, and the question of whether it is truly carbon negative is even more controversial. To answer the question, Treehugger had a conversation with Peter Moonen, the National Sustainability Manager for Wood Works, a Canadian wood promotion organization.

Moonen noted that you can start with simple chemistry and biology; wood is about 50% carbon, which is removed from the Carbon Dioxide in the air. When you do the chemistry, it turns out that a ton of wood is basically storing the carbon from a ton of CO2. (It actually stores 1.83 tonnes but after fabrication, it nets out at about a tonne). Note that it is storing it, it is not magically sucking more carbon out of the air. The only way you can consider it "negative" is if it is replaced by more trees that keep converting CO2 into wood, and that keep doing it for as long as it takes to replace a ton of wood, which might be 50 or 60 years–"the building has to last as long as the tree." If the Pavilion was thrown away at the end of six months and burned, there would be no storage and no negative carbon. So the use of the term "carbon negative" is pretty questionable in the first place.

AECOM sustainability director David Cheshire says the building was designed to last sixty years, so there is that. But he also said that the building absorbed 91 tonnes of CO2 emissions. Buckminster Fuller may have asked "how much does your building weigh?" but if a tonne of wood is equivalent to a tonne of carbon, then this Serpentine pavilion is an awfully heavy building; no wonder it needed such a big foundation.

A sheet of 25mm (1 inch) plywood weighs about 50kg, so that 91 tonnes translates into 1820 sheets of plywood, which laid end to end would run a little less than three miles. I cannot help but look at that pavilion and think that something is wrong with this calculation.

We have always tried to avoid overselling the benefits of wood construction; there is no question that has a far lower carbon footprint than steel or concrete, both of which have chemistry that emits CO2 when it is made, while wood has a chemistry that absorbs it. When it comes to the upfront carbon emissions, the embodied carbon that matters now when we have a carbon budget we need to stay under to keep the globe from heating more than 1.5°C, there is no comparison between wood and other materials.

But while I am an architect, not an engineer, my gut and my experience tell me that building a billion Serpentine Pavilions will not solve climate change and that this building did not suck up 31 tonnes of CO2 by virtue of it being built, and it's not absorbing any now; it is sitting there in a park.

It's why I always return to this diagram from the World Green Building Council showing how one should think about building, where you start with trying to build nothing, then to build less, then to build clever, and finally, to look at using low carbon construction technologies. And unfortunately, this Serpentine Pavilion fails in all of these.

(Sources: Treehugger)