Originally published in Metropolis, December 2023
In an interview earlier this year, Thomas Heatherwick described his vision for 1,000 Trees, a mountain-sized shopping center that opened in Shanghai in 2022. Waxing rhapsodic about the mall’s namesake trees, most of which are perched atop soaring concrete columns, some as tall as 200 feet, Heatherwick casually noted that the trees will “absorb tons of carbon dioxide every year”—the subtext being that this would somehow offset the CO2 emitted by the 675,000-square-foot building. In reality, the trees likely won’t sequester even a fraction of the CO2 produced by the columns alone.
Heatherwick’s statement speaks to a broader cultural assumption that because vegetation can, in the right circumstances, sequester carbon, designed landscapes are inherently less carbon-intensive than buildings—that parks and other green spaces are automatic allies in the race to reduce the emission of greenhouse gasses into the atmosphere. This is far from the truth, says Chris Hardy, a landscape architect and senior associate at Sasaki. “I’ve come to the realization that landscape architecture can be just as carbon intensive as architecture on a per unit area basis,” he says. This is because urban landscapes often include plenty of hardscape—manufactured pavers or precast concrete, materials that, depending on where and how they’re made, can have significant carbon footprints.
The carbon emitted in the production of a construction material is known as embodied carbon, and until recently, there wasn’t a good way for landscape architects to measure it. (Building carbon calculators such as Tally don’t include landscape and site infrastructure, Hardy says, and they often assign a generic value that underestimates the true figure.) But the past few years have seen the emergence of several carbon calculators aimed at helping landscape architects both reduce the carbon footprints of their projects and increase their carbon sequestration potential.
Among them is Carbon Conscience, a free tool developed by Hardy that allows designers to quantify the carbon emissions of various design choices, from land use to materials. Pathfinder, developed by Pamela Conrad, the founder of Climate Positive Design, works similarly, but where Carbon Conscience is geared toward the earliest stages of design, Pathfinder is better suited to later phases. The goal, Conrad and Hardy both say, is for the tools to eventually integrate with one another, so that a project started in Carbon Conscience can be seamlessly imported into Pathfinder.
Conrad launched Pathfinder in 2019, along with the Climate Positive Design Challenge, which establishes carbon targets for designed landscapes, as measured by the time it takes for them to sequester more carbon than they emitted in construction: 5 years for parks and green spaces, 20 years for streetscapes and plazas. Since then, nearly 2,000 projects in 183 countries have been uploaded to the database.
Conrad’s former firm, the San Francisco–based CMG, was among the first to use Pathfinder to set embodied carbon targets. The project, a new tech campus for an unspecified tech company in San Bruno, California, was completed earlier this year. Corbett Belcher, an associate principal at CMG, says that carbon-reduction strategies drove nearly every design decision, including a commitment to reuse 100 percent of the nearly 600 eucalyptus trees that had to be cut down to make way for the campus expansion. (The trees had been planted on existing building entitlements.) The resulting timber became the raw material for custom retaining walls and terraced seating areas. As a replacement for concrete, Belcher says, every piece of wood was a little more carbon sequestered and a little less emitted. The reuse of salvaged wood ultimately helped avoid 117 metric tons of carbon dioxide emissions.
These sorts of numbers and their apparent exactitude are understandably seductive, and designers who have used Pathfinder and Carbon Conscience describe them as compelling tools for engaging clients on the environmental impacts of design decisions. But they also belie the messy nature of attempting to manage living systems. For the tech campus in San Bruno, for instance, CMG needed to fit 500 trees into 3 acres to hit its carbon sequestration targets. The landscape architects devised a mechanically successional planting strategy in which trees are planted more densely than is typical, with faster-growing trees providing a burst of carbon-sequestration early on, then harvested to make room for the slower-growing oaks and buckeyes.
“The strategy is a good one if we can get the maintenance to work,” Belcher says. “We need to remove the trees and keep them whole so that we don’t just release the carbon.” And yet design teams have little control over site maintenance, much less what it looks like in 30 years: Will the same tech company own the property in 2050? Will the company even exist?
The drive toward quantifying landscape performance can also overshadow other critical considerations, like immediate environmental justice concerns or communities’ cultural associations with a site. Ross Altheimer and Maura Rockcastle, the founders of Minneapolis-based landscape architecture studio TENxTEN, say they’re excited by the potential of carbon calculators and hope to integrate them into their practice. But they also see a need to place the question of decarbonization within larger issues of governance, power, and justice. “How does [decarbonization] intersect with what communities actually need, and is this supporting larger conversations about justice and equity?” Altheimer asks.
Rockcastle points to TENxTEN’s work at Minneapolis’s Fort Snelling, a historic 19th-century fort at the confluence of the Minnesota and Mississippi rivers. The designers worked with a council of Dakota elders to reclaim as much of the site as possible for a restored native prairie made up of culturally significant plants. Native prairies happen to be remarkable carbon sinks, but Rockcastle says that any ecological functionality is “the outcome of a strategy that was about honoring the ground and honoring community and rebuilding relationships between the Dakota people and the land.”
It’s all but certain that the adoption of carbon calculators will shift behavior and gradually reshape the look and feel of outdoor spaces. This will be particularly true if more and more cities begin to regulate the embodied carbon of public projects, as Toronto recently did for city-owned buildings. Civic spaces, like parks and plazas, are likely next, Conrad says, which is an opportunity for designers to challenge their conception of what urban landscapes are—and what they’re for.
“Anytime a project comes up and somebody says, ‘We want a plaza,’ we need to ask the question, ‘Why do you need a plaza? Can it actually be a forest in the city? Can it be a park?’” Conrad says. “I think there’s a really interesting juxtaposition that’s possible when you start thinking about how cities need to evolve in that way—to think about, what’s actually best for the city? What’s best for the planet?” 🌾