“Embodied carbon accounts for 75 to 95 percent of the greenhouse gas emissions from landscape architecture projects,” said Chris Hardy, ASLA, PLA, senior associate at Sasaki, during the third in a series of webinars organized by the ASLA Biodiversity and Climate Action Committee. But by selecting locally made low-carbon materials, landscape architects can significantly reduce the climate impacts of their work.
Embodied carbon emissions are generated from the extraction, manufacturing, transportation, and construction of landscape materials. The other share of project emissions come from operating and maintaining a landscape.
Hardy created Carbon Conscience to help designers understand the embodied carbon emissions that would result from their early stage design concepts. It also shows the climate impacts of different materials.
For built projects, Hardy said a good way to evaluate emissions is to undertake a lifecycle assessment (LCA). This approach measures the “collective carbon impact of a project.” Whole project LCAs include the landscapes surrounding buildings.
Landscape architects can also ask for environmental product declarations (EPDs), which outline the carbon impact of a particular product or category of products.
Running through a range of materials and their impacts, Hardy said local materials that are closer to a site will be better from a climate point of view, because there will be fewer emissions transporting them.
Local stone is more sustainable than concrete or asphalt. Emissions from “bricks can be tricky” to calculate as it depends on the type of material and how they are dried. Asphalt is “surprisingly good in terms of its climate impacts” because it incorporates bitumen, a waste product.
Natural hardwood products store carbon so they offer “net negative emissions.” Hardy is a fan of Amish-harvested domestic Black Locust hardwood.
He said other kinds of manufactured wood products like bamboo composite, pressure treated softwoods, and thermally modified woods store carbon but also have higher levels of chemicals and emissions from their manufacturing.
Metals, like steel, aluminum, and bronze, are very high in embodied carbon so should be avoided. Plastics and foams are not only laden with chemicals but also high in embodied carbon. “Where possible, maximize products with recycled content,” he said.
Satyam Maharaj, an associate with the Rocky Mountain Institute, explained how the emissions from concrete, typically the single largest source of embodied carbon in landscapes, varies by state.
State departments of transportation have different requirements for the percentage of cement or fly ash or slag that needs to be in a concrete mix. And they have different design standards, which outline how much concrete needs to be poured for a given use.
He said updating these requirements and standards to account for carbon can make a significant difference. “Michigan lowered the emissions from their concrete by 37 percent.”
There is a pathway to cut emissions from concrete by 100 percent, through a combination of new approaches like:
- Lean design
- Shifting towards 50 percent fly ash or slag in concrete mixes
- Using agroconcrete
- Incorporating carbon-storing aggregates
- And purchasing concrete from facilities run on renewable energy and that capture emissions
Meg Calkins, FASLA, professor of landscape architecture at NC State University, explained how material decisions impact the emissions of standard elements like seat walls, pavement, retaining walls, and decks.
Through in depth analyses of different material mixes, she found a few key ways to reduce emissions.
For seat walls, it’s important to use less material overall. “Smaller is better.” And shifting to wood and local stone products is best.
For pavement, gravel with stabilized binder has a far lower carbon impact than porous pavers, concrete pavers, and poured concrete. If a designer has to use pavers, use the thinnest ones possible.
With retaining walls, she wants to see more gabions and compressed earth blocks. They have a much lower carbon impact than concrete retaining walls. “As designers, we need to shift the aesthetic.”
When designing decks, not all wood products are the same. The harvesting and extraction of Ipe and other tropical hardwoods has disastrous effects on rainforests because they are “keystone species in their ecosystem.”
“1.6 acres of rainforest are cut down for one little deck.” And while that Ipe wood deck stores carbon, an acre of rainforest stores 27 tons of carbon per acre per year, meaning all that future natural sequestration is lost.
“Even avoid FSC-certified tropical hardwoods, given there is so much fraud in the Brazilian lumber industry,” she said.
Calkins concluded that thermally modified U.S. yellow pine or ash have far lower ecosystem and carbon impacts than tropical hardwoods.
