“Landscape architects have started conversations about embodied carbon. There is a realization that we can no longer ignore the grey parts,” said Stephanie Carlisle, Senior Researcher, Carbon Leadership Forum and the University of Washington, during the first in a series of webinars organized by the ASLA Biodiversity and Climate Action Committee.

The grey parts are concrete, steel, and other manufactured products in projects. And the conversations happening are laying the foundation for a shift away from using these materials. The landscape architect climate leaders driving these conversations are offering practical ways to decarbonize projects and specify low-carbon materials.

“The built environment now accounts for 40 percent of global greenhouse gas emissions,” Carlisle said. And with population growth, the “global building stock is expected to double over the next 40 years. That means a new New York City every month.” That also means “embodied carbon is expected to account for more than half of construction emissions from now to 2050.”

For landscapes, approximately 75 percent of emissions come from embodied carbon. These are generated by the extraction, manufacturing, transportation, and installation of landscape materials. The other 25 percent come from operations — lighting, water systems, and maintenance.

“Landscape projects are infrastructure. They are highly engineered. They share the same materials with buildings, roads, and bridges,” Carlisle said. “Parks and other landscapes can also be hardscapes that use concrete and steel.”

The ASLA Climate Action Plan calls for eliminating embodied carbon emissions from projects by 2040. The way landscape architects can do this is by tracking the global warming potential (GWP) of the materials they specify.

The lifecycle assessment (LCA) is the global standard for measuring the GWP of a project. It covers energy and emissions from the manufacturing of materials, the construction process, use of the materials, and their end of life reuse, recycling, or disposal.

In the past, LCAs have typically focused on buildings, but Carlisle and landscape architects are leading a shift to whole project LCAs, which also include the energy and water use and emissions from landscapes and infrastructure that surround buildings.

There are a range of tools for measuring project impacts, including professional LCA tools, carbon calculators, design-integrated whole building LCA tools, and product databases.

Another way to measure GWP is through environmental product declarations (EPDs). These need to be developed by product manufacturers. EPDs identify the carbon emissions from products and are complementary to whole site LCAs. “Designers can use both models.”

Carlisle said LCAs should “not be for special projects” alone but also be part of the core design services of landscape architects. “This is the path to zero emissions.”

She also urged landscape architects to:

• “Build less and reuse more
• Design lighter and smarter
• Use low-carbon alternatives
• Procure lower-carbon products
• Minimize site disturbances
• And increase carbon sequestration.”

But she noted landscape architects should be realistic about how long it takes to store carbon in soils and plants.

In all these efforts, “landscape architects are behind the game,” argued Chris Hardy, ASLA, PLA, Senior Associate at Sasaki, founder of Carbon Conscience, and a landscape architect leading the decarbonization of the profession. “Architects are about 10 years ahead of us.”

While the whole building LCA process has been codified for more than five years, the whole project LCA approach has only recently been developed through Climate Positive Design‘s Pathfinder tool.

Hardy recommended landscape architects focus on the embodied carbon from products and their replacements; the construction process; and the circular economy, including how products are reused or recycled at the end of their use in landscapes.

Landscape architects also have the unique ability to store carbon in landscapes through soils, plants, and trees. This presents a great opportunity.

But he noted that carbon storage capacity varies widely by ecosystem type. “Wetlands, salt marshes, and mangroves have high carbon storage capacity, followed by forests and prairies.”

At Sasaki, he developed the Carbon Conscience tool to “change the conversation during the concept and planning stages” of a project, when the opportunity to reduce emissions is greatest.

The tool enables landscape architects to see the carbon impacts of different site scenarios. There are 260 landscape and 250 building uses available.

Soon, landscape architects will be able to transfer their concept designs from Carbon Conscience into Climate Positive Design’s Pathfinder, where more detailed carbon calculations can be made, rooted in specific material choices.

“We are on a mission to decarbonize,” said Pamela Conrad, ASLA, PLA, founder of Climate Positive Design, who has been the leading the decarbonization of landscape architecture for five years.

“Seven years ago, when I was working on the Treasure Island project off the coast of San Francisco, I realized landscape architects were having conversations about climate impacts like sea level rise, but not the carbon footprint of our projects.”

When Conrad starting running the numbers, she discovered a landscape she designed would take 200 years to offset. But with a few tweaks that maintained the integrity of the design, that could be brought down to 20 years.

“It was a moment of awakening. I realized we need to change business as usual.”

Conrad chaired the task force that created the ASLA Climate Action Plan in 2022. In it, she outlined science-based targets landscape architects need to hit.

“To keep the 1.5°C (2.7°F) global warming limit within reach, we need to cut our project emissions by half by 2030. And then we need to reach zero emissions and double our current rates of carbon sequestration by 2040.”

Conrad has been tracking the carbon performance of landscape architecture projects. More than 10,000 projects have been submitted to Pathfinder to date, and together they will result in 1.9 million trees planted, which is equal to taking 400,000 cars off the road.

But much more needs to be done. Landscape architects need to further adapt how they design to take the GWP of projects into account. Conrad encouraged them to apply practical strategies:

• “Incorporate walking and biking infrastructure
• Use reclaimed, reused materials
• Substitute cement with other materials with lower embodied carbon
• Reduce site disturbances that impact carbon stored in soils
• Restore ecosystems
• Increase plantings
• Be creative with greening”

“And just going local for products can cut emissions from transportation by 15-20 percent.”

As Pathfinder and Carbon Conscience further develop, landscape architects will also need to collaborate more with architects and engineers on decarbonization. With their ability to store carbon in landscapes, they can play an even greater role in reducing the climate impacts of the built environment.