The American Institute of Architects (AIA) recently published their latest annual report on the AIA 2030 Commitment. Titled 2030 By the Numbers, the document summarizes the annual performance of the architecture and design community towards its goal of carbon neutral buildings by 2030—along with suggestions for improving performance year-to-year. 

It stands to reason that the AIA would spearhead climate action germane to the Institute's disciplinary purview: buildings. However, buildings occupy sites, and those sites have the potential to harbor vegetation capable of sequestering carbon. Certain species of trees can sequester and store significant amounts of carbon over the course of its lifecycle—and the lifecycle of our buildings. The ecosystem service of carbon sequestration through site "softscapes" is one of the precious few practical ways that we can actually "dial back" the carbon impacts of our building projects.

Conversely, our site "hardscapes" contribute to a project's embodied carbon intensity through the creation, transportation, and installation of materials such as asphalt, concrete paving, stone pavers, wood decking, steel grating, and so on. 

Additionally, the operations and maintenance of our sites come with significant energy-intensive processes such as site lighting, water feature, irrigation systems, fire pits, as well as the regular use of maintenance equipment (think mowers, trimmers, leaf-blowers) and the deployment of fertilizers, weed treatment, and the like. The energy resources utilized will shift the site's contribution to a project's operational carbon intensity as well. 

 

ASLA Climate Action Plan

In recognition of the carbon intensity of our landscape, the American Society of Landscape Architects (ASLA) have made several recent moves toward deeper climate action:

 

How much carbon might a site be responsible for on a project? Here is a case study.

In order to better understand the magnitude of landscape carbon relative to building-related carbon, let us consider a case study: a recent LEED v4 Gold public library project in Indianapolis, Indiana.

The basics: this library is a roughly 20,000 square-foot single level facility on approximately 4.5 acres of developed land. The figure below exhibits the site plan and offers a depiction of the site hardscape and softscape. 

 

Figure: Site plan for a public library case study examined for lifecycle building and site CO2e emissions. Figure by Browning Day.

 

Anecdotally, one might argue that—proportionally—this site is fairly epitomic of a suburban building project. In fact, the site is designed to manage on-site runoff for a 98th percentile rainfall event using low-impact development.

The building achieved a 50 percent energy performance improvement over its baseline. Energy-efficiency measures and on-site renewable energy combined for a predicted energy cost savings of over 80 percent. As such the building's operational carbon intensity was reduced from 60 to 33 kgCO2e/m2 per year. 

The structure and enclosure were optimized to reduce the building’s embodied carbon intensity from 486 to 412 kgCO2e/m2.

In total, the optimized 60-year "lifecycle" operational carbon settled at 3.4 million kgCO2e versus the structure and enclosure's embodied carbon of just over 714,000 kgCO2e.

 

What about the site?

Initially, the operational carbon of the library over a 60-year lifecycle was assessed at just over 960,000 kgCO2e. However, after optimizing operational and maintenance procedures, the operational carbon could be reduced to nearly 600,000 kgCO2e. 

The embodied carbon of the site hardscape rivals the embodied carbon of the building itself at just under 775,000 kgCO2e. Let that sink in. The embodied carbon of this fairly well-vegetated site is of the same order of magnitude as the one-story building that occupies the site. After optimization, the embodied carbon intensity of the site hardscape could be reduced to just under 575,000 kgCO2e.

 

Figure: 60-year building and site CO2e emissions for public library case study located in Indianapolis, Indiana. Figure by Daniel Overbey.

 

The sequestration potential of trees.

The sequestration potential of the trees planted as part of project development was just over 100,000 kgCO2e. However, not all trees sequester CO2 at the same rate. Should the species of trees be optimized for sequestration, collectively they could absorb nearly 260,000 kgCO2 over the 60-year lifecycle of the building. 

This is just one case study. There are many qualifiers and caveats for this example. My colleague Brett Schlachter, ASLA, has been working with me on this case study and we have received some tremendous peer feedback. For instance, Darby Simpson of Indiana University pointed out that grasses in perennial systems are the fastest way to sequester carbon. Indeed, one big caveat is that the assessment tools leveraged for this case study were focused on trees. We should take other vegetation into account. 

This is just the beginning of an important conversation that should take place between AIA and ASLA. The big take-away here is that the site's operational and embodied carbon intensity is quite significant. Moreover, landscape architects wield the "superpower" of ecosystem services to dial back carbon on a building project through vegetation. 

To my fellow building professionals, it is time to take our carbon footprint conversation outside.