Evaluating the Pathfinder tool

Mayfield Park © Jarrell Goh

Studio Egret West, designers of Manchester’s Mayfield Park, explore the usefulness of Pathfinder and discuss nature-based carbon sequestration as a metric for sustainable design.

There is a carbon footprint to everything we design and build, from small residential courtyards to public squares and urban parks. How should we measure the carbon footprint of our schemes to include the carbon storage potential of trees, plants and soils?

With policies for Whole Life Carbon and Circular Economy now mandatory for major applications in London, and with other national authorities to follow suit, there is a recognised need for landscape architects to make use of tools available for carbon accounting. While much of the data, policy and guidance on emissions in our industry is specific to buildings (embodied and operational), nature-based solutions are unique in their ability to absorb emissions on-site.

Fortunately, a web-based tool called Pathfinder can be used to weigh up embodied emissions against the potential carbon absorbed by trees and plants over a period. Pathfinder, also known as Climate Positive Design, was launched in 2019 by Pamela Conrad, Principal at CMG Landscape Architects in San Francisco. Designed to help landscape architects to deliver schemes which sequester more greenhouse gas emissions than they embody, it is freely available from www.climatepositivedesign.com.

At Studio Egret West, we are trialling Pathfinder in the hopes of finding a meaningful approach to climate-conscious design. It is not perfect, with many variables unaccounted for such as species specific tree sequestration, and high-level assumptions made on the embodied and end-of-life process emissions for hardscape materials. Nevertheless, the tool can be useful for designers to model their schemes and think about carbon holistically and numerically.

To demonstrate this, we have tested Pathfinder ‘retrospectively’ on two case studies: Mayfield Park, a new public green space in Manchester (featured in the last publication of the Landscape Journal ) and Brewery Square, a public courtyard within a mixed-use development in Clerkenwell (where our studio is based). To reduce the variability of the test, we decided to only measure paving, subbase, trees and soft landscape elements within the studies.

Mayfield Park is a 6.5-acre destination green space in the heart of the future Mayfield Neighbourhood. Using Pathfinder, we know that the total embodied carbon of all paving and subbase, measured in ‘carbon dioxide equivalent’ or ‘C02e’ (a common metric used to describe different greenhouse gases), equates to 755,000kg CO2e, with reinforced concrete paving making up 38% of this. Conversely, all soft landscape elements, including 140 trees, absorb a combined amount of 19,500 CO2e per year, with marginal/aquatic planting contributing 52% of this.

With all the metrics factored in, Pathfinder suggests it will take 39 years for Mayfield Park to absorb its embodied carbon. This doesn’t tell us much when we don’t have a benchmark to compare with, though it does shine a light on the reality of emissions from development, even for a pioneering city park design such as Mayfield. So, we ask ourselves, what would Mayfield Park look like if it was purely dictated by carbon performance? A newly planted woodland river corridor perhaps (with no paths, bridges, lawn space or play area) or left undeveloped as it was prior to 2021.

In fact, Mayfield Park has carried through a climate-aware approach to placemaking from the beginning of the project through to delivery. The park is imbued with a post-industrial character, lending itself to the ethos of seeing beauty and opportunity in existing, leftover spaces. This is best demonstrated by the refurbishment of the river walls, recycling of site won material and re-use of the hog-back beams. We were also able to overcome logistical challenges on site and justify design moves with the client and contractor by aligning cost-saving with carbon-saving. These factors have helped to shape Manchester’s first city centre park in almost 100 years, providing accessible green space and a destination play area, with over 10,000m² of new planting and a revitalised river unlocking an important biodiverse habitat.

Our second case study is Brewery Square, a public space within a mixed use development in Clerkenwell. Completed in 2005, the square is the centrepiece of a masterplan by GML Architects and Erik van Egaraat, with Hamiltons Architects as plot architects. The building is also home to our studio, where the natural stone paving and a raised planting bed with non-native trees and shrubs make for a pleasant outdoor space to rest, take a break and socialise in the sun.

At Studio Egret West, we are trialling Pathfinder in the hopes of finding a meaningful approach to climate conscious design.

Pathfinder tells us the embodied carbon of the stone paving totals 63,600kg C02e. Conversely, the trees and shrubs absorb a combined amount of 400kg CO2e per year; equating to 159 years for Brewery Square to absorb its own carbon footprint. It may come as no surprise that a square that is 85% hardscape and 15% planting (on plan) would yield a high number of years to reach net positive. Once again, we do not have a benchmark to compare with.

To find out more, we conducted several scenarios to examine the potential enhancement of carbon performance by tweaking the ratio of stone paving and soft landscape. Our findings indicated that with each 15–20% increase in the presence of shrubs and trees, the time required to achieve net positive status was nearly cut in half.

We should feel inspired in having naturebased skills and tools to create better environments for people and nature, while also addressing emissions on-site again, we do not have a benchmark to compare with.

Scenario A has a 70% to 30% hard-to-soft landscape split = 91 years to net positive. Scenario B (50% to 50%) = 36 years. Scenario C (30% to 70%) = 16 years. Scenario D (15% to 85%) = 9 years. These findings suggest there is a correlation between soft landscape area cover and carbon absorption over a period of time. As obvious as this may seem, we see the potential in using a time-based metric to measure the climate positive value of a landscape proposal.

While Pathfinder does not give us all the answers, it provides designers with a conceptual understanding of climate positive design, and a framework that over time could be developed into something which offers greater accuracy, site specificity and more reliable metrics for carbon performance. Life-carbon assessments are still in their infancy for major planning applications in London, yet we are already modelling landscapes in BIM for these purposes. As we recognise the influence and success of policies such as Biodiversity Net Gain (BNG), Urban Greening Factor and Play Space Requirements on new developments, perhaps there is room for another quantifiable indicator of sustainable design using a climate positive timeframe.

The findings have given our studio members (landscape and non-landscape) some insight into the metrics for our profession. The next step is to encourage clients, developers, local authorities and other designers to be more transparent about the carbon impact of development and how to address it properly. Furthermore, we should feel inspired in having nature-based skills and tools to create better environments for people and nature, while also addressing emissions on-site.

Martin Lee is an experienced Project Landscape Architect working on a range of large-scale mixed-use hybrid planning applications, through to delivery and on-site involvement. He was involved in the detailed design for Mayfield Park and is now leading on sustainability for landscape architecture within Studio Egret West.