Abstract

As cities grow, ensuring access to clean air requires policies that both lower pollution loads and expand urban forests, which in turn provide air filtration ecosystem services (e.g., nature’s contributions to people). Approaches for monitoring ecosystem services and attributing drivers are needed to ensure demand for clean air is met. The Group on Earth Observations Biodiversity Observation Network has recently proposed the Essential Ecosystem Service Variables (EESVs) framework. EESVs seek to standardize and harmonize methods for monitoring ecosystem services among frameworks, including the System of Environmental-Economic Accounting Ecosystem Accounting (SEEA EA) framework. Through a case study of Canadian cities, including Toronto and Vancouver, we propose approaches for monitoring three EESV classes (1) Ecological Supply, (2) Demand, and (3) Use for the service of air quality regulation (e.g., pollutant removal of particulate matter 2.5, PM2.5, by urban forests). Here, we define Ecological Supply in terms of both ecosystem properties and capacities. First, we estimate the average leaf area of the urban forest within each city (an ecosystem property), which accounts for changes in forest extent, condition, and phenology. Second, we estimate the amount of PM2.5 that can be removed while meeting an air quality guideline (the ecosystem capacity to deliver the service). We define Demand as the number of days where PM2.5 is greater than an air quality guideline established by the World Health Organization (WHO). Meeting this guideline reduces the health impacts associated with air pollution. We define Use as the pollution removal of PM2.5 by urban forests using a dry deposition model. We then assess trends and attribute drivers, including increased wildfires under climate change and changing policies. It is important to track many facets of a given ecosystem service, especially for regulating services, where Use of the service could decline for either of two reasons: (1) a decrease in Ecological Supply of the service or (2) a decrease in pollution loads (e.g., Demand). For example, in Toronto, despite increases in Ecological Supply, Use is decreasing over time, because pollution loads have declined, although present PM2.5 concentrations are still higher than WHO guidelines. Alternatively, Use in Vancouver is stable; however, Demand is currently unmet due to increases in wildfires. Furthermore, we found that when Use is greater than the ecosystem capacity to filter pollutants to levels that meet air quality guidelines (Ecological Supply), then Use is no longer sustainable and unmet Demand will likely have health impacts. Lastly, we demonstrate the potential of expanding urban forests to meet Demand, which in some cases will require dual policy strategies targeting both the expansion of urban forests and their associated benefits as well as the reduction of pollution levels. Under continuing climate change, Demand for air quality regulation may either grow with the increasing frequency of wildfires or decline as cities implement pollution reduction policies. We demonstrate the potential of EESVs to monitor these changes and attribute drivers to support decision-making and to ensure cities meet the Demand for clean air in growing population centers.