The UHI effect describes the higher day and night temperatures experienced in urban and suburban areas in comparison to their surrounding rural areas. This temperature gap results from solar heat trapped and absorbed by the built environment – roads, pavements, buildings, and roofs – as well as waste heat released as a byproduct of human activity such as combustion engines and air-conditioners.

The District and the LGC led a project to address the UHI effect in the Capital Region and identify the most effective urban heat reduction measures. The project developed an advanced model of the UHI effect in the six-county Sacramento region at local and regional scales for today’s conditions as well as 2050 land use and climate scenarios. The model identifies the areas with the most severe UHI effect and evaluates the effectiveness of potential heat mitigation measures deployed at various scales. These findings were then distilled into transportation sector-focused recommendations and strategies for local governments. The project team also conducted community outreach and engagement throughout the Capital Region to understand community concerns around extreme heat, transportation needs, and priorities for neighborhood and transportation improvements.

Key Findings

The key findings are summarized broadly here but are elaborated upon in the Project Summary Report (Attachment A) and described fully in the Urban Heat Mitigation Plan (Attachment B), which sets out implementation strategies for local agencies, and the Urban Heat Technical Analysis Report Parts 1 and 2 (Attachment C and D), which describe the modeling of the heat island and mitigation measures.

• The areas with the most severe urban heat island effect and absolute air temperatures are North Highlands (an AB 617-identified community), Citrus Heights, Folsom, El Dorado Hills, Roseville, Rocklin, Lincoln, Yuba City, Marysville, and parts of Auburn. The second most affected areas include Natomas, North Sacramento, Del Paso Heights (another AB 617-identified community), and the stretch of unincorporated Sacramento County from I-80 to Folsom. These locations should be prioritized for urban heat pollution reduction measures.

• This project evaluated urban heat mitigation strategies that are reasonable, realistic, and can be deployed today, rather than hypothetical or based on unrealistic implementation levels. The main cooling measures modeled in this study include cool roofs and pavements, increases in tree canopy coverage, vehicle electrification, and solar photovoltaic panels. 

- The most effective strategies to reduce daytime urban temperatures are cool roofs and cool pavements, the first of which can help save building energy, while cool pavements can help reduce maintenance costs and improve visibility and safety, among other benefits. Cool roofs can be installed on both new buildings and existing buildings, as part of roof maintenance or replacement, and deliver energy savings for both applications. Similarly, cool pavements can also be installed as both new road projects and maintenance/repair projects.

- An increase in tree canopy cover has the greatest cooling impacts at night and on the 24-hour temperature average.

- A combination of increases in cool roofs and pavements and tree canopy coverage provides the largest cooling effect regardless of time of day. This measure increases the average albedo of roofs and pavements by 0.35 and tree canopy by 12 percentage points.

• The cooling impacts can be nearly doubled if a community’s upwind neighboring communities also implement urban cooling measures, compared to that community acting alone. In other words, regional coordination can help expand and amplify cooling benefits. In some cases, this coordination can offset the UHI index entirely.

• These heat mitigation strategies are modeled to successfully reduce temperatures from one National Weather Service Heat Index warning threshold to a lower, less dangerous threshold. For the North Highlands community, the mitigation scenario of a 0.35 increase in roof and pavement albedo and a 12 percentage point increase in tree canopy was able to shift 100% of the temperatures in the Dangerous threshold (heat index of 106°F) to the lower Extreme Caution threshold.

These project findings demonstrate that communities and neighborhoods can achieve tangible cooling benefits through local implementation of increased tree canopy, cool roofs and pavements, and other measures. While the analysis focuses on urban heat islands, the recommendations for heat reduction strategies can be applied in all neighborhoods, not just urban areas or heat islands.

Many things remain uncertain about the future, but the rise in temperatures as a result of climate change is inexorable, at least until mid-century. By 2050, the urban heat island effect is projected to increase to 9-12°F for North Sacramento, North Highlands, Folsom, Roseville, and El Dorado Hills. As the infrastructure and buildings constructed today are likely to still be here in 30 years, it is critical to ensure that they are designed and built to help cool – not heat – communities. Forthcoming recovery and stimulus funding offer local jurisdictions the opportunity to develop and design shovel-ready road construction and maintenance, complete streets, and other infrastructure projects. Incorporating heat resilience and urban heat-reduction measures into these projects will help to protect the long-term health of these investments – preventing pavements from buckling in extreme heat, for example – as well as the health of communities and businesses.

The full Capital Region Transportation Sector Heat Pollution Reduction Plan provides implementation mechanisms, case studies, best practices, and other resources to support local jurisdictions in adopting urban heat reduction policies and programs. The District hopes that this project will serve as an effective foundation to galvanize the implementation of urban heat reduction strategies throughout the region, and looks forward to working with regional public works and planning departments to do so.