Urban Heat Island Effect: The Fine-grained Picture

An excellent study out of Madison, Wisconsin, about the effect of tree canopy and impervious surfaces on air temperature in an urban environment:

Scale-dependent interactions between tree canopy cover and impervious surfaces reduce daytime urban heat during summer


Carly D. Ziter, Eric J. Pedersen, Christopher J. Kucharik, and Monica G. Turner

PNAS, April 9, 2019, vol. 116, no. 5, pp. 7575-7580 (doi/10.1073/pnas.1817561116)

The lead author is currently at Concordia University (carly.ziter@concordia.ca).

The findings have immediate application to urban forest policy and practices. The method of data collection is carefully explained and could be replicated elsewhere.

Abstract (underlining added)

“As cities warm and the need for climate adaptation strategies increases, a more detailed understanding of the cooling effects of land cover across a continuum of spatial scales will be necessary to guide management decisions. We asked how tree canopy cover and impervious surface cover interact to influence daytime and nighttime summer air temperature, and how effects vary with the spatial scale at which land-cover data are analyzed (10-, 30-, 60-, and 90-m radii). A bicycle-mounted measurement system was used to sample air temperature every 5 m along 10 transects (∼7 km length, sampled 3–12 times each) spanning a range of impervious and tree canopy cover (0–100%, each) in a midsized city in the Upper Midwest United States. Variability in daytime air temperature within the urban landscape averaged 3.5 °C (range, 1.1–5.7 °C). Temperature decreased nonlinearly with increasing canopy cover, with the greatest cooling when canopy cover exceeded 40%. The magnitude of daytime cooling also increased with spatial scale and was greatest at the size of a typical city block (60–90 m). Daytime air temperature increased linearly with increasing impervious cover, but the magnitude of warming was less than the cooling associated with increased canopy cover. Variation in nighttime air temperature averaged 2.1 °C (range, 1.2–3.0 °C), and temperature increased with impervious surface. Effects of canopy were limited at night; thus, reduction of impervious surfaces remains critical for reducing nighttime urban heat. Results suggest strategies for managing urban land-cover patterns to enhance resilience of cities to climate warming.”

Some additional findings:

+ “Air temperature varied substantially within the city… Mean within-ride daytime temperature range (i.e., difference between the hottest and coolest areas of each transect) was 3.5 °C …, whereas temperature varied by only 0.2 °C, on average, for fixed reference sensors during the same measurement periods.

+ “The presence of lakes decreased adjacent temperatures by only ∼0.25 °C on average, and lake effects were largely restricted to shoreline locations. Influence declined quickly with increasing distance from the lake, with no effect remaining at distances more than ∼700 m from shore”.

+ “Effects of impervious cover were as or more important than canopy cover for nighttime air temperature.”

+ “Our results suggest that the most effective strategies for urban heat mitigation will involve modifications to both green and gray infrastructure.”

+ “…lower cover of impervious surfaces remained critical for reducing summer air temperatures at night, given the amount of heat stored and radiated back during nighttime. Reduction of heat at night is particularly important from a health perspective, as high overnight temperatures contribute significantly to heat-related illness and mortality, as the body has no opportunity to recover from daytime heat exposure.”

+ “Although prioritizing areas ≥40% canopy may increase cooling the most, it is important to ensure that planting efforts do not occur exclusively in areas in which tree cover is already high. Climate adaptation efforts must also consider the social and environmental (in)justice issues embedded within many cities.”

+ “…increasing tree canopy cover within only a 10–30-m radius [an area comparable in Madison to a single downtown lot (10 m) or two to three suburban properties (30 m)] still yielded measurable cooling. … [but] these planting decisions should recognize that benefits may be small if the surrounding area is low canopy.”

+ “Because of the long-lived nature of trees and persistence of pavement, current decisions (from homeowner preferences to urban planning choices and urban forest policy) are setting up the urban heat riskscape of the future. Thoughtful choices today are needed to ensure the resilience of our future cities, and will rely at least in part on city programs, homeowner education, or other incentives. … Our methods provide guidance for affordable, low-impact measurement of the intraurban heat island, and could be replicated to test mitigation strategies in cities that vary in urban form, population, or geographic region. Methods may also be amenable to citizen science sampling programs, offering a mechanism to further engage urban populations in the development of climate adaptation measures that will be critical as our cities warm.”

E.D. 15 July 2019