Greater aridity increases the magnitude of urban nighttime vegetation-derived air cooling

Peter C Ibsen,Dorothy Borowy,Sheri A Shiflett,David M Hondula,Mary V Santelmann,Christopher M Swan,Mary K Wright,Michael C Sukop,Hattie Greydanus,Tyler Dell,Michelle L Talal,Miguel Valencia,Thomas Meixner,G Darrel Jenerette,Neha Gupta

doi:10.1088/1748-9326/abdf8a

Abstract

High nighttime urban air temperatures increase health risks and economic vulnerability of people globally. While recent studies have highlighted nighttime heat mitigation effects of urban vegetation, the magnitude and variability of vegetation-derived urban nighttime cooling differs greatly among cities. We hypothesize that urban vegetation-derived nighttime air cooling is driven by vegetation density whose effect is regulated by aridity through increasing transpiration. We test this hypothesis by deploying microclimate sensors across eight United States cities and investigating relationships of nighttime air temperature and urban vegetation throughout a summer season. Urban vegetation decreased nighttime air temperature in all cities. Vegetation cooling magnitudes increased as a function of aridity, resulting in the lowest cooling magnitude of 1.4 °C in the most humid city, Miami, FL, and 5.6 °C in the most arid city, Las Vegas, NV. Consistent with the differences among cities, the cooling effect increased during heat waves in all cities. For cities that experience a summer monsoon, Phoenix and Tucson, AZ, the cooling magnitude was larger during the more arid pre-monsoon season than during the more humid monsoon period. Our results place the large differences among previous measurements of vegetation nighttime urban cooling into a coherent physiological framework dependent on plant transpiration. This work informs urban heat risk planning by providing a framework for using urban vegetation as an environmental justice tool and can help identify where and when urban vegetation has the largest effect on mitigating nighttime temperatures.

Highlights

Average nighttime air temperatures (Tair) have been steadily increasing across the US in recent years, with implications for reduced human health and well-being [1, 2]
Was higher aridity during heat waves associated with greater cooling, but a consistent pattern occurred in the two cities that experienced a shift between a hot and dry pre-monsoon and a more humid monsoon weather pattern
Our results, spanning spatial scales of intra-urban to continental and temporal scales from individual evenings to the summer season, suggest widespread urban vegetation cooling that is consistently sensitive to aridity, implying greater latent heat fluxes in more arid cities at the same levels of vegetation coverage

Summary

Introduction

Average nighttime air temperatures (Tair) have been steadily increasing across the US in recent years, with implications for reduced human health and well-being [1, 2]. Within cities, developed areas with little vegetation result in hot spots where local temperatures are substantially greater than the citywide average, exacerbating UHI effects [9,10,11]. Does urban warming elevate health risks, high temperatures increase energy costs due to air conditioning [8, 12]. In Madison, WI, recent work showed only 1 ◦C air cooling associated with urban vegetation, while studies in Los Angeles and Palm Springs, CA, reported a mean 2.5 ◦C and 4.9 ◦C Delta Tveg, respectively [9, 17, 19]. A more comprehensive evaluation of Delta Tveg is needed, both within and among cities

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Results

Conclusion