Cool Cities: Counteracting Potential Climate Change and its Health Impacts

Curr Clim Change Rep (2015) 1:163–175 DOI 10.1007/s40641-015-0019-1 CLIMATE CHANGE AND HUMAN HEALTH (LS KALKSTEIN AND R DAVIS, SECTION EDITORS) Cool Cities: Counteracting Potential Climate Change and its Health Impacts Haider Taha 1 Published online: 16 July 2015 # Springer International Publishing AG 2015 Abstract Cool cities can produce a range of multi-scale, multi-dimensional impacts on the atmospheric environment. One area of increasing interest is the potentially beneficial impacts of cool cities in reducing heat stress and improving air quality health during hot weather and heat events. While the overriding effects of cool cities are beneficial in terms of thermal environment, cooling-energy use, air pollutant emissions, atmospheric chemistry, and air quality, some inadvertent effects can also arise. The goal of this paper is to present and compare the magnitudes of local heat and air quality effects induced by (1) climate change, (2) urban heat islands, and (3) cool cities. From the review of past and more recent findings, this paper concludes that cool cities have the potential to offset the negative local health impacts of climate change and/or their exacerbation by urban areas. To achieve these benefits, the measures must be tailored to region-specific characteristics and needs. Keywords Air quality . Air quality index . Atmospheric modeling . Climate change . Cool cities . Heat health . Heat index . Heat island control . Urban climate . Urban cooling . Urban heat islands This article is part of the Topical Collection on Climate Change and Human Health * Haider Taha 1 Altostratus Inc., 940 Toulouse Way, Martinez, CA 94553, USA Introduction One anticipated outcome of climate change is an increase in frequency and duration of heat events [1, 2]. While these events are defined differently in different regions [3], all require certain temperature thresholds to be exceeded, the compounding effect of humidity be considered, and that they have certain intensity, duration, and spatial-extent characteristics [4]. Many studies project significant health implications from such conditions. For example, a study of 12 U.S. cities estimates that by the end of the century, up to 200,000 heatrelated deaths will occur in these urban areas because of climate change [5, 6]. During the California heat wave of 2006, there was a 9 % increase in daily mortality rate for each 5.5 °C (10 °F) increase in apparent temperature [7]. The climate change impacts on heat stress, emissions, and atmospheric pollution can be further exacerbated by urban heat islands (UHIs) because of the higher canopy- and boundary-layer temperatures, transport of heat and pollutants, reduced surface moisture, and modified cloudiness. These pathways, separately and synergistically, impact heat and air quality health. UHIs can be defined or diagnosed in different manners including, for example, as instantaneous or cumulative metrics, point-specific or area-averaged, relative to fixed or timevarying (wind-dependent) temperature reference points, and with or without pre-defined temperature thresholds. UHIs also vary in intensity and diurnal/seasonal profiles as well as in spatiotemporal characteristics. Instantaneous UHIs are most often in the range of 0.5–3 °C [8, 9•, 10•, 11, 12]. Larger UHIs, e.g., 4–8 °C, have been observed [13, 14] but are not considered typical. Cool islands (UCI), when cities are cooler than non-urban surroundings, can occur during certain parts of the day, e.g., early morning [11], or on seasonal time scales [15], but UHIs occur much more frequently [8]. And while UHIs 164 can be an asset in high-latitude climates, they are a liability in terms of energy, heat, emissions, and air quality in the midand low latitudes. Thus, where UHIs exist in these regions, it is desirable to mitigate them. Presently, there exists a global interest in implementing cool cities [16]. Whether these measures can offset part or all of the urban and local climate effects will depend on each region’s characteristics. However, cool cities can produce desired environmental benefits (cooling) regardless of the existence of a UHI, i.e., even when urban areas are UCIs [11]. Figure 1 depicts the general pathways discussed in this paper. While climate change will impact all local meteorological fields, the main focus in this paper is on the changes in air temperature. Urban areas can locally exacerbate (+) the climate effects and increase temperature further. Cool cities can lower temperatures (−) and mitigate the health effects. The dashed line suggests that cool cities can also indirectly impact the global climate via reducing emissions of greenhouse gases and their subsequent intercontinental transport. Curr Clim Change Rep (2015) 1:163–175 The beneficial effects (cooling and improved air quality) of the measures in Table 1 are mainly localized effects, i.e., experienced directly at and near where cool cities measures are deployed. Downwind of the modified areas, effects are generally positive but sometimes also negative, e.g., warming and/ or increased ozone [8, 11]. Furthermore, the effectiveness of cool cities varies significantly from one geographical area to another, depending on climate, emissions profiles, technical potential, and size of modified urban area. Accordingly, the deployment of these technologies should be tailored to areaspecific characteristics so as to maximize health benefits. Positive and Negative Impacts of Cool Cities As with many environmental control measures, cool cities can exert both positive (beneficial) and negative (inadvertent) effects. The benefits include reduced cooling energy demand, improved thermal comfort, reduced emissions of air pollutants, and improved air quality [35]. Inadvertent effects include winter heating energy penalties and increased temperature and/or pollutant concentrations downwind of cool cities [18]. Cool Cities Measures Regional Scale While many technologies could be considered part of the cool cities portfolio of strategies, Table 1 lists some of the more common ones, along with their influence pathways [17•, 20•]. These measures, whether standalone or in combinations, can affect regional meteorology, microclimate, emissions, and chemistry in varying degrees and, thus, directly and/or indirectly heat and air quality health. Thus, urban areas or neighborhoods that deploy some or all of these measures are defined as cool cities. In terms of atmospheric and environmental impacts, cool cities are defined as the horizontal and vertical domains within which the effects from these measures are detected, e.g., on micrometeorology (temperature, moisture, and wind), emissions (anthropogenic and biogenic), and chemistry (air quality). Fig. 1 Pathways discussed in this paper Modeling studies at the regional, urban, and microscales [11, 18, 23] show that the inadvertent effects are smaller than the beneficial ones and limited in spatial extent. For ex (...truncated)