We investigated the spatial and temporal variation in June mean minimum temperatures for weather stations in and around metropolitan Phoenix, USA, for the period 1990 to 2004. Temperature was related to synoptic conditions, location in urban development zones (DZs), and the pace of housing construction in a 1 km buffer around fixed-point temperature stations. June is typically clear and calm, and dominated by a dry, tropical air mass with little change in minimum temperature from day to day. However, a dry, moderate weather type accounted for a large portion of the inter-annual variability in mean monthly minimum temperature. Significant temperature variation was explained by surface effects captured by the type of urban DZ, which ranged from urban core and infill sites, to desert and agricultural fringe locations, to exurban. An overall spatial urban effect, derived from the June monthly mean minimum temperature, is in the order of 2 to 4 K. The cumulative housing build-up around weather sites in the region was significant and resulted in average increases of 1.4 K per 1000 home completions, with a standard error of 0.4 K. Overall, minimum temperatures were spatially and temporally accounted for by variations in weather type, type of urban DZ (higher in core and infill), and the number of home completions over the period. Results compare favorably with the magnitude of heating by residential development cited by researchers using differing methodologies in other urban areas.

Global environmental change and sustainability science increasingly recognize the need to address the consequences of changes taking place in the structure and function of the biosphere. These changes raise questions such as: Who and what are vulnerable to the multiple environmental changes underway, and where? Research demonstrates that vulnerability is registered not by exposure to hazards (perturbations and stresses) alone but also resides in the sensitivity and resilience of the system experiencing such hazards. This recognition requires revisions and enlargements in the basic design of vulnerability assessments, including the capacity to treat coupled human–environment systems and those linkages within and without the systems that affect their vulnerability. A vulnerability framework for the assessment of coupled human–environment systems is presented.

Research on global environmental change has significantly improved our understanding of the structure and function of the biosphere and the human impress on both (1). The emergence of “sustainability science” (2–4) builds toward an understanding of the human–environment condition with the dual objectives of meeting the needs of society while sustaining the life support systems of the planet. These objectives, in turn, require improved dialogue between science and decision making (5–8). The vulnerability of coupled human–environment systems is one of the central elements of this dialogue and sustainability research (6, 9–11). It directs attention to such questions as: Who and what are vulnerable to the multiple environmental and human changes underway, and where? How are these changes and their consequences attenuated or amplified by different human and environmental conditions? What can be done to reduce vulnerability to change? How may more resilient and adaptive communities and societies be built?

Answers to these and related questions require conceptual frameworks that account for the vulnerability of coupled human–environment systems with diverse and complex linkages. Various expert communities have made considerable progress in pointing the way toward the design of these frameworks (10, 11). These advances are briefly reviewed here and, drawing on them, we present a conceptual framework of vulnerability developed by the Research and Assessment Systems for Sustainability Program (http://sust.harvard.edu) that produced the set of works in this Special Feature of PNAS. The framework aims to make vulnerability analysis consistent with the concerns of sustainability and global environmental change science. The case study by Turner et al. (12) in this issue of PNAS illustrates how the framework informs vulnerability assessments.

Transitions towards sustainability are urgently needed to address the interconnected challenges of economic development, ecological integrity, and social justice, from local to global scales. Around the world, collaborative science-society initiatives are forming to conduct experiments in support of sustainability transitions. Such experiments, if carefully designed, provide significant learning opportunities for making progress on transition efforts. Yet, there is no broadly applicable evaluative scheme available to capture this critical information across a large number of cases, and to guide the design of transition experiments. To address this gap, the article develops such a scheme, in a tentative form, drawing on evaluative research and sustainability transitions scholarship, alongside insights from empirical cases. We critically discuss the scheme's key features of being generic, comprehensive, operational, and formative. Furthermore, we invite scholars and practitioners to apply, reflect and further develop the proposed tentative scheme – making evaluation and experiments objects of learning.

Phoenix and neighboring municipalities, like many in the South and West, pursued a growth strategy based on annexation in the decades after the second world war. This article explores the link between annexation and competition for tax revenues. After discussing arguments for annexation, it traces the history of annexation in the Phoenix metropolitan area. A long‐running series of ‘border wars’ entailed litigation, pre‐emptive annexations and considerable intergovernmental conflict. The article argues that tax revenues have been a key motivation for municipalities to seek annexation, particularly since the 1970s. The timing of annexation was an important component of the strategies of municipal officials. Developers sought urban economic growth, but did not always favor political expansion of municipal boundaries through annexation. The article then considers several related policy issues and argues that while opportunities for annexation are becoming more limited, competition for tax revenues (particularly sales‐tax revenues) continues to be fierce, creating dilemmas for municipalities in the region.

We conducted microclimate simulations in ENVI-Met 3.1 to evaluate the impact of vegetation in lowering temperatures during an extreme heat event in an urban core neighborhood park in Phoenix, Arizona. We predicted air and surface temperatures under two different vegetation regimes: existing conditions representative of Phoenix urban core neighborhoods, and a proposed scenario informed by principles of landscape design and architecture and Urban Heat Island mitigation strategies. We found significant potential air and surface temperature reductions between representative and proposed vegetation scenarios:

1. A Park Cool Island effect that extended to non-vegetated surfaces.
2. A net cooling of air underneath or around canopied vegetation ranging from 0.9 °C to 1.9 °C during the warmest time of the day.
3. Potential reductions in surface temperatures from 0.8 °C to 8.4 °C in areas underneath or around vegetation.

The City of Phoenix’s Cool Urban Spaces Report (2014) investigated the impact of the Phoenix Cool Roofs and Tree and Shade Master Plan initiatives on the city. The study evaluated how these heat mitigation efforts affect microclimates and human thermal comfort in the Phoenix metropolitan area. These findings are especially relevant as rapid and extensive urbanization has led to an urban heat island (UHI) effect that has increased steadily at approximately 0.9°F per decade. The city’s questions guiding this research were: 1. What are the cooling benefits achieved by increasing tree canopy from 10% (current) to 25% (2030 goal) and/or implementing cool roofs under existing conditions and projected warming? 2. What is the diurnal thermal benefit of tree canopy shade for a typical heat wave day during pre-monsoon summer?

The impacts of cool roofs and trees on near-ground air temperatures were modeled through 54 scenarios for a typical residential neighborhood in Phoenix. We ran the model for a combination of three tree-planting scenarios (no trees, current canopy cover and 2030 canopy goal) and three landscaping scenarios (mesic, oasis and xeric) with regular roofs and cool roofs under current climate conditions and two climate change projections. Two significant results of the tree and shade initiative are: 1. Increasing tree canopy cover to 25% leads to an additional temperature reduction of 4.3°F, which is a total cooling benefit of 7.9°F as compared to a bare neighborhood, and 2. Switching landscaping from xeric to oasis, i.e., adding grass patches to residential backyards, reduces average neighborhood temperatures by 0.4°F to 0.5°F.

The scenario with the lowest air temperatures is the residential neighborhood with mesic landscaping, 25% tree canopy cover and cool roofs under current climate conditions with an average neighborhood temperature of 99.5°F. In contrast, the xeric neighborhood with no tree cover and regular roofs under the high-emissions climate change scenario is the hottest. This indicates that the combination of increased tree canopy cover and cool roofs does lower temperatures as well as reduce the demand for air conditioning, thereby reducing anthropogenic heat. However, trees and cool roofs are only part of the solution and need to be included in a broader, more comprehensive mitigation and adaptation plan.

Across all climate and tree scenarios, the effect of cool roofs alone on local daytime temperatures is relatively low. Air temperature reduction only amounts to 0.5°F in the neighborhood. Regarding the city’s cool roofs initiative, results show little benefit for extending this project to commercial and residential properties based on its cooling impacts alone. Our research thus far indicates that there is no simple solution to mitigating the UHI, but a complex balance of strategies will be necessary so that efforts to lower the daytime temperatures do not increase nighttime temperatures or shift UHI impacts to more vulnerable populations.