© 2012 Macmillan Publishers Limited. All rights reserved. LETTERS PUBLISHED ONLINE: 12 AUGUST 2012 | DOI:10.1038/NCLIMATE1656 Summer-time climate impacts of projected megapolitan expansion in Arizona M. Georgescu 1,2 * , M. Moustaoui 2 , A. Mahalov 2 and J. Dudhia 3 Efforts characterizing the changing climate of southwestern North America by focusing exclusively on the impacts of in- creasing levels of long-lived greenhouse gases omit funda- mental elements with similar order-of-magnitude impacts as those owing to large-scale climate change 1,2 . Using a suite of ensemble-based, multiyear simulations, here we show the intensification of observationally based urban-induced phe- nomena and demonstrate that the direct summer-time climate effects of the most rapidly expanding megapolitan region in the USA—Arizona’s Sun Corridor—are considerable. Although urban-induced warming approaches 4 ◦ C locally for the max- imum expansion scenario, impacts depend on the particular trajectory of development. Cool-roof implementation reduces simulated warming by about 50%, yet decreases in summer- time evapotranspiration remain at least as large as those from urban expansion without this mode of adaptation. The contribution of urban-induced warming relative to mid- and end-of-century climate change illustrates strong dependence on built environment expansion scenarios and emissions path- ways. Our results highlight the direct climate impacts that result from newly emerging megapolitan regions and their significance for overcoming present challenges concerning sus- tainable development 3,4 . Direct effects of urbanization-induced land-use and land-cover change (LULCC) are an important driver of local to global change, with considerable implications for air quality, climate and natural resource sustainability 5–7 . Rapid population growth (Nevada: 66.3%, 35.1% and Arizona: 40.0%, 24.6% recorded the fastest na- tional population growth rates between 1990–2000 and 2000–2010, respectively; the two states are the only ones in the USA to maintain a decadal population growth rate exceeding 20% since 1950; ref. 8) and associated urbanization-induced landscape modification 9 place the American southwest in a particularly vulnerable situation as mounting concerns related to large-scale anthropogenic climate change are layered on top of water-resource sustainability con- straints resulting from rising demand 7 . An important question is whether, and to what extent, direct climatic impacts associated with rapidly urbanizing megapolitan regions in the US southwest are as important as those resulting from large-scale global climate change. Located in the American southwest, the states of the Colorado River Basin are expected to add 23 million new residents between 2000 and 2030, with Arizona’s burgeoning population accounting for roughly a quarter of projected growth, facilitating a top ten national ranking as one of the most populous states in the USA (ref. 10). The emergence of the Sun Corridor (megapolitan region stretching from the Arizona–Mexico border to northern Arizona; see Supplementary Information) as one of the largest megapolitan areas in the US (ref. 11), underscores the importance of well-timed 1 School of Geographical Sciences and Urban Planning, Arizona State University, PO Box 875302, Tempe, Arizona 85287-5302, USA, 2 School of Mathematical and Statistical Sciences, Global Institute of Sustainability, Arizona State University, Tempe, Arizona 85287-1804, USA, 3 Mesoscale and Microscale Meteorology Division, National Center for Atmospheric Research, PO Box 3000, Boulder, Colarado 80307-3000, USA. *e-mail: Matei.Georgescu@asu.edu. 30 1950 1955 1960 1965 1965 1970 1975 1980 1985 1990 1995 10 12 14 16 18 20 Diurnal range (°C) Diurnal range (°C) 22 24 32 34 36 30 1950 1955 1960 Mean temperature (°C) Mean temperature (°C) 1970 Year Year 1975 1980 1985 1990 1995 10 12 14 16 18 20 22 24 32 34 36 T AVG T MAX ¬ T MIN T AVG T MAX ¬ T MIN a b Figure 1 | Observed time series of the mean summer-time temperature and diurnal temperature range at an urbanizing and non-urbanizing station. Observed time series of the mean summer-time temperature (T AVG ) and diurnal temperature range (T MAX − T MIN ) at a, Phoenix Sky Harbor International Airport, Arizona and b, Sacaton, Arizona. Straight lines represent trend of time series using a linear least squares fitting technique. and managed growth. Phoenix, Arizona, the largest city in the Colorado River Basin and at the heart of the emerging Sun Corridor, finds itself at a crossroads, in dire need of science-based policy decisions ensuring sustainable growth with minimal consequences for the natural environment 12 . Arizona’s projected population increase is likely to encourage further landscape modification in future decades (2050 state estimates range between 8 and 16 million people 13 ). Such drastic conversion to engineered structures has had and is expected to continue having significant impacts on local to regional scale climate (for example, by exacerbating an already significant urban heat island). For example, Fig. 1a shows the historically observed summer-time (June–August, JJA) average temperature and diurnal range (maximum minus minimum temperature) at Phoenix’ Sky Harbor International Airport. Also illustrated are trends for both the average temperature and diurnal range, obtained with the linear regression method using a least squares fitting technique. The diurnal range progressively decreases with time, a manifestation of relentless urbanization driving the increase in urban-heat-island magnitude and rise in overall mean temperature, a feature not evident at a nearby non-urbanizing rural station (Fig. 1b). Evaluation of LULCC resulting from expansion of the built environment is necessary to address policy-relevant NATURE CLIMATE CHANGE | ADVANCE ONLINE PUBLICATION | www.nature.com/natureclimatechange 1