Guest Weblog by Dr. Matt Georgescu Of Rutgers University
More Evidence of LULCC Impact
Assessment of anthropogenic influences on climate has primarily focused on changes in globally-averaged metrics (e.g., temperature, tropospheric radiation balance) resulting from emissions of well-mixed greenhouse gases. However, sub-global-scale forcings and their impacts are also important. Understanding regional climate change is essential in its own right, as this is the scale of many impacts of concern for human and natural systems. In addition to drivers at the global scale, thorough attribution of anthropogenic climate change must take into account an additional number of factors. It is my belief that in addition to increases in the concentration of atmospheric greenhouse gases, landscape change plays a significant role on the evolving climate system. This message, of which Prof. Pielke and climatesci.org have been outspoken proponents of (e.g., Pielke et. al., 2002), in my opinion, deserves further attention.
The Greater Phoenix area (i.e., central Arizona) serves as a strategic study region that may be used to better understand the climatic consequences of landscape change – the area has undergone rapid population increase since the initial permanent settlement was established in 1868. Throughout the next 50 to 75 years, or so, agriculture became the mainstay of the area’s growing economy. An increasingly diverse economy, attractive climate, and relatively low-cost housing led to a shift in economic priorities during the latter half of the century and urban/sub-urban expansion began to dominate. The city continued to expand, and by 1995 Phoenix’ (the city itself, rather than the metropolitan area as a whole) population increased to nearly 1.2 million.
My Ph.D. work at Rutgers University [with Advisor Chris P. Weaver] has focused on assessing the impact of landscape change on the summer climate of one of the nation’s most rapidly expanding metropolitan complexes, the Greater Phoenix, AZ, region. The specific importance for research investigations of this area is two-fold:
1. The area has been undergoing, and continues to undergo, rapid landscape change as sprawl continues nearly unabated, thereby offering scientists a valuable opportunity to observe and relate modeling results to the actual evolving situation on the ground.
2. Sprawl and landscape change continue in numerous additional semi-arid locales, all serving as centers of human migration, both within the United States (e.g., Las Vegas) and without (e.g., Riyadh). Therefore, lessons learned regarding possible negative effects over the Greater Phoenix, AZ, region, may lead to improved mitigation strategies in other areas undergoing similar landscape change.
Of particular significance to the Greater Phoenix area and it’s relentlessly growing population and metropolitan expansion is the impact on precious natural water resources. This region is stressed to begin with and the climatological scarcity of water, together with increasing expansion, may pose significant impacts on the public sector down the road.
New work, recently published (online) in the Journal of Arid Environments (Georgescu et al., 2008), demonstrates the important dual roles of two specific patterns of land-use over the Greater Phoenix area. The summary of the paper reads as following:
This work evaluates the first-order effect of land-use/land-cover change (LULCC) on the summer climate of one of the nation’s most rapidly expanding metropolitan complexes, the Greater Phoenix, AZ, region. High-resolution – 2-km grid spacing – Regional Atmospheric Modeling System (RAMS) simulations of three ‘‘wet” and three ‘‘dry” summers were carried out for two different land-cover reconstructions for the region: a circa 1992 representation based on satellite observations, and a hypothetical land-cover scenario where the anthropogenic landscape of irrigated agriculture and urban pixels was replaced with current semi-natural vegetation. Model output is evaluated with respect to observed air temperature, dew point, and precipitation. Our results suggest that development of extensive irrigated agriculture adjacent to the urban area has dampened any regional-mean warming due to urbanization. Consistent with previous observationally based work, LULCC produces a systematic increase in precipitation to the north and east of the city, though only under dry conditions. This is due to a change in background atmospheric stability resulting from the advection of both warmth from the urban core and moisture from the irrigated area.
Analysis of results show a dipole pattern of temperature differences between the pair of landscape reconstructions that is magnified during the “dry” simulations as compared to the “wet” simulations; that is to say, during “dry” runs (each run, using a triply nested grid configuration, with the fine grid containing a 2-km grid spacing, lasted for 1 entire July month), the maximum (urban) temperature increases are enhanced for the 1992 landscape relative to the pre-settlement landscape (urban area differences during the “dry” simulations are in excess of 0.7°C, while urban area temperature differences between the pair of landscapes are closer to 0.5°C for the “wet” simulations). Similarly, the maximum temperature decreases are also enhanced (that is, more cooling over plots of irrigated agriculture) during the “dry” simulations when compared to the “wet” simulations.
In addition, this paper (i.e., Georgescu et al., 2008) is the first to present numerical modeling results, to our knowledge, consistent with prior observational analysis (e.g., Shepherd et al., 2006) showing an enhancement of precipitation due to the presence of the Greater Phoenix area.
The combined effect of warming (over areas that underwent urbanization) and cooling (over plots of irrigated agriculture) tend to counteract one another. This result is especially critical when considering that during the last three or so decades, coverage of irrigated agriculture has declined sharply at the expense of urbanization (suggesting a significantly greater warming effect due to recent LULCC). Two additional manuscripts detailing the radiative, dynamical, and thermodynamical effect(s), also through a numerical modeling framework, depict the evolution of Greater Phoenix’ regional climate in response to the observed changes in landscape (since the dawn of the satellite era to, roughly, today), are nearing completion.
Georgescu, M., G. Miguez-Macho, L. T. Steyaert, and C.P. Weaver, 2008: Sensitivity of summer climate to anthropogenic land cover change over the Greater Phoenix, AZ, Region, J. Arid Env., doi: 10.1016/j.jaridenv.2008.01.004.
Pielke Sr., R.A., G. Marland, R.A. Betts, T.N. Chase, J.L. Eastman, J.O. Niles, D. Niyogi, and S. Running, 2002: The influence of land-use change and landscape dynamics on the climate system-relevance to climate change policy beyond the radiative effect of greenhouse gases, Phil. Trans. A. Special Theme Issue, 360, 1705-1719.
Shepherd, J. M., 2006: Evidence of urban-induced precipitation variability in arid climate regions, J. Arid. Env., 67, 607-628.