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ScienceWeek
CLIMATOLOGY: URBANIZATION, LAND-USE, AND CLIMATE CHANGE
The following points are made by Eugenia Kalnay and Ming Cai (Nature 2003 423:528):
1) The most important anthropogenic influences on climate are the emission of greenhouse gases(1) and changes in land use, such as urbanization and agriculture(2). But it has been difficult to separate these two influences because both tend to increase the daily mean surface temperature(3,4). The impact of urbanization has been estimated by comparing observations in cities with those in surrounding rural areas, but the results differ significantly depending on whether population data(5) or satellite measurements of night light are used to classify urban and rural areas.
2) Two methods used in the US to classify meteorological stations into urban and rural to "correct" the observed surface temperature trends for urbanization effects are based on population data(5) and satellite measurements of night-light, respectively, and the corresponding estimates of the impact of urbanization differ in magnitude (0.06 and 0.15 °C per century). The finding that atmospheric temperatures as measured by satellites and weather balloons have smaller warming trends than surface observations has been the subject of much discussion centered mostly on the quality of the data, but it could be partially explained by a predominance of land-use effects over greenhouse warming near the surface.
3) The authors report they use the difference between trends in observed surface temperatures in the continental US and the corresponding trends in a reconstruction of surface temperatures determined from a reanalysis of global weather over the past 50 years, which is insensitive to surface observations, to estimate the impact of land-use changes on surface warming. The author suggest their results indicate that half of the observed decrease in diurnal temperature range is due to urban and other land-use changes. Moreover, their estimate of 0.27 °C mean surface warming per century due to land-use changes is at least twice as high as previous estimates based on urbanization alone.
4) The authors conclude: "Our method can incorporate updated observations as they become available, can be applied to land stations throughout the world, to other variables such as humidity and winds, detect seasonal trends, and signal changes in station locations that are otherwise difficult to identify."
References (abridged):
1. IPCC Climate Change 2001: The Scientific Basis (Cambridge Univ. Press, Cambridge, UK, 2001)
2. Pielke, R. A. Sr et al. The influence of land-use change and landscape dynamics on the climate system: Relevance to climate-change policy beyond the radiative effects of greenhouse gases. Phil. Trans. R. Soc. Lond. A 360, 1-15 (2002)
3. Gallo, K. P. & Owen, T. W. Satellite-based adjustments for urban heat island temperature bias. J. Appl. Meteorol. 38, 806-813 (1999)
4. Owen, T. W., Gallo, K. P., Elvidge, C. D. & Baugh, K. E. Using DMSP-OLS light frequency data to categorize urban environments associated with US climate observing stations. Int. J. Remote Sensing 19, 3451-3456 (1998)
5. Easterling, D. R. et al. Maximum and minimum temperature trends for the globe. Science 277, 364-367 (1997)
Nature http://www.nature.com/nature
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URBANIZATION EFFECTS ON TREE GROWTH IN THE VICINITY OF NEW YORK CITY
The following points are made by J.W. Gregg et al (Nature 2003 424:183):
1) Urbanization of the globe is accelerating, with potentially large impacts on vegetation in cities and surrounding areas. Urban air contains high concentrations of many gaseous, particulate and photochemical pollutants (such as NOx, HNO3, SO2, H2SO4, O3 and volatile organic compounds)(1,5); and urban soils are high in heavy metals and can be more hydrophobic and acidic than surrounding rural environments(2). Although many of these contaminants have detrimental effects on plant growth, urban environments also have higher rates of nutrient and base-cation deposition(1,5), warmer temperatures (urban "heat-island" effect)(3), and increased CO2 concentrations(4) -- factors that often, but not invariably, enhance plant growth.
2) Given the potential for interactions among all factors and the relative absence of studies examining more than two or three factors in combination, understanding the net effect of multiple anthropogenic environmental changes in an urban environment and the relative importance of the individual factors remains a major challenge.
3) In summary: Plants in urban ecosystems are exposed to many pollutants and higher temperatures, CO2 and nitrogen deposition than plants in rural areas(1-5). Although each factor has a detrimental or beneficial influence on plant growth, the net effect of all factors and the key driving variables are unknown. The authors grew the same cottonwood clone in urban and rural sites and found that urban plant biomass was double that of rural sites. Using soil transplants, nutrient budgets, chamber experiments and multiple regression analyses, the authors demonstrate that soils, temperature, CO2, nutrient deposition, urban air pollutants and microclimatic variables could not account for increased growth in the city. Rather, higher rural ozone (O3) exposures reduced growth at rural sites. Urban precursors fuel the reactions of O3 formation, but NOx scavenging reactions resulted in lower cumulative urban O3 exposures compared to agricultural and forested sites throughout the northeastern USA. The authors suggest their study shows the overriding effect of O3 despite a diversity of altered environmental factors, reveals "footprints" of lower cumulative urban O3 exposures amidst a background of higher regional exposures, and shows a greater adverse effect of urban pollutant emissions beyond the urban core.
References (abridged):
1. Lovett, G. M. et al. Atmospheric deposition to oak forests along an urban-rural gradient. Environ. Sci. Technol. 34, 4294-4300 (2000)
2. Pouyat, R. V., McDonnell, M. J. & Pickett, S. T. A. Soil characteristics of oak stands along an urban-rural land-use gradient. J. Environ. Qual. 24, 516-526 (1995)
3. Peterson, J. T. The Climate Of Cities: A Survey Of The Recent Literature. NAPCA Pub. No. AP-59 (US Department of Health, Education and Welfare, 1969)
4. Idso, C. D., Idso, S. B. & Balling, R. C. Jr An intensive two-week study of an urban CO2 dome in Phoenix, Arizona, USA. Atmos. Environ. 35, 995-1000 (2001)
5. Gatz, D. F. Urban precipitation chemistry: A review and synthesis. Atmos. Environ. B Urban Atmos. 25, 1-16 (1991)
Nature http://www.nature.com/nature
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ATMOSPHERIC DEPOSITION TO OAK FORESTS ALONG AN URBAN-RURAL GRADIENT
The following points are made by G.M. Lovett (Environ. Sci. Technol 2000 34:4294):
1) To determine the patterns of atmospheric deposition and throughfall in the vicinity of a large city, bulk deposition, oak forest throughfall, and particulate dust deposition, the authors measured at sites along a transect within and to the north of New York City. Concentrations and fluxes of NO3-, NH4+, Ca2+, Mg2+, SO42-, and Cl- in throughfall all declined significantly with distance from the city, while hydrogen ion concentration and flux increased with distance from the city. Most of the change in concentrations and fluxes occurred within 45 km of the city.
2) Throughfall deposition of inorganic N was twice as high in the urban sites as compared to the suburban and rural sites. Bulk deposition patterns were similar to those of throughfall, but changes along the transect were much less pronounced. The water-extractable component of dust deposition to Petri plates also was substantially higher in the urban sites for Ca2+, Mg2+, SO42-, NO3-, and Cl-. The dust particles had little alkalinity, suggesting that alkaline aerosols were neutralized by acidic gases in the atmospheric.
3) The authors propose that dust emissions from New York City act like an "urban scrubber", removing acidic gases from the atmospheric and depositing them on the city as coarse particle dry deposition. Despite the urban scrubber effect, most of the dry deposition of nitrate was from gaseous nitrogen oxides, which were in much higher concentration in the city than in rural sites. The authors suggest that excess deposition of nutrients and pollutants could be important for the nutrient budgets of forests in and near urban areas.
Environ. Sci. Technol http://pubs.acs.org/journals/esthag/
ScienceWeek http://www.scienceweek.com
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