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An Extreme Urban Surrogate of Projected Global Warming
Reference
Shen, W., Wu,. J., Grimm, N.B. and Hope, D. 2008. Effects of urbanization-induced environmental changes on ecosystem functioning in the Phoenix metropolitan region, USA. Ecosystems 11: 138-155.

Background
Because urban environments are affected by urban heat islands, carbon dioxide domes, and high-level nitrogen deposition, the authors say that "to some extent they portend the future of the global ecosystem," and that they "provide a unique 'natural laboratory' to study potential ecosystem responses to anthropogenic environmental changes."

What was done
Shen et al. used a version of the Patch Arid Land Simulator-Functional Types (PALS-FT) process-based ecosystem model -- originally developed for the Chihuahuan Desert but modified to represent the Larrea tridentata-dominated ecosystem characteristic of the Sonoran Desert within which Phoenix is located -- to investigate impacts of previously documented city-to-desert gradients of atmospheric CO2 concentration, air temperature (TA), and nitrogen deposition (Ndep) on aboveground net primary productivity (ANPP) and soil organic matter (SOM).

What was learned
In response to the mean maximum rural-to-urban increases in CO2 (160 ppm), Ndep (24 kg per ha/year) and TA (4.0C) characteristic of Phoenix, mean ANPP changes of +52.5, +42.7 and -7.8 g dry matter (DM) per m2/year were obtained, respectively, from the 76.3 g DM per m2/year characteristic of desert conditions, when each of the three factors was increased individually. And when all three parameters were increased together, the net increase in ANPP was found to be even greater than the sum of the three individual results: 108 vs. 87.4 g DM per m2/year, which numbers translate to respective percentage increases of 142% vs. 115%. In the case of SOM, increases of 18.5, 12.3 and 1.2 g C per m2/year were obtained for mean maximum individual increases in CO2, Ndep and TA, respectively, while the combined increase was 30.9 g C per m2/year.

What it means
Even in a desert region as hot as Phoenix, the types of CO2, temperature and nitrogen deposition increases predicted for the years ahead portend huge increases in indigenous ecosystem productivity and soil organic matter buildup.

Reviewed 28 May 2008