Urban forest contribution to the global carbon budget: A meta-analysis

Dustin L. Herrmann, Stuart T. Schwab and Darrel Jenerette, Botany and Plant Sciences, University of California, Riverside, Riverside, CA, Meghan Avolio, Department of Earth & Planetary Sciences, Johns Hopkins University, Baltimore, MD, Diane Pataki, School of Biological Sciences, University of Utah, Salt Lake City, UT

Background/Question/Methods
Climate change mitigation requires reductions in atmospheric carbon dioxide and other greenhouse gases achieved through cuts in emissions or removal by earth systems. At present, anthropogenic carbon dioxide emissions are 35.9 Gt CO2ꞏy-1, a level that is substantially greater than carbon sinks in the earth system. As such, we preface that carbon sequestration by earth systems, the focus here, must be accompanied by emission cuts in a global strategy for climate change mitigation rather than approached as an offset for business-as-usual emission rates. Urban ecosystems are being examined as a knowledge frontier for their contributions to carbon storage and sequestration. Their global impact is expected to be constrained by their relatively small global coverage at ~0.5-3% of the earth’s land surface and potentially minor sequestration rates. Despite a small footprint globally, if urban areas, characterized here as urban forest ecosystems, function as aggrading forests, they act as a carbon sink. Here we synthesize the peer-reviewed literature on carbon storage and sequestration by urban trees through quantitative meta-analysis to provide a robust empirical grounding for tackling a contended and outstanding question. That is, can urban forest ecosystems be a meaningful part of a global strategy to mitigate climate change?
Results/Conclusions
At about 60% of potential records examined, we recorded over 70 scientific journal articles reporting carbon storage and sequestration by urban trees with an acceleration in output with more than half published in the last five years. Importantly, this growth in records adds coverage of less represented regions. Wide variation across cities existed with means of 21.9 ± 11.5 Mg Cꞏha-1 (± 1 sd) and 0.65 ± 0.41 Mg Cꞏha-1ꞏy-1 for citywide estimates of carbon storage and sequestration rates, respectively. Based on these preliminary analysis means and approximating global urban extent at 1x106 km2, trees in urban forests globally store 2.19 Gt C and sequester 0.065 Gt Cꞏy-1. If urban forests increase in biomass by 0.065 Gt C per year, this carbon sink represents 0.7% of contemporary carbon dioxide emissions globally. Preliminarily, we conclude carbon storage and sequestration by urban forests is arguably too marginal to shape local land use decisions. However, to the extent that maintaining and growing trees in cities is pursued for multiple local objectives, we present the existing evidence for the degree to which urban forests could contribute to global carbon budgets. Refinement of results anticipated.