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Urban Warming vs. Global Warming in East China
Reference
Yang, X., Hou, Y. and Chen, B. 2011. Observed surface warming induced by urbanization in east China. Journal of Geophysical Research 116: 10.1029/2010JD015452.

Background
The authors note that the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) states that Urban Heat Island (UHI) effects are real; but they say the Report says the effects are only "local and have a negligible influence on global warming trends." However, they indicate that the UHI effect is regarded by others "as one of the major errors or sources of uncertainty in current surface warming studies," citing Gong and Wang (2002) and Heisler and Brazel (2010). And they say that "some research results indicate that this effect may play a more significant role in temperature trends estimated at multiple geographic scales," noting that Pielke (2005) and Stone (2009) suggest that "such results should be accorded more consideration in the mitigation of climate change."

What was done
As their contribution to the subject, Yang et al. used monthly mean surface air temperature data from 463 meteorological stations, including those from the 1981-2007 ordinary and national basic reference surface stations in east China and from the National Centers for Environmental Prediction and National Center for Atmospheric Research (NCEP/NCAR) Reanalysis, in order to investigate the effect of rapid urbanization on temperature change for six different categories of population size/density -- metropolis, large city, medium-sized city, small city, suburban and rural -- as determined from satellite-measured nighttime light imagery and census data.

What was learned
The three researchers say their findings indicate that "rapid urbanization has a significant influence on surface warming over east China," noting that "overall, UHI effects contribute 24.2% to regional average warming trends," and that "the strongest effect of urbanization on annual mean surface air temperature trends occurs over the metropolis and large city stations, with corresponding contributions of about 44% and 35% to total warming, respectively," with UHI trends of 0.398°C and 0.26°C per decade. And they say that due to other considerations, the UHI warming trends and their contributions to the overall warming over east China provided in their paper "can still be regarded as conservative."

What it means
The Chinese scientists conclude that if such UHI trends continue, "certain metropolitan areas may experience a rate of warming well beyond the range projected by the global climate change scenarios of the IPCC," referencing Stone (2007), while adding that "the increasing divergence between urban and rural surface temperature trends highlights the limitations of the response policy to climate change [that] focus only on greenhouse gas reduction," citing Stone (2009). And, of course, their findings call into serious question some of the basic conclusions of the IPPC, such as the organization's claim that UHI effects "have a negligible influence on global warming trends."

References
Gong, D.Y. and Wang, S.W. 2002. Uncertainties in the global warming studies. Earth Science Frontiers 9: 371-376.

Heisler, G.M. and Brazel, A.J. 2010. The urban physical environment: Temperature and urban heat islands. In: Aitkenhead-Peterson, J. and Volder, A. (Eds.) Urban Ecosystem Ecology. American Society of Agronomy, Crop Science Society of America, Soil Science Society of America, Madison, Wisconsin, USA, pp. 29-56.

Pielke Sr., R.A. 2005. Land use and climate change. Science 310: 1625-1626.

Stone Jr., B. 2007. Urban and rural temperature trends in proximity to large US cities: 1951-2000. International Journal of Climatology 27: 1801-1807.

Stone Jr., B. 2009. Land use as climate change mitigation. Environmental Science and Technology 43: 9052-9056.

Reviewed 28 September 2011