What was done
The authors compared "the possible effectiveness of heat island mitigation strategies to increase urban vegetation, such as planting trees or incorporating vegetation into rooftops, with strategies to increase the albedo of impervious surfaces."

What was learned
With respect to the magnitude of the problem they were seeking to address, Rosenzweig et al. report that "surface air temperatures elevated by at least 1°C have been observed in New York City for more than a century (Rosenthal et al., 2003; Gaffin et al., 2008), and the heat island signal, measured as the difference between the urban core and the surrounding rural surface air temperature readings taken at National Weather Service stations, averages ~4°C on summer nights (Kirkpatrick and Shulman, 1987; Gedzelman et al., 2003; Gaffin et al., 2008)," with the greatest temperature differences typically being sustained "between midnight and 0500 Eastern Standard Time (EST; Gaffin et al., 2008)." And on a day that they studied quite intensively (14 August 2002), they report that at 0600 EST, "the city was several degrees warmer than the suburbs, and up to 8°C warmer than rural areas within 100 km of the city."

With respect to mitigation strategies, the twelve researchers determined that "the most effective way to reduce urban air temperature is to maximize the amount of vegetation in the city with a combination of tree planting and green roofs." Based on modeling studies of these approaches, for example, they estimated that this strategy could reduce simulated citywide urban air temperature by 0.4°C on average, and 0.7°C at 1500 EST, while simulated reductions of up to 1.1°C at 1500 EST could be expected in some Manhattan and Brooklyn neighborhoods, "primarily because there is more available area in which to plant trees and install vegetated roofs."

What it means
These several findings reveal that New York City has already experienced an urban-induced warming equivalent to what is predicted to occur by the end of the current century as a result of business-as-usual greenhouse gas emissions, and that planting additional vegetation throughout the city would likely moderate its thermal environment more than all of the greenhouse-gas emissions reductions the world's governments are ever likely to make.

References
Gaffin, S.R., et al. 2008. Variations in New York City's urban heat island strength over time and space. Theoretical and Applied Climatology 94: 1-11.

Gedzelman, S.D., Austin, S., Cermak, R., Stefano, N., Partridge, S., Quesenberry, S. and Robinson, D.A. 2003. Mesoscale aspects of the urban heat island around New York City. Theoretical and Applied Climatology 75: 29-42.

Kirkpatrick, J.S. and Shulman, M.D. 1987. A statistical evaluation of the New York City-northern Jew Jersey urban heat island effect on summer daily minimum temperature. National Weather Digest 12: 12.

Rosenthal, J., Pena Sastre, M., Rosenzweig, C., Knowlton, K., Goldberg, R. and Kinney, P. 2003. One hundred years of New York City's "urban heat island": Temperature trends and public health impacts. EOS, Transactions of the American Geophysical Union 84 (Fall Meeting Supplement), Abstract U32A-0030.