Just published. I haven't read it -- just passing it on.
Canbing Li, Yijia Cao, Mi Zhang, Jianhui Wang, Jianguo Liu, Haiqing Shi & Yinghui Geng. Hidden Benefits of Electric Vehicles for Addressing Climate Change. Scientific Reports, 2015 DOI: 10.1038/srep09213
Hidden Benefits of Electric Vehicles for Addressing Climate Change : Scientific Reports : Nature Publishing Group
There is also an article about the paper here:
Hidden benefits of electric vehicles revealed -- ScienceDaily
"... They show that the cool factor is real -- in that electric vehicles emit significantly less heat. That difference could mitigate the urban heat island effect, the phenomenon that helps turn big cities like Beijing into pressure cookers in warm months. ..."
But, of course the ultimate effect on the climate depends on the carbon emissions (if any) of the electricity source.