It seems to me you worry too much. The fact we think we have global warming is obviously due at least in part to the heat island effect.
Check out this web site. http://www.epa.gov/heatislands/
As someone who spent nearly 4 hours chewing on a Nerf football during last Sunday's Patriots-Chargers thriller, I'm in no position to address my anxiety. (Irrelevant Google-induced sidebar of the week: Nerf balls are apparently a must for sailing.)
The Urban Heat Island effect is popular because it makes sense. I know I've never questioned it. It's hotter on the pavement than on the grass. There's lots of pavement in the city. The weather people on TV are always saying it's a few degrees warmer at Reagan than it is in, say, Frederick. And there are lots of cartoons like this one that make a colorful (if not quite scientifically rigorous) case.
But after reading up on Urban Heat Islands, there seems to be only one thing everyone agrees on: It's really hard to objectively measure the temperature in a given place. If you take a temperature two feet off the pavement on a summer day, it's going to be much hotter than a temperature reading taken 100 feet above the pavement. Which is accurate? Unfortunately from a scientific standpoint, urban weather stations tend to be in/on/near buildings, while rural weather stations tend to be in/on/near dirt, so it's hard to make an accurate comparison. For details, scroll down to "Disadvantages of Surface and Air Temperature Measurements" at this site.Because of all that disparity, there's some skepticism about whether the Urban Heat Island effect even exists, nevermind its effect on global warming. The UN's Intergovernmental Panel on Climate Change states, "Extensive tests have shown that the urban heat island effects are no more than about 0.05°C up to 1990 in the global temperature records used in this chapter to depict climate change."
So it looks like the conventional wisdom about the effect of Urban Heat Islands is wrong on the global level, and a little shaky even on the local level. In fact, the very EPA website that Tom refers to discourages any direct connection between urban heat islands and global warming, and says that if anything, it's the byproducts of the heat island effect that could contribute to global warming ...
Heat islands describe local-scale temperature differences, generally between urban and rural areas. In contrast, global warming refers to a gradual rise of the earth's surface temperature.
While they are distinct phenomena, summertime heat islands may contribute to global warming by increasing demand for air conditioning, which results in additional power plant emissions of heat-trapping greenhouse gases. Strategies to reduce heat islands, therefore, can also reduce the emissions that contribute to global warming.