The Urban heat island affect can add as much as 20 F to average temperatures in a city. :

"Phoenix area temperatures could continue to increase - perhaps as much as 15 to 20 degrees above historic averages - as buildings, roads and parking lots march across former croplands and virgin desert.
...
Interestingly, average daytime summer highs in Balling's study remained about the same - hovering between 102 and 104 degrees. This relative constant is due to the wind's dispersal of the sun's energy during the day. Is this still the case? Balling isn't sure, having switched his study to global warming."
http://www.sepp.org/reality/arizrepub.html

"On hot summer days, urban air can be up to 10°F hotter than the surrounding countryside. Not to be confused with global climate change, scientists call this phenomenon the "heat island effect." Heat islands form as cities replace natural land cover with pavement, buildings, and other infrastructure.
Increased urban temperatures can affect public health, the environment, and the amount of energy that consumers use for summertime cooling."
http://yosemite.epa.gov/oar/globalwarming.nsf/content/ActionsLocalHeatIslandEffect.html

Ok, so most affect is in night time temperature [which btw is also true in regards to Global warming] but a significant portion of the increase temperature is in daytime temperature [unlike global warming].
Now, the urban heat island [UHI] affect is causing 10 times the increase in temperatures and is affecting billions of people, yet it seems there isn't much attention spent addressing this issue. Let me put this way, UHI has probably killed thousands of people [mostly elderly people, disabled, or people generally impaired in some way] and global warming has killed no one, nor killed polar bears or anything else [despite various attempts of people lying about this].

UHI is something we could actually do something about, whereas the world has spent 100s billions on global warming with no significant result, nor any hope of any significant result.

It is my contention that little scientific work has been devoted to studying the Urban Heat Island affect. And that on top of this, it might be possible that potential work that could been spent studying this has been to some degree been politically thwarted by those trying to promote "the dangers from Global warming". I could talk at length about this possiblity, but I think the more important point is the apparent lack of attention devoted to UHI.

I draw your attention to what govt offers a solution to UHI:
"These "heat island reduction strategies" include:

* Installing cool or vegetated green roofs
* Planting trees and vegetation
* Switching to cool paving materials "
http://yosemite.epa.gov/oar/globalwarming.nsf/content/ActionsLocalHeatIslandEffect.html

"Trees and vegetation cool the air by providing shade and through evapotranspiration (the evaporation of water from leaves). Shade reduces the amount of solar radiationTo definition provided on the glossary page transmitted to underlying surfaces, keeping them cool. Shaded walls may be 9 to 36°F (5° to 20°C) cooler than the peak surface temperatures of unshaded surfaces."
http://www.epa.gov/heatisland/strategies/vegetation.html

Oh, I found this rather strange [and probably incorrect] item:
"Trees sequester – or store – between 35 and 800 pounds annually depending on their size and growth rate. The total quantity of carbon stored in mature trees may be 1,000 times more than the storage in small, young trees. The U.S. Forest Service estimates that carbon storage by urban forests is between 400 and 900 million metric tons nationally. "

How can just trees in urban areas be sucking up so much CO2? How many god damn trees are in urban areas. It's no brainer that there is at least hundreds of times more trees in all the forest which isn't in urban areas. Anyhow, I'll have check further into this- starting I suppose with how they are defining "urban forests".

Anyhow, the main solution seems to be plant more trees. Now, I'm certainly not against planting trees, but doubt it would be very effective in reducing UHI. Great way to get cooler temperature in local areas- a shaded street is certainly cooler, a home with plenty of shade tree will be cooler- that is obvious. But this is not the same thing as solving UHI. Trees offer shade and cool by evaporation:
"A mature tree with a 30-foot crown transpires approximately 40 gallons of water per day."
http://www.epa.gov/heatisland/strategies/vegetation.html

So suppose you had 5 of these small trees per person living in a city of 10 million- that's 2 billion gallons of water being put into the air- just from the trees (about 7.5 million tons per day- 2.7 billion tons per year). This going to add significantly to the local humidity and H2O is a greenhouse gas. Just these trees are adding somewhere around 100 times the greenhouse gas of water as compared to the CO2 emitted by this population of people. As far as total amount water being added to air from various human activity such as watering lawns, ect, is probably at least twice maybe ten times as much.