As plants grow, they transpire, releasing water into the atmosphere.  During the summer in a city, trees help to cool the immediate surroundings through transpiration.

New research from Carnegie’s Global Ecology department, published last month in Environmental Research Letters, concludes that transpiration has a global effect as well.

How does this happen?  Water vapor is a greenhouse gas, so one might expect that more water vapor in the atmosphere would lead to higher temperatures.

But water vapor also condenses into clouds, which reflect sunlight, resulting in a cooling effect.  The increased transpiration from plants, combined with evaporation from bodies of water, results in lower-level clouds.  Lower clouds tend to reflect more sunlight, hence the cooling effect.

So you can plant trees locally, reap the cooling effect locally, and also help to cool globally!

Learn more about the relationship between clouds and climate in the High-Adventure Science climate investigation.

http://www.sciencedaily.com/releases/2011/09/110914161729.htm