Urban ‘Green’ Spaces May Contribute to Global Warming. As it turns out, “green spaces” doesn’t mean pocket parks or wooded areas. It refers to grass. Grass in parks and grass covering athletic fields.
And, although the study – from the University of California Irvine – looked at grass in parks, the conclusions may give pause to lawn-proud homeowners, too:

Dispelling the notion that urban “green” spaces help counteract greenhouse gas emissions, new research has found – in Southern California at least – that total emissions would be lower if lawns did not exist.

Why?It’s not so much the grass — which does remove CO2 from the air and store carbon in the soil — but the care that the lawn needs: applying fertilizer, mowing, irrigation, leaf blowing, etc., all of which produce emissions (four times greater than the amount of carbon stored).
Read the whole article: http://www.csmonitor.com/Environment/Bright-Green/2010/0122/Lawns-may-contribute-to-global-warming