A new Internet tool to help individuals and communities curb their role in adding global-warming carbon emissions will be announced today at a conference in New York of mayors from around the world, said a person who built the Web technology. Many environmental groups offer simple carbon calculators on the Web, which allow people to figure the carbon dioxide production from daily routines like driving a car or lighting a house. “But this is serious software, serious quantitative methods and social networking technology brought to the green world,” said Ron Dembo, the chief executive of Zerofootprint, a nonprofit group that...