The United States is not part of the Kyoto Protocol.But since 2005 more than 1,000 mayors across the country have agreed to sign their own version of the protocol. It is called the United States Conference of Mayors' Climate Protection Agreement.Local leaders have agreed to follow the suggestions of the Kyoto Protocol in their communities.They have also agreed to urge state and federal governments to follow these suggestions.One goal is to reduce air pollution to the 1990 levels by 2012. A "green" city might work on several kinds of environmental goals and programs.These include air quality,reducing electricity use, green building, public health, the reuse of materials, water quality and clean transportation. The Natural Resources Defense Council works to protect the environment through action, law and science.One of its online projects is called Smarter Cities, which lists some cities that have made important environmental steps. Washington,D.C.is also a leading city for green roofs,with over 92,000 square meters of green rooftops.The city also launched a program called "Skip the Bag,Save the River."Stores charge people a fivecent  tax for using plastic or paper shopping bags instead of reusable bags.Money from the tax will be used to clean up a local river. New York City is using water as a renewable energy source.For several years, the city has experimented with water in the East River to create energy.Officials operating the program recently asked for permission to put into place 30 river turbine devices. The people of Portland, Oregon are among the top recyclers in the nation.People living there recycle over half of the waste they throw out.The city has also worked to provide green transportation by providing safe bike paths and free parking spaces where electric cars can get recharged. In Oakland, California, you can ride on one of several public hydrogenpowered  buses.These buses release zero pollution into the air.However,they cost about five times more than common buses. One obvious disadvantage of adopting hydrogenpowered buses is  _  .
its high cost side effect zero pollution large space