Several years ago, the mayor of Chicago, Illinois, visited Germany, a leader in green roof design. Later he decided to have vegetation planted on Chicago City Hall. Today, the tops of many other buildings also look like parks. In Canada, the Coast Plaza Hotel in Vancouver has a forest on its roof. New York City has an unusual example farm for hydroponics. The farm is on a boat called the Science Barge. Rainwater and river water are used to grow lettuce, tomatoes, cucumbers and peppers. The farm shows the possibilities for designing rooftop systems. Green roofs are designed to save energy and capture rainwater. Rooftops covered with plants help keep buildings cool. They can extend the life of a roof. And they can reduce water running off into streets and storming water systems. Experts say green roofs usually do not get much above 27degC. So temperatures might not be a problem for growing. But rooftop gardeners have to be careful not to put too much weight on the roof. This is true whether plants are grown in soil or water. Earlier in 2008, researchers in Texas reported that green roofs can reduce a building's air conditioning costs by about onefifth compared to blacktop roofs.  But they also reported that not all green roofs perform equally well. They found that designs differ in their ability to keep buildings cool and to capture rainwater. The researchers put experimental roofs on top of metal insulated   boxes, to recreate green roof conditions. The study compared products from six companies to help businesses understand how to improve their designs. Each rooftop had sixteen different kinds of plants native to Texas. The researchers say the presence of native plants likely helped all the green roofs capture water better compared to sedums. Sedums are plants that need little water and often are used on green roofs. The native plants could take in more water and release more of it to the atmosphere. The advantages of green roofs are mentioned in the passage EXCEPT   _  .
changing the local weather prolonging the roof's life keeping the house cool reducing the house's air conditioning costs