Many of the 45,000 big dams that have been built around the world cost too much and have had negative impacts on the environment and the poor, according to a report released yesterday by the World Commission on Dams. The commission, which was set up in 1998 by the World Bank and World Conservation Union, also questioned the claim that all hydropower dams provide electricity without increasing greenhouse gas emissions, noting that reservoirs created by dams have drowned forests and released carbon dioxide that had been stored in trees. About half of the world’s wetlands have been lost to dams, the report says, and from 40 million to 80 million people have been displaced by dams. Dam critics are calling for big funders, such as the World Bank and export credit agencies, to halt support for all new dams unless the projects follow the report’s recommendations, which include involving locals in the planning stage and weighing environmental concerns much more heavily.