Seven of the world’s major corporations committed themselves yesterday to making significant reductions in their greenhouse gas emissions, a voluntary effort to combat climate change ahead of any government requirements. Working in partnership with the nonprofit Environmental Defense, the companies — DuPont, BP, Royal Dutch/Shell Group, Suncor Energy, Ontario Power Generation, Alcan Aluminum, and Pechiney SA — pledged to reduce their combined emissions, estimated at 360 million metric tons in 1990, to 280 million metric tons by 2010. The companies will adopt more efficient technologies and trade emission reduction credits amongst themselves in order to meet their goals. In an unrelated agreement, Polaroid announced last week that it will cut its carbon dioxide emissions 25 percent from 1994 levels by 2010, similar to commitments made earlier this year by Johnson & Johnson and IBM, part of a program sponsored by the World Wildlife Fund and the Center for Energy and Climate Solutions.