Tired of waiting for the U.S. federal government to act, New England governors, including three Republicans, joined yesterday with Eastern Canadian premiers to vow to cut the region’s greenhouse gas emissions to 1990 levels by 2010, and 10 percent below that by 2020. No regional or international agency exists to enforce the deal, but the pact outlines in some detail how the goals could be met. The governors and premiers also agreed to reduce mercury emissions from power plants and incinerators by 75 percent by 2010. Enviros accused the Canadian coal industry of watering down the goal, which had been to eliminate mercury emissions by 2010.