President Bush and Canadian Prime Minister Brian Mulroney signed an accord Wednesday pledging their governments to cooperative efforts to sharply reduce pollutants that cause acid rain.

"Pollution is never stopped by a line on the map," Bush said at the signing ceremony on Parliament Hill. "Progress begins when we act on the understanding that transboundary pollution requires cooperative global stewardship among all nations."Under the agreement, the United States would reduce emissions of sulfur dioxide and nitrogen oxides, which react with moisture and become acid rain. Acid rain damages lakes and forests, kills fish and is a suspected health hazard. The accord also requires Canada to adopt a permanent national emissions cap of 3.2 million tons of sulfur dioxide by the year 2000.

The agreement, under negotiation since 1987, sets a timetable and targets for reducing emissions of those pollutants. Coal-burning and high-sulfur oil power plants are a principal source of sulfur dioxide; automobile exhaust is a chief source of nitrogen oxides.

That act would cut sulfur dioxide emissions by 50 percent to 8.9 million tons a year by the year 2000. Canadian legislation passed in 1985 also is expected to halve emissions north of the border, to 2.3 million tons by 1994.

Mulroney said the agreement "corrects many errors of the past." Canada has said that marine life in as many as 14,000 lakes is in jeopardy from U.S.-caused acid rain.

About 20 million tons of sulfur dioxide are emitted annually in the United States, mostly from power plants that burn coal or high-sulfur oil, the Environmental Protection Agency says.