SANTA MONICA, Calif., March 21, 2000 -- Not enough work in Hollywood these days, apparently. How else to explain the celebrity triumvirate currently on the stump for causes ranging from global warming to nukes to circus elephants. Leonardo DiCaprio On the global-warming front is Leonardo DiCaprio. The "Titanic" star (and "Beach" washout) issued a statement today calling for governments to give up fossil fuels in order to save the world from, you know, getting too stuffy and stuff. "There is a global emergency going on," DiCaprio said in a statement issued by the U.S. environmental group EarthAction. "Earth is heating up, and everyone's future is at risk." On the nuclear-weapons front is Michael Douglas. In a speech before British lawmakers in London on Monday, the "China Syndrome" star called on the United States and Russia to thin out their stockpile of bombs. Doug