A week before President Bush travels overseas to meet with European leaders already frustrated with his stance on global warming, a report released yesterday by the U.S. National Academy of Sciences states that global warming is real and getting worse and that human activity is largely responsible for the problem. The report — an evaluation of the work of the U.N. Intergovernmental Panel on Climate Change requested by the White House — will provide ammo to the European leaders and environmentalists, who are calling for mandatory limits on carbon dioxide emissions. As recently as March, Bush said he wasn’t sure global warming was occurring. But yesterday his national security adviser, Condoleezza Rice, said, “This is a president who takes extremely seriously what we do know about climate change, which is essentially that there is warming taking place.”