LONDON - The world will have to spend an extra $500 billion to cut carbon emissions for each year it delays implementing a major assault on global warming, the International Energy Agency said on Tuesday. At United Nations climate talks in Barcelona last week negotiators from developed countries said the world would need an extra six to 12 months to agree a legally binding, global deal to cut carbon emissions beyond a planned December deadline. The IEA, energy adviser to 28 industrialized countries, said the world must act urgently to put greenhouse gases on a track to limit global warming to no more than 2 degrees Celsius.