Associated Press: Ditching its past cautious tone, the nation’s top scientists urged the government Wednesday to take drastic action to raise the cost of using coal and oil to slow global warming. The National Academy of Sciences specifically called for a carbon tax on fossil fuels or a cap-and-trade system for curbing greenhouse gas emissions, calling global warming an urgent threat. The academy, which advises the government on scientific matters, said the nation needs to cut the pollution that …