WASHINGTON (AP) — President Barack Obama is calling on the world to end massive government subsidies that encourage the use of fossil fuels blamed for global warming. The president, who is set to host the G-20 economic summit opening Thursday in Pittsburgh, will propose a gradual elimination, with the time frame to be determined, according to White House officials.”Later this week, I will work with my colleagues at the G-20 to phase out fossil fuel subsidies so that we can better address our climate challenge,” Obama said Tuesday at the United Nations global warming summit.