A ruling by the US government that greenhouse gases threaten human health has raised hopes that further progress can be made at the Copenhagen climate summit.The White House has already pledged to cut its CO2 emissions by 17 percent by 2020 based on 2005 levels. But news that America’s Environmental Protection Agency could order carbon cuts without the approval of congress is being seen as a major breakthrough. ‘‘Today’s action is a step towards enduring, pragmatic solutions to the enormous challenge of climate change. It is a step toward innovation, investment and implementation of technologies that reduce harmful emissions,’‘ said Lisa Jackson from the US Environmental Protection Agency. Hugely reliant on coal for its energy needs, after China, the US is the world’s biggest emitter of CO2. Around 50 percent of all America’s electricity comes from the fossil fuel. Some in Congress believe the move will be detrimental to the economy. US Republican senator for Oklahoma James Inhofe said: ‘‘It would have a devastating effect on employment, it would cost in taxes millions and billions and in terms of jobs probably millions of jobs.’‘ The new ruling applies to six gases scientists say contribute to global warming. Many believe it will strengthen President Obama’s hand for his visit to the Danish capital later this month.