WASHINGTON D.C. — The United States has called for urgent action to combat climate change, saying in a report that human-caused warming is already having a serious impact across the country.The report released is a four-year study by leading scientists warning of the risks of rising sea levels, droughts, fires and pest outbreaks if the world does not tackle the repercussions of greenhouse gas emissions.In a televised interview shortly after the report was published, President Barack Obama emphasised the need for action to curb carbon pollution before it is too late.