President Obama has vowed to chop US greenhouse gas emissions in a series of stages, starting with a 17 percent cut by 2020.The White House announcement comes ahead of December’s climate change talks in Copenhagen. Starting from levels measured in 2005, US officials say after 2020 that target to slice man-made CO2 levels will be raised from 17 percent to about a third by 2025. Further cuts are expected by 2030. Obama confirmed he’d join some other 64 world leaders for the Copenhagen summit. News the US is prepared to give clear targets will raise hopes that a plan to save the planet can be found.