Cutting U.S. greenhouse gas emissions in half is doable but hard, experts say, and some of the biggest barriers are political, not technical.President Joe Biden on Thursday FILE – A wind turbine is pictured, Jan. 13, 2021, near Spearville, Kan.U.S. emissions are declining, but far too slowly to reach Biden’s target. They would have to fall on a scale that has happened only three times since 2005, Rossetti noted, and not for good reasons — during the COVID-19 pandemic, the 2008-09 financial crisis and during an exceptionally mild winter in …