The U.S. aims to cut greenhouse gas emissions in half by 2030 as part of its new commitment to the Paris climate agreement. U.S. emissions of heat-trapping greenhouse gases plunged last year, but that was an anomaly owing to the coronavirus pandemic, which put the brakes on large segments of the economy. As the country rebounds, emissions are expected to rise once again, and the Biden administration is racing to find ways to put the U.S. on track to meet even more ambitious targets that scientists say are needed to avert the worst effects of global warming.