In 2013, US CO2 emissions increased by 2.5%. Emissions are a function of four variables: population, GDP/person, energy intensity and CO2 intensity. Population growth boosted emissions by 0.7%, rising GDP/person boosted emissions by 1.5%, energy intensity surprisingly rose by 0.5% due to the Polar Vortex and CO2 intensity fell by just 0.3%, less than usual, because the share of electricity coming from coal rose as natural gas prices increased.