Under RGGI, sources are required to possess CO2 allowances equal to their CO2 emissions over a three-year control period. A CO2 allowance represents a limited authorization to emit one ton of CO2. The first three-year control period took effect on January 1, 2009 and extended through December 31, 2011. The second three-year control period took effect on January 1, 2012 and extended through December 31, 2014. The third three-year period took effect on January 1, 2015 and extends through December 31, 2017.