Serving as a key piece of President Obama's Climate Action Plan, the Clean Power Plan sets goals for reducing the US's GHG emissions 32% by 2030 (which exceeds the US's COP21 NDC target of 26-28%). Under Section 111(d) of the Clean Air Act, the US EPA established interim and final CO2 emissions performance rates for two subcategories of fossil fuel-fired electric generation units: Fossil fuel-fired electric steam generating units (generally, coal- and oil-fired power plants); Natural gas-fired combined cycle generating units To maximize the range of choices available to states in implementing the standards and to utilities in meeting them, the EPA established interim and final statewide goals in three forms: A rate-based state goal measured in pounds per megawatt hour (lb/MWh); A mass-based state goal measured in total short tons of CO2; A mass-based state goal with a new source complement measured in total short tons of CO2. States then develop and implement plans that ensure that the power plants in their state - either individually, together or in combination with other measures - achieve the interim CO2 emissions performance rates over the period of 2022 to 2029 and the final CO2 emission performance rates, rate-based goals or mass-based goals by 2030. Through the best system of emissions reduction available, the rule determined three "Building Blocks" for states to achieve the interim and final emissions reductions targets. These blocks are: Building Block 1: reducing the carbon intensity of electricity generation by improving the heat rate of existing coal-fired power plants; Building Block 2: substituting increased electricity generation from lower-emitting existing natural gas plants for reduced generation from higher-emitting coal fired power plants; Building Block 3: substituting increased electricity generation from new zero-emitting renewable energy sources (like wind and solar) for reduced generation from existing coal-fired power plants. Once the state has developed a plan to reduce emissions and the EPA issues approval, the state has 15 years for full implementation. Within the time frame, there will be three interim goals that the states must meet to ensure compliance with the final emissions reduction goal. On February 9, 2016, the Supreme Court stayed implementation of the Clean Power Plan pending judicial review. On October 10, 2018, EPA proposed to repeal the Clean Power Plan, after completing a thorough review as directed by the Energy Independence Executive Order.