Renewable Portfolio Standards
研究了美国各州可再生能源组合标准（RPS）对可再生能源发展和温室气体排放的实际影响，发现效果虽存在但很小，与政策宣称的目标不符。
State-level renewable portfolio standards (RPSs) aim to encourage renewable energy and discourage greenhouse gas (GHG) emissions from the electric power sector in the United States. Do they work? Some prominent government agencies and advocacy groups assert that U.S. renewables growth has been largely due to RPSs. That seems unlikely, given that in most regions, renewables exceed RPS requirements. But it is not an easy question to answer, thanks to interstate trading and the possibility that states with abundant renewable resources might set the most ambitious RPS goals. We combine the best features of four recent academic studies, using ordinary least-squares and instrumental variables approaches. In some specifications, RPSs do appear to reduce the use of natural gas to generate electricity and decrease GHG emissions, while boosting the use of wind and solar power. But the effects are small—consistent with the academic findings and in contrast to the public claims and policy goals.