In 2015, the Obama administration’s Environmental Protection Agency (EPA) issued its Clean Power Plan (CPP) that prescribed detailed regulations for the control of carbon dioxide emissions from existing coal-powered power plants as part of its effort to control climate change. Earlier this month, the Trump EPA under Scott Pruitt issued its own proposed rule to undo the Obama administration’s guidelines without a commitment to replace them with a substitute set of rules dedicated to the same end. In response to Pruitt’s major shift in policy direction, states like Massachusetts and New York are suing to prevent the new legal regime from going into effect.

Pruitt’s reversal in environmental policy raises two issues—one scientific and one legal. The scientific issue revolves around the 2009 endangerment findings from an Obama administration study, which determined that carbon dioxide emissions are a pollutant whose emissions levels must be regulated under the Clean Air Act (CAA) because “greenhouse gases in the atmosphere may reasonably be anticipated both to endanger public health and to endanger public welfare.” Other chemicals on the list of six designated pollutants—like methane and nitrous oxide, with known toxicities—surely deserve that designation, but the Obama report overstates the risks of carbon dioxide to the environment.

Although that report acknowledges that the relevant science is evolving, it does not recognize that new information about climate change could weaken the case for regulating carbon dioxide. For example, the 2009 report assumes that modest increases in temperature are likely to create dangers to crops by shortening the growing season. But more careful studies since that time have shown that the increase in carbon dioxide has resulted, as Matt Ridley reports, in a dramatic increase in the greenery on the earth’s surface of about 14 percent over the last 30 years. This far outpaces any supposed harm that might come, as the EPA report suggests, from “weed and pest growth,” which are best controlled by specific technologies and not by top-down policies addressing climate change generally.