In recent years, plaintiffs’ attorneys and public-interest groups have brought common law actions seeking injunctive relief or damages for air emissions they claim cause climate change. Because climate change is a global phenomenon, these actions have targeted both in-state and out-of-state sources. Does state common law reach this far?

A state’s common law is founded in its police powers, which are among the powers that the Constitution generally reserved to the states. By contrast, the Constitution specifically delegates to Congress the power to regulate interstate commerce. A state’s police powers therefore do not extend beyond its borders. For this reason, the Supreme Court in the last century discovered a limited “federal” common law to address interstate pollution at a time when there were no federal laws regulating such interstate concerns. Missouri v. Illinois, 180 U.S. 208, 241 (1901). As the Court observed, “[i]f state law can be applied, there is no need for federal common law; if federal common law exists, it is because state law cannot be used.” City of Milwaukee v. Illinois, 451 U.S. 304, 314 n.7 (1981) (Milwaukee II).

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