MEGAN THOMPSON:

The Public Trust Doctrine is a legal theory that essentially says, government should hold certain natural resources in trust for the public. It can be traced back to ancient Roman law and English common law.

In the US, it's mostly been used to guarantee public access to waterways, and became part of American case law back in the 1890's in a Supreme Court ruling that private developers in Chicago couldn't prevent public access to Lake Michigan.

But in the 1970's, environmental lawyers began arguing that the Public Trust Doctrine should be extended to other resources, like wildlife or even the air – and that it should compel governments to protect these resources, too. And now, Professor Wood says, it should be extended to protect the atmosphere as well.