Originally published in The Revelator. Reprinted under Creative Commons license CC BY-NC-ND 3.0.

For five decades California and the federal government have worked together in an innovative exercise in federalism aimed at achieving cleaner air. California has played an important role in controlling greenhouse gas emissions that contribute to climate change, particularly from motor vehicles.

But now, contrary to law and in a massive departure from past practice, President Donald Trump has announced that his administration is pulling the rug out from under California's feet by divesting it of its longstanding authority to adopt auto emission controls more stringent than the Environmental Protection Agency's.

The action, implemented jointly by the EPA and the National Highway Traffic Administration, couldn't come at a worse time. Less than a year ago, the Intergovernmental Panel on Climate Change called "ambitious mitigation actions" indispensable to limiting warming to 1.5 degrees Celsius and avoiding the most disruptive and potentially irreversible effects of climate change.

California, supported by more than 20 other states, is already challenging the administration in court. But if the administration's effort succeeds, the world will be deprived of California's vital leadership, and the Trump administration will have shelved another important tool …