When California’s Gov. Jerry Brown signed a 10-year extension of the state’s cap-and-trade program this summer, it was heralded as a rebuke of President Trump, who had just announced he would withdraw the U.S. from the Paris Climate Accord. While the nation was failing on climate change, the story went, states could succeed. The trouble is that California could leak—like a sieve.

In the decade since Mr. Brown’s predecessor, Gov. Arnold Schwarzenegger, first signed the Global Warming Solutions Act, the cap-and-trade program...