The state’s Air Resources Board must tackle this crucial issue. Later this week, the board is expected to approve a framework for regulating emissions to 2030, and so far no fix for the permit surplus is included. Getting it right is critical, because the program is expected to account for nearly 40 percent of emissions reductions under the new framework.

Fortunately, the fix is straightforward. The Air Resources Board simply needs to adjust future emissions limits downward to take account of the surplus permits already in the marketplace. A consortium of states in the Northeast, which has a more limited cap-and-trade program that applies only to the power industry, took exactly that approach in 2014 when it faced a similar oversupply of permits.

But, you ask, won’t tightening the limits offend those companies and investors who, like dutiful Costco shoppers, had the good sense to buy the permits when they were cheap?

No. The minute the board adjusts the limits, the price of permits is likely to rise as demand for them increases. It will be like somebody telling you the paper towels stacked in your garage are worth more than you paid for them.

It is true, however, that companies expecting to have to buy a lot of future permits will not like the change. The oil industry, in particular, has savagely fought California’s emissions limits and is likely to warn that tighter emissions limits in the 2020s will raise the price of gasoline.

If so, the effect is likely to be no more than a few cents a gallon. (The main factor determining the cost of gas is the price of oil in global markets.) And by the 2020s, dozens of electric car models will be on the market, and gasoline cars will get better mileage under rules that were also pioneered in California and later adopted by the Obama administration.

The Air Resources Board must take on the risk posed by this oversupply of permits, sooner rather than later. Governor Brown has only a year left in office. We simply do not know if his successor will be as intelligent about the climate problem, or as committed to action, as he has been.

California has been a world leader in reducing greenhouse emissions, and the governor is rightly treated as a climate hero on the global stage. But that record will be at risk if he leaves office without tackling this issue — potentially tarnishing not just his legacy but also that of the eight Republican lawmakers who were brave enough to defy their party and vote for rational action to save the planet.