The nine states require power plants to buy permits to pollute. Over time, these permits decline in quantity. The idea of such cap-and-trade systems is to put a price on emissions, giving utilities an incentive to figure out ways to reduce emissions. Utilities can trade permits with one another. Since it began, the initiative has raised $2.7 billion, which the states have invested in energy efficiency, in helping low-income people pay electricity bills and in renewable sources of power like wind and solar. (The other R.G.G.I. states are Delaware, Maine, Maryland, New Hampshire, Rhode Island and Vermont.)

Even as emissions have come down, electricity rates have fallen by an average of 3.4 percent in the nine states, according to the Acadia Center, an energy research and advocacy organization. And the economies of the nine states have grown faster than the economy of the rest of the country. The program has had the added benefit of reducing ground-level pollution that causes respiratory illnesses and other diseases, providing nearly $6 billion in public health benefits, according to Abt Associates, a research company.

All this serves to chastise Mr. Christie, who pulled New Jersey out of the program in 2011, complaining about its costs and saying it had “no discernible or measurable impact upon our environment.” Mr. Christie’s tenure will soon be history, and the Republican and Democratic candidates to replace him say they want New Jersey to rejoin the initiative. Virginia may also decide to participate if the Democratic candidate for governor wins this November.

Other states in the area, like Pennsylvania, ought to consider joining, as well. And given the potential for interstate trading of permits, R.G.G.I. might over time connect to California’s recently strengthened cap-and-trade program. The R.G.G.I. agreement will be the subject of a public meeting in Baltimore on Sept. 25, after which states will incorporate it into their laws and regulations.

As important as it is, R.G.G.I., which applies only to power plants, is but one of several steps states can take to achieve even more dramatic reductions. New York, Massachusetts and several other R.G.G.I. states have laws (or have issued executive orders) that commit them, on paper, to reducing all greenhouse gas emissions 80 percent from 1990 levels by 2050. To meet ambitious targets like these, the states will need to reduce emissions from cars and trucks while ramping up solar and wind power to produce the electricity required by hybrid and electric vehicles and new mass transit fleets. (Nationally, the transportation sector is now a bigger source of climate pollution than power plants, according to the Energy Information Administration.)