The Regional Greenhouse Gas Initiative, which caps power plant carbon emissions in participating states, is expected to expand to New Jersey and Virginia.
Currently, all New England states ar members of the program, along with New York, Maryland and Delaware.
The 2017 carbon emissions cap for RGGI states is about 84 million tons a year. That cap gets reduced by 2.5 percent each year until 2020.
Since the program's start in 2009, carbon emissions have gone down in RGGI states by about 40 percent.
State regulators in Virginia have been taking steps since November to develop a plan to join the initiative and cap emissions for most power plants starting in 2020. New Jersey, which was originally a member of the program but pulled out in 2011, is expected to rejoin early next year.
Jordan Stutt, policy analyst at the Acadia Center, a clean energy advocacy organization, said more states joining RGGI would make the program better.
"(RGGI would be) more powerful in the sense that we would be reducing more emissions; we would be accelerating the transmission to a clean energy economy in a broader area," Stutt said. "When you look at a bigger region with similar policy, you’re better able to achieve that policy goal."
This fall, RGGI states agreed to reduce carbon emissions by another 30 percent by 2030, relative to 2020 emission levels. That's equivalent to one year's worth of pollution from more than 25 million cars, according to the Natural Resources Defense Council.
Starting in 2021, RGGI states will cap emissions at 75 million tons. That cap will decrease by three percent each year until 2031.