After years as a holdout, Pennsylvania is poised to join the Regional Greenhouse Gas Initiative and implement a cap-and-trade program for carbon dioxide emissions from power plants.

Gov. Tom Wolf signed an executive order Thursday directing the Department of Environmental Protection to join the initiative as a step to combat climate change; regulators have through July 2020 to develop proposed rules for Pennsylvania’s sign-on.

Pennsylvania was one of two states among the 11 in the Northeast and Mid-Atlantic that has never joined RGGI, and environmentalists have long called on the legislature to sign on.

» READ MORE: Here’s what Pennsylvania is doing to address climate change — and why politics is making it difficult

“Given the urgency of the climate crisis facing Pennsylvania and the entire planet, the commonwealth must continue to take concrete, economically sound and immediate steps to reduce emissions," Wolf said in a statement. “Joining RGGI will give us that opportunity to better protect the health and safety of our citizens.”

Wolf used authority under the Air Pollution Control Act to order the step, but House Republicans on Thursday called it illegal. Their dispute over whether the air-pollution law gives the governor proper authority could set the state up for a court fight if the DEP moves ahead.

Just two weeks ago, Wolf’s office said that he wanted to do RGGI with the legislature and that he was waiting to hear whether lawmakers would support it before looking at other options.

“Gov. Wolf clearly does not have the authority to take this reckless action, which will cost average Pennsylvanians more of their hard-earned money through their energy bills, without legislative approval," said Rep. Daryl Metcalfe (R., Butler), chair of the House committee on energy and environment.

Joining the initiative, known as RGGI, will mean the Keystone State will make a carbon dioxide budget that limits emissions. After the state sets a cap on carbon dioxide emissions from electric power generators, power plants go to auctions to purchase allowances for their emissions.

Connecticut, Delaware, Maine, Maryland, Massachusetts, New Hampshire, New York, Rhode Island, and Vermont are members of the initiative. Pennsylvania and Virginia are not members; New Jersey, which was a founding member but pulled out during Gov. Chris Christie’s administration, is in the process of rejoining.

Pennsylvania produces 0.5% of all global carbon emissions, and the largest share of the state’s emissions come from the energy sector. DEP Secretary Patrick McDonnell said the state has a “significant opportunity” to reduce emissions through RGGI.

The RGGI states reported in 2015 that they had reduced power-sector carbon dioxide pollution by 45% since 2005. The region’s economy has grown and in the first six years, RGGI investments returned $2.31 billion in lifetime energy-bill savings to more than 161,000 households and 6,000 businesses that participated in programs funded by RGGI proceeds, the group reported.

Environmental advocates praised Wolf’s step.

“RGGI has a proven track record of success for limiting carbon emissions from participating states’ power sectors and we are excited that Pennsylvania will be joining its ranks," said a statement from the advocacy group Clean Power PA Coalition. “We must begin the transition away from fossil fuel energy here in Pennsylvania right now, and we must do so in a manner that protects Pennsylvania ratepayers and ensures a just transition for workers.”