Updated Sept. 13 with Connecticut announcement.

New York, Maryland and Connecticut all announced plans this week for statewide phase-outs of hydrofluorocarbons, or HFCs, potent greenhouse gases used in air conditioners and refrigeration systems.

Their moves, following a similar step earlier this year by California, will help fill a breach in climate policy created by the federal government. New York's plan, the most advanced of the three, closely follows a 2015 Obama-era regulation that was partially struck down by a federal court and that the Trump administration announced last April it would no longer enforce at all.

"While the Trump administration denies climate change and rolls back efforts to protect our planet, New York is picking up the mantle of climate leadership and forging a path forward," Gov. Andrew Cuomo said on Monday in announcing his state's plan. "In New York, we believe denial is not a life strategy, and we will continue to fight climate change to protect our economy, our planet and our future."

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HFCs are short-lived climate pollutants that are thousands of times more potent than carbon dioxide at trapping heat in the atmosphere, though they have a shorter life span.

The New York regulations would phase out HFCs in new and retrofitted equipment, including commercial and residential refrigerators, air-conditioning equipment and light-duty vehicle air-conditioning systems, by 2024. Once finalized, expected next year, the regulations would be a crucial component of the state's climate policy.

Maryland Environment Secretary Ben Grumbles announced his state's plans on Tuesday ahead of the Global Climate Action Summit in San Francisco. He said Maryland plans to develop regulations to phase out the use of the certain HFCs in foam products and refrigeration. "These fast-acting super-pollutants are a major threat to our climate progress," he said.

Connecticut Gov. Dannel Malloy followed by announcing at the summit on Thursday that he had directed his state Department of Energy and Environmental Protection to develop HFCs regulations. "If the federal government will not act to mitigate the impacts of climate change, it is incumbent upon states to act to protect the one planet that we have," he said.

HFCs: Low-Hanging Fruit for Climate Action

Environmental advocates praised the decisions.

"This is an important step of leadership," David Doniger of the Natural Resources Defense Council said. "HFCs are an important climate pollutant and a low-hanging fruit, there is easy stuff to do there."

A federal rule finalized by the EPA in 2015 would have reduced HFC emissions nationwide by as much as the equivalent of 72 million metric tons of carbon dioxide by 2025, a reduction equal to the pollution of 21 coal-fired power plants.

That rule was struck down by a federal appeals court in August 2017 after it was challenged by foreign HFCs manufacturers who argued the EPA had no authority to regulate the gas under the Clean Air Act. Major U.S. chemical companies that manufacture alternatives less damaging to the atmosphere had supported the rule, but the Trump administration has not sought other ways to limit HFC emissions, creating a void now being filled by states, said Jason Anderson of ClimateWorks Foundation, an advocacy group.

Could California + New York Shift the Industry?

In addition to New York, Maryland and Connecticut, California adopted regulations earlier this year that ban HFCs in new air-conditioning and refrigeration.

Bans in two of the most populous states could push manufacturers to phase out HFCs across the U.S. market, Anderson said. "I suspect this might tip the balance in favor of a standardization even without regulation in other states," he said.

Phasing out HFCs nationwide would play a key role in achieving greenhouse gas reduction goals set by the United States in the Paris climate agreement, as well as the Kigali Amendment to the Montreal Protocol, which seeks to eliminate the use of HFCs worldwide.

"The U.S. and Europe, which are on a more accelerated phased-down schedule compared to developing countries, really need to be taking this kind of step to meet that calendar," Anderson said. "Developing countries have a bit of a delay in their requirements for implementation, but it's much easier to meet those requirements if they see it being done elsewhere like in Europe and North America."