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US President Joe Biden's efforts to enforce environmental regulation in the world's second-most polluting country have been dealt a major blow by a Supreme Court ruling on air pollution.
The decision from the currently right-leaning court rules in favour of challengers seeking to curb the power of the Environmental Protection Agency (EPA) to regulate pollution from power plants.
The policy in question, former president Barack Obama's Clean Power Plan, would have paved the way for the transition from fossil fuels to clean energy across the US, which opponents say would do severe economic damage.
But the Supreme Court (SCOTUS) suspended it in 2016, meaning there are no regulations now in force that would reduce carbon emissions from power plants - even though they are responsible for at least a quarter of US greenhouse gas emissions.
SCOTUS, with a 6-3 conservative majority, has been sceptical of the power of federal regulatory agencies like the EPA - which exists within the executive branch - to function without relying on the legislative branch of Congress for approval.
The EPA is the key authority responsible for protecting the public from pollution and other environmental toxins by regulating industry.
The debate epitomises the challenge for Mr Biden, a Democrat, to implement climate policies without the support of legislators in Congress, which is harder to accomplish given partisan divisions, though more binding in the long-run.
His administration set a goal to decarbonise the power sector by 2035, and he came into office with hopes of using the EPA as the main tool to do so.
Last week, the same justices voted to overturn the constitutional right to choose abortion, which has existed for almost 50 years, paving the way for half the country to severely restrict or completely ban the practice.
Getting hamstrung by the court risks the credibility of the US's claim under Mr Biden to be a climate leader.
Speaking to Sky News two weeks ago, EPA chief Michael S Regan said while the case "is very important," the president has "a number of tools in his toolbox" to tackle pollution.
Besides going through Congress, the president could regulate greenhouse gases indirectly via rules on other air pollutants that could speed up closure of older coal-fired plants - though this step could face stiff legal challenges.
"Companies will take a look at that suite of regulations and determine if they want to double down and invest in the past, or if they want to invest in clean energy solutions for the future," Mr Regan said.
West Virginia vs. EPA
Two coal companies and 17 Republican US states - led by West Virginia, a major coal producer - asked the justices to limit the EPA's ability to regulate greenhouse gas emissions from existing coal-and-gas-fired power plants under the landmark Clean Air Act environmental law.
Their challenge came after the EPA under then president Barack Obama in 2015 sought to require power plants to slash greenhouse gas emissions - under a sweeping rule called the Clean Power Plan - mainly by shifting toward clean energy sources like solar and wind.
The states argued the EPA exceeded its authority by trying to regulate carbon dioxide under the Clean Air Act, and by doing so in a way that would have vast economic impacts.
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