Donald Trump has just gone for the jugular of climate legislation.
The US president has scrapped a scientific finding by the US Environmental Protection Agency (EPA) that greenhouse gas emissions endanger human health - the so-called "endangerment finding".
If you want to gut federal climate action in the US, this is the big one to go for, because it is the legal basis for many other rules.
Accepting that greenhouse gas emissions harm health - and scientists are clear they do, by way of air pollution, and extreme weather like heavy flooding - is what has justified subsequent rules to clean up the sources of those emissions.
He has also repealed rules on emissions standards for all vehicles and engines.
Transport is the biggest polluting sector in the US, famously a nation of drivers.
If US transport was a country, it would be the sixth biggest polluter in the world.
That's bigger than the entire economies of Brazil or Indonesia.
The big motivation for these changes is to cut costs. The White House stresses that these moves will save carmakers about $2,400 (£1,754) a car, and the economy $1.3trn (£954bn) a year.
Environmentalists dispute these figures, saying that if cars are less fuel efficient, petrol bills will go up, and so will healthcare bills for asthma sufferers, or households cleaning up wildfire or flood damage.
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But the news has been welcomed by the fossil fuel industry, and Republicans who say the rules went too far by regulating emissions in a manner that side-stepped approval by Congress.
Climate policies in the US always yo-yo under the Democrats and Republicans, the latter being far more sceptical.
Environmentalists are now planning to challenge these rollbacks in court.
But if they go through, it will make it much harder for anyone to ever bring the limits on climate pollution back in again. .
(c) Sky News 2026: Trump 'guts climate laws' by abandoning acceptance that greenhouse gases endanger health
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