Big day indeed.
Some points I liked:
1. Justices said that the Clean Air Act, one of America's central environmental protection laws, specifically barred considerations of cost.
2. The court ruled that the EPA cannot sidestep its authority to regulate the greenhouse gases that contribute to global climate change unless it can provide a scientific basis for its refusal.
3. This decision is a rebuke to the Bush administration, which has maintained that it does not have the right to regulate carbon dioxide and other greenhouse gases under the Clean Air Act, and even if it did, it would not use the authority. The ruling does not force the Environmental Protection Agency to regulate auto emissions, but it would almost certainly face further legal action if it fails to do so.