The EPA is challenged in the Supreme Court over greenhouse gas regulations

On December 7, 2009, the Administrator signed two distinct findings regarding greenhouse gases under section 202(a) of the Clean Air Act:
•Endangerment Finding: The Administrator finds that the current and projected concentrations of the six key well-mixed greenhouse gases — carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6) — in the atmosphere threaten the public health and welfare of current and future generations.
•Cause or Contribute Finding: The Administrator finds that the combined emissions of these well-mixed greenhouse gases from new motor vehicles and new motor vehicle engines contribute to the greenhouse gas pollution which threatens public health and welfare.
These findings do not themselves impose any requirements on industry or other entities. However, this action was a prerequisite for implementing greenhouse gas emissions standards for vehicles. In collaboration with the National Highway Traffic Safety Administration, EPA finalized emission standards for light-duty vehicles (2012-2016 model years) in May of 2010 and heavy-duty vehicles (2014-2018 model years) in August of 2011.

http://www.epa.gov/climatechange/endangerment/
Signed by the Administrator Dec. 7, 2009 ( a date that will live in Infamy)
Final rule published in the Federal Register, Dec. 15, 2009
Rule Effective Jan. 14, 2010.
I think the one of the most vulnerable aspects of the EPA rule is that the Clean Air Act specifies that emissions above 100 and 250 Tons per year for pollutants be regulated. This level, specified by Congress in the law, is an absurdly low figure in regards to CO2. So the EPA is “tailoring” the rule, i.e. rewriting the CAA requirements, to allow them to regulate the big stuff, and gradually tighten the noose.
Thanks to Justice Roberts, what words are used and not used in laws, is increasingly becoming irrelevant these days.

Existing Clean Air Act statutory thresholds for criteria pollutants are 100 and 250 tons per year (TPY). If GHG emissions were subject to those thresholds, then, according to EPA estimates, 82,000 PSD permits would be required each year and 6 million facilities would require Title V operating permits. Many commercial establishments, apartment buildings, hospitals and schools could find themselves subject to EPA regulation under current thresholds. Thus, on the basis of administrative necessity, EPA is “tailoring” its requirements to reach the largest GHG emitters first.
Under Step 1 of the “tailoring” rule (January 2-June 30, 2011), sources already subject to PSD requirements would be required to implement GHG BACT requirements if a modification results in an emissions increase of 75,000 tpy or more of GHGs. Under Step 2 (July 1, 2011 to June 30, 2013), PSD permitting requirements will cover new construction projects that emit GHG emissions of at least 100,000 tpy even if they do not exceed the permitting thresholds for any other pollutant. Modifications of existing facilities that increase GHG emissions by at least 75,000 tpy will be subject to permitting requirements even if they do not significantly increase emissions of any other pollutant…….
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The tailoring rule also commits EPA to conduct another rulemaking that will determine whether certain smaller sources can be permanently excluded from permitting. EPA will not require permits for smaller sources until at least April 30, 2016.
Current Status
On June 26th, 2012, the U.S. Court of Appeals for the D.C. Circuit unanimously upheld EPA’s authority to set emissions limits for GHGs. The Court affirmed EPA’s Endangerment Finding, Mobile Source rules and upheld the Tailoring Rule on the grounds that plaintiffs lacked standing. While this litigation was pending, EPA initiated a number of steps necessary to regulate GHGs.
(Source: http://businessroundtable.org/studies-and-reports/regulation-of-greenhouse-gas-emissions-under-the-clean-air-act/)