On 23 June the US Supreme Court partially upheld and partially overturned federal rules regulating greenhouse gases emitted from new and modified major stationary sources as defined under the Clean Air Act. As a result, both sides are claiming victory.
As adopted by the US Environmental Protection Agency, these federal rules required new and modified major sources of GHG emissions to obtain Prevention of Significant Deterioration (PSD) and Title V operating permits and to implement emission limitations for all emitted pollutants including GHGs. 
The challenge was mounted by the Utility Air Regulatory Group, a coalition of power companies, and this is the latest battle in a conflict that started seven years ago. In 2007, the US Supreme Court ruled (Massachusetts v EPA) that the Environmental Protection Agency has the authority to regulate greenhouse gas emissions, if it determined that these emissions endangered public health. Following that, in 2010, the EPA introduced a new set of regulations designed to control carbon dioxide emissions from light and heavy vehicles as well as generators and industrial and utility sources. A coalition of power companies challenged the legality of the regulations, arguing that the science used by the EPA in deciding the regulations was inaccurate.
In 2012, a three-judge panel of the D.C. Circuit court unanimously rejected the challenges, but in 2013 the case was accepted for review by the US Supreme Court.
On 23 June the US Supreme Court announced its decision in the case. Its judgment was that the earlier decision was affirmed in part, and reversed in part. Writing for different majorities among the five Justices of the court, on the two key issues in the case, Justice Scalia rejected EPA’s broadest interpretation of its authority, but largely kept intact the practical effect of GHG regulation under the PSD programme by upholding EPA’s authority to require GHG emission limitations based on BACT for most covered sources.
The court concluded that the Clean Air Act neither compels nor permits the Environmental Protection Agency to adopt an interpretation of the Clean Air Act requiring a stationary source of pollution to obtain a PSD or Title V permit on the sole basis of its potential greenhouse-gas emission: however, it judged that the EPA had reasonably interpreted the Clean Air Act to require sources that would need permits based on their emission of chemical pollutants to comply with ‘best available control technology’ (BACT) for greenhouse gases.
Justice Scalia delivered an opinion which included the following key sentences:
"The power of executing the laws necessarily includes both authority and responsibility to resolve some questions left open by Congress that arise during the law’s administration. But it does not include a power to revise clear statutory terms that turn out not to work in practice."
"An agency has no power to ‘tailor’ legislation to bureaucratic policy goals by rewriting unambiguous statutory terms."
The two primary holdings in the opinion each relied on a different collection of Justices to reach a majority.  First, a 5-4 majority of the court determined that an increase in emissions of GHGs at a stationary source not already subject to PSD and Title V permit requirements could not on its own trigger those requirements.  However, a separate majority held that once a source becomes subject to PSD  permitting requirements by virtue of emissions of other pollutants (such as criteria pollutants), EPA may require BACT for any non-trivial GHG emissions from that source. 
Analysts van ness Feldman commented: ‘This decision will have limited practical impact on the scope of GHG emissions covered under the PSD programme, and it has no direct impact on EPA’s recent proposed rules setting emission standards for new and existing power plants under section 111 of the Clean Air Act.  However, the reasoning in this case could support an argument for limiting the extent to which EPA may claim authority to regulate GHG emissions through new paradigms and nontraditional measures for reducing GHG emissions under section 111 of the Clean Air Act.’

For a full discussion of the effect of the Ruling read the Van Ness Feldman analysis, http://www.vnf.com/2991