In 2007, the Supreme Court rendered one of its most important environmental decisions. The case, Massachusetts v. EPA , was a challenge to the denial by the Environmental Protection Agency (EPA) of a petition asking it to take two actions—(a) find under the Clean Air Act (CAA) that greenhouse gases (GHGs) emitted from new motor vehicles "cause, or contribute to, air pollution which may reasonably be anticipated to endanger public health or welfare," through their climate change effects, then (b) issue standards for those GHG emissions. EPA's petition denial was based in part on its claim that it lacked authority to regulate GHGs. To the contrary, said the Supreme Court by 5-4, GHGs constitute "air pollutants" under the CAA, hence EPA does indeed have the authority to regulate GHG emissions. The Court gave EPA three options: (a) determine that GHG emissions from new motor vehicles "cause, or contribute to, air pollution which may reasonably be anticipated to endanger public health or welfare," then promulgate standards limiting those emissions; (b) determine that such GHG emissions do not cause or contribute to such endangerment; or (c) explain adequately why the agency cannot or will not make a determination under either (a) or (b). EPA chose option (a)—that is, to make a positive "endangerment finding" for GHG emissions from new motor vehicles. That finding was made in December 2009, and under CAA section 202 required EPA to promulgate standards for new motor vehicle emissions to address the endangerment. That action, or rather actions, the agency also has taken, beginning in May 2010. In EPA's view, the 2010 motor vehicle standards, in turn, triggered CAA duties for stationary sources (factories, power plants) of GHG emissions as well. More specifically, the agency adopted regulations under which major new stationary sources and major modifications of existing stationary sources, when proposed for Prevention of Significant Deterioration (PSD) areas, must undergo "new source review" based on their expected CO 2 emissions. If subject to PSD new source review, the CAA required those sources to install "best available control technology" (BACT) to control GHG emissions. Reading the CAA literally, EPA believed, new source review and BACT would be required for any major new source or modification that emitted or increased CO 2 emissions by the same low threshold amount applied by the CAA to non-GHG emissions. And the BACT requirement, EPA argued further, triggered CAA Title V operating permitting requirements—again, on a literal reading of the CAA, with the same low threshold as for non-GHG emissions. Given the huge amount of CO 2 emitted by even some small stationary sources, this literal reading of the CAA brought a correspondingly huge number of sources under the act's new source review, BACT, and Title V permitting requirements. In an effort to shrink this large number to administratively practical levels, EPA construed the act to allow a phase-in of the statutory thresholds through a so-called "tailoring rule." In 2014, the Supreme Court in Utility Air Regulatory Group v. EPA rejected EPA's argument that regulation of motor vehicle GHG emissions necessarily triggers PSD new source review of the same emissions from stationary sources. As the Court saw it, PSD new source review and BACT cannot be triggered solely by a source's GHG emissions. On the other hand, EPA's application of CO 2 BACT to "anyway" sources—sources subject to new source review based on their non-GHG emissions—was seen by the Court as a reasonable reading of the CAA. In a related holding, Utility Air Regulatory Group voided EPA's tailoring rule as an impermissible rewriting of the CAA's emission thresholds. This report is a chronology of the major climate change-related actions taken by federal agencies, principally EPA, in the wake of Massachusetts v. EPA . It does not include executive orders and presidential proclamations. Most of the listed actions trace directly or indirectly back to the Massachusetts decision. In contrast, a few were included solely because of their relevance to climate change and their occurrence post- Massachusetts . That is, they were not legally compelled by Massachusett s v. EPA or EPA actions in compliance therewith. More analytical treatment of the listed agency actions may be found in other CRS reports. Please note— Dates used are those of Federal Register publication wherever a Federal Register citation is given. In most instances , however, the agency action was signed and publicly announced weeks (or more) earlier. Once an agency promulgates a final rule, the report's entry for the proposed rule has been deleted. A rule is not deleted for any other reason, such as subsequent invalidation by the courts or withdrawal by the proposing agency—developments noted in footnotes that usually follow the boldface heading. Mar ch 6: EPA denies California's request for waiver of CAA preemption . 73 Fed. Reg. 12156. The CAA preempts state controls on new motor vehicle emissions, but offers California, and California alone, the opportunity to request a waiver of CAA preemption. EPA must grant the preemption waiver if certain conditions are met. The importance of this "California waiver" is magnified by the fact that once EPA grants the waiver, states that adopt motor vehicle emission standards identical to California's also partake of the preemption waiver for the same vehicles. In the present case, California sought a waiver of CAA preemption for its GHG emissions limits for 2009 and later model year motor vehicles. EPA denied the waiver on finding that the state did not need those emission limits to meet "compelling and extraordinary conditions," as required by the CAA. (See "July 8, 2009" for EPA's reversal of this denial.) July 30: EPA issues advance notice of proposed rulemaking regarding GHG emissions from new motor vehicles. 73 Fed. Reg. 44354. This document sets out EPA's view of the legal implications were EPA to make a positive endangerment finding for GHGs from new motor vehicles—as discussed in the introduction of this report, "option (a)" offered by the Supreme Court. It is purely an informational document, prepared after the George W. Bush Administration decided in late 2007 not to issue an endangerment finding for new motor vehicle GHG emissions, but rather to leave that decision to the next Administration. Dec ember 31 : EPA Administrator publishes interpretive memo randum ("Johnson memorandum"). 73 Fed. Reg. 80300. EPA Administrator Stephen Johnson issued this memorandum, titled "EPA's Interpretation of Regulations that Determine Pollutants Covered by Federal Prevention of Significant Deterioration (PSD) Permit Program." The memorandum narrowly interprets the CAA phrase "pollutant subject to regulation under this act" to include only pollutants regulated by actual, not potential future , emission limits under the CAA or its regulations. At the time, much hung on this distinction between actual, and potential future, emission limits. In PSD areas of the country, the CAA requires only pollutants "subject to regulation under [the CAA]" to be controlled by potentially expensive BACT—when emitted by new major emitting facilities or major modifications of existing facilities. Since there were no "actual" GHG regulations under the CAA when the Johnson memorandum was issued, this meant that for the near term at least, new major emitting facilities and major modifications of existing facilities proposed for PSD areas did not have to install BACT for GHG emissions. July 8: EPA grants California's request for waiver of CAA preemption. 74 Fed. Reg. 32744. This rule reversed EPA's prior denial of California's request for a preemption waiver (see "March 6, 2008"). As noted, its effect is to allow California's GHG emissions limits for 2009 and later model year motor vehicles to go into effect, and to allow the identical emission standards for the same vehicles promulgated by other states to do likewise. Such "other states" now number 13, plus the District of Columbia. Oct ober 30: EPA finalizes mandatory GHG monitoring rule . 74 Fed. Reg. 56260. This rule, known as the Greenhouse Gas Reporting Rule, was required by the FY2008 Consolidated Appropriations Act, which instructed EPA to develop a rule "to require mandatory reporting of GHG emissions above appropriate thresholds in all sectors of the economy"—using EPA's existing CAA authority. The rule was to take effect January 1, 2010, with the first monitoring reports due in 2011. This report does not list EPA's many amendments and expansions of the Greenhouse Gas Reporting Rule. See, for example, recent amendments including an alternative verification approach plus final confidentiality determinations for the newly added data elements, at 79 Fed. Reg. 63750 (October 24, 2014). To stay abreast, the reader is referred to http://www.epa.gov/ghgreporting/reporters/notices/index.html . Dec ember 15: EPA finalizes endangerment finding for GHG emissions from new motor vehicles . 74 Fed. Reg. 66496. This endangerment finding, under CAA section 202(a), was option (a) offered EPA by Massachusetts v. EPA , as noted in the report's introduction. Parsing the language of section 202(a), EPA actually concluded that two component endangerment findings were required. Based on the "air pollution that may reasonably be anticipated to endanger ... ," phrase, the first finding was that six GHGs otherwise present in the atmosphere are reasonably likely to endanger both public health and welfare through their climate change impacts.