	Introduction

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 chose option (a)—that is, to make a positive "endangerment finding" for GHG emissions from new motor vehicles. 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. 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 . 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. 2008

Mar ch 6: EPA denies California's request for waiver of CAA preemption . 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. Dec ember 15: EPA finalizes endangerment finding for GHG emissions from new motor vehicles . The memorandum directs EPA to issue a revised version of its 2012 proposed new source performance standards for GHG emissions from fossil fuel-fired power plants. November 4: EPA proposes emission guidelines for CO 2 emissions from existing fossil fuel-fired electric generating units in Indian country and U.S. territories. December 24 : Council on Environmental Quality issues revised draft guidance . As asserted in the revised draft guidance, its purpose is "to provide Federal agencies direction on when and how to consider the effects of greenhouse gas (GHG) emissions and climate change in their evaluation of all proposed Federal actions in accordance with [NEPA and CEQ regulations]."
In 2007, the Supreme Court rendered one of its most important environmental decisions. In Massachusetts v. EPA, the Court held 5-4 that greenhouse gases (GHGs), widely viewed as contributing to climate change, constitute "air pollutants" as that phrase is used in the Clean Air Act (CAA). As a result, said the Court, the U.S. Environmental Protection Agency (EPA) had improperly denied a petition seeking CAA regulation of GHG emissions from new motor vehicles by citing, among other reasons, the agency's lack of authority over such emissions.
This report offers a chronology of major federal agency actions, mainly by EPA, that involve GHGs or climate change and that occurred after Massachusetts v. EPA. Most of the listed actions trace directly or indirectly back to the decision. Examples include EPA's "endangerment finding" for GHG emissions from new motor vehicles; the resulting EPA standards (issued on multiple occasions) for GHG emissions from new motor vehicles; EPA's proposal of performance standards (again on separate occasions) for CO2 emissions from new, and modified or reconstructed, fossil fuel-fired power plants; and EPA's proposal of emission guidelines for CO2 emissions from existing fossil fuel-fired power plants. Several listed EPA actions, taken on the agency's view that regulation of GHG emissions from new motor vehicles triggers new source review of GHG stationary sources, are now either void or will have to be limited slightly in scope, owing to the 2014 decision of the Supreme Court in Utility Air Regulatory Group v. EPA.
A few agency actions were included in the report solely because of their relevance to climate change and their post-Massachusetts occurrence—that is, they were not legally compelled by Massachusetts v. EPA or EPA actions tracing back to that decision. Examples include EPA's responses to California's request for a waiver of CAA preemption allowing that state to set its own limits for GHG emissions from new motor vehicles; OMB's "social cost of carbon" dollar amount to be used in agency cost-benefit analyses; the Council on Environmental Quality's draft guidance on how climate change is to be considered in environmental impact statements; and EPA's monitoring rule for GHG emissions.
More analytical treatment of the government actions in this report may be found in other CRS reports listed in footnote 16 herein.