by Catarina Conran

October 12, 2024

For decades, the Chevron doctrine, established by Chevron U.S.A., Inc. v. Natural Resources Defense Council, Inc., 467 U.S. 837 (1984), has been the law of the land. Under Chevron, courts were required to defer to an agency’s “reasonable” interpretation of ambiguous statutory language. Agencies like EPA, FWS, and the Army Corps of Engineers had discretion to define key terms and implement regulatory frameworks under laws like the Clean Air Act, the Clean Water Act, the Endangered Species Act, and the National Environmental Policy Act. In practice, this meant that agencies were not just enforcing environmental statutes, they were also shaping them—filling in gaps left by ambiguity and adapting broad statutory mandates to address emerging issues.

That system has now been upended. In Loper Bright Enterprises v. Raimondo, 603 U.S. 369 (2024), the Supreme Court ended Chevron deference, removing the requirement that courts credit agency expertise when interpreting ambiguous statutes. Instead, with no obligation to defer to the technical or scientific judgments of agencies, judges will now decide statutory meaning for themselves. However judges, for all their wisdom, are not scientists or engineers. Agencies employ biologists, hydrologists, and chemists to interpret data and implement policy. But judges will now be asked to make those technical calls themselves, without the benefit of deference to agency expertise.

Implications for Environmental Conservation

This seismic shift raises immediate challenges for environmental advocates. Statutes that were once interpreted flexibly in favor of agency expertise are now vulnerable to restrictive readings in court. For example, EPA’s authority under the Clean Air Act to regulate emerging pollutants like greenhouse gases or PFAS could be narrowed if judges reject interpretations that go beyond the statute’s explicit text. Or, under the Endangered Species Act, the meaning of terms like “significant portion of its range” could be decided by judges rather than biologists.

For advocates, this likely means that environmental litigation strategies must evolve. Arguments can no longer rest on the assumption that courts will uphold agency interpretations as long as they are “reasonable.” Instead, they must persuade judges directly, through statutory text, legislative history, and carefully built factual records. Advocacy will likely need to be tailored to a judiciary that is more diverse in philosophy and less anchored in scientific expertise.

This regime shift will significantly affect advocacy efforts under several landmark environmental laws. A few of these are discussed briefly below.

NEPA

NEPA has long required agencies to take a “hard look” at the environmental impacts of major federal actions. Until now, agencies had discretion under Chevron in deciding what counted as “indirect” or “cumulative” effects of proposed actions. In other words, agencies could decide whether they needed to analyze downstream greenhouse gas emissions, long-term climate impacts, or the combined effects of multiple projects in the same region. Without Chevron, courts are more likely to second-guess those judgments, potentially narrowing reviews to immediate and obvious impacts while ignoring long-term or climate-related consequences.

Clean Air Act

The Clean Air Act has some of the broadest statutory language, which agencies like EPA have historically interpreted to regulate new pollutants as science evolves. For example, in Massachusetts v. EPA, 549 U.S. 497 (2007), Chevron deference played a key role in the Court’s affirmation of EPA’s authority to treat greenhouse gases as “air pollutants.” Now, without Chevron, courts may be less willing to accept EPA’s expansive reading of its regulatory powers and future efforts to regulate emerging pollutants (like PFAS) may face tougher judicial scrutiny.

Clean Water Act

The Clean Water Act is also full of terms that invite interpretation, such as “point source,” “pollutant,” and “navigable waters.” Under Chevron, EPA and the Corps had room to define and adapt these terms as new issues arose (e.g., nutrient pollution or agricultural runoff). Without Chevron, courts may narrow those definitions, potentially excluding entire categories of discharges or waterways from federal protection.

Sackett v. EPA, 598 U.S. 651 (2023) also recently delivered a major blow to the Clean Water Act, sharply limiting the scope of “waters of the United States” and holding that wetlands are only protected if they have a “continuous surface connection” to navigable waters. In other words, this decision significantly limited the number of wetland ecosystems in the U.S. that can be protected under the Clean Water Act. Without Chevron, EPA has little interpretive room left to reclaim that ground.