Why the US Clean Air Act Still Matters for State Climate Policy

The Clean Air Act as the Backbone of State Climate Policy

When we talk about environmental policy in the United States, the Clean Air Act (CAA) is the foundation upon which almost everything else is built. Since its major amendments in 1970 and 1990, the CAA has authorized the Environmental Protection Agency (EPA) to set national ambient air quality standards (NAAQS) and to require states to implement them through State Implementation Plans (SIPs). What many people miss is that the CAA was deliberately designed as a partnership between federal and state governments—a structure that, today, gives states enormous leverage to pursue climate policy even when federal leadership stalls.

This cooperative federalism means that states are not mere bystanders waiting for Washington. They are active laboratories of democracy, testing policy tools that often become templates for national action. California's special authority under Section 209 of the CAA—to request a waiver from the EPA to set stricter vehicle emission standards—is the most famous example, but it is hardly the only one. States like Massachusetts, New York, and Washington have used SIPs and section 111(d) of the act to regulate greenhouse gases from power plants, and courts have repeatedly upheld these actions.

How Section 111(d) Became a Climate Battleground

Section 111(d) of the Clean Air Act directs the EPA to regulate air pollutants from existing stationary sources that are not covered by NAAQS or hazardous air pollutant standards. In 2015, the Obama administration used this section to issue the Clean Power Plan, which would have set state-specific carbon emission reduction targets for existing power plants. The plan was stayed by the Supreme Court in 2016 and later repealed by the Trump administration, but the legal questions it raised never fully disappeared.

The key issue is the so-called "outside the fence line" debate: can the EPA consider measures like shifting generation to cleaner sources or implementing carbon trading when setting emission guidelines under 111(d)? In 2022, the Supreme Court's decision in West Virginia v. EPA curtailed the agency's ability to use such system-wide approaches without clear congressional authorization. However, the decision did not strip states of their own authority to regulate. In fact, the Court explicitly noted that states have independent legal authority to address carbon emissions from power plants, which is why we now see states adopting their own carbon pricing or clean electricity standards.

The State Implementation Plan: A Tool for Innovation

The SIP process is where state policy meets federal law in the most tangible way. Under the CAA, each state must submit a SIP that demonstrates how it will attain and maintain the NAAQS. While SIPs are usually associated with conventional pollutants like ozone and particulate matter, they are also the vehicle through which states can implement innovative measures that have climate co-benefits.

For example, the South Coast Air Quality Management District in California has used SIP-related regulations to require zero-emission trucks at warehouses and ports, simultaneously cutting diesel particulate matter and CO2. Similarly, the Regional Greenhouse Gas Initiative (RGGI) in the Northeast is a cap-and-trade program for power plant emissions that operates under the broad umbrella of state and regional climate policy, but its legal grounding is in state law, not the CAA. Still, the CAA provides the procedural framework—public hearings, EPA review, and federal enforceability—that gives these state programs legitimacy and durability.

California's Waiver Power and the Federal Preemption Question

No discussion of state climate policy under the CAA is complete without examining California's special status. Section 209 of the act allows California to request a waiver from the EPA to set stricter vehicle emission standards than the federal ones, and other states can then adopt those standards under Section 177. This has created a powerful dynamic: California and the 17 other states that follow its standards represent about 40% of the U.S. new car market, giving them significant influence over what vehicles automakers produce.

The Trump administration revoked California's waiver in 2019, but the Biden administration reinstated it in 2022. The legal back-and-forth underscores a crucial point: the waiver is a policy tool, not a permanent right. If a future administration decides to revoke it again, states would lose their ability to set stricter standards, unless Congress amends the act. This fragility has prompted some states to codify their own greenhouse gas standards at the state level, but those are still subject to preemption if the federal government regulates vehicle emissions under the CAA. The tension between state ambition and federal supremacy remains unresolved, and it will likely require a legislative fix—or a Supreme Court ruling—to settle.

Comparing State Climate Policy Approaches

To see how the CAA framework shapes real-world outcomes, compare the approaches of three states: California, Texas, and New York. California uses its waiver authority to regulate vehicle emissions and has also implemented a cap-and-trade program that covers electricity, industry, and transportation fuels. Texas, on the other hand, has historically relied on voluntary programs and has not adopted a comprehensive climate plan, but it does have a strong renewable portfolio standard that was set in 1999 and later strengthened. New York has adopted a legally binding Climate Leadership and Community Protection Act (CLCPA) that mandates 100% zero-emission electricity by 2040 and economy-wide carbon neutrality by 2050, but it does not rely on the CAA for enforcement; instead, it uses state law and state agencies.

Here is a comparison table highlighting key differences:

This comparison shows that the CAA is not a one-size-fits-all tool. States can choose to use it aggressively, as California does, or they can rely on state-specific legislation. The choice depends on political will, legal capacity, and the nature of the emissions sources in each state.

Practical Lessons for State Policymakers

If you are a state legislator, regulator, or policy advocate, there are concrete steps you can take to maximize the CAA's potential for climate action, while staying within legal bounds.

1. Inventory your SIP. Review your state's current SIP to identify where you can integrate climate co-benefits, such as requiring best available control technology for greenhouse gases at major sources.
2. Leverage Section 126 petitions. Section 126 allows states to petition the EPA to address emissions from upwind states that interfere with attainment of NAAQS. While not climate-specific, it can be used to reduce ozone precursors, which often come from the same sources as CO2.
3. Adopt California's vehicle standards. If your state has not already done so under Section 177, adopting California's Advanced Clean Cars II rule can cut transportation emissions significantly. As of 2024, 17 states have done so.
4. Use state law to fill gaps. Where the CAA is silent or preempted, consider enacting state statutes like New York's CLCPA to set binding targets and create enforcement mechanisms.
5. Participate in EPA rulemaking. Submit comments and intervene in litigation to shape EPA's 111(d) guidelines for power plants, which are currently being developed under the Biden administration's 2023 proposed rule.

The Litigation Landscape and What It Means for You

Court rulings are not abstract; they directly affect the tools available to state policymakers. The 2022 Supreme Court decision in West Virginia v. EPA narrowed EPA's authority, but it also opened the door for states to argue that they are not bound by the same limits. As a result, we are seeing a wave of state-level lawsuits that seek to compel utilities to decarbonize under state public utility law, and some that even attempt to hold fossil fuel companies liable for climate damages, as in the suits filed by Honolulu and other municipalities.

For state agencies, this means that legal risk is now a core part of policy design. Any climate rule you write should include a thorough legal analysis that anticipates challenges based on preemption, commerce clause, and non-delegation doctrines. It is also wise to build a record that justifies your rule as a means of protecting public health and welfare, which is the CAA's core purpose.

Conclusion: The CAA Is Not Perfect, But It Is What We Have

The Clean Air Act is decades old, and its mechanisms were not designed with climate change in mind. Yet it remains the most powerful federal statute for addressing greenhouse gases that we currently have. States that understand its nuances can use it to advance ambitious climate policy, even in the face of federal inaction or hostility. The key is to approach the CAA as a framework for creative problem-solving, not as a static set of rules. Whether you are drafting a new SIP, petitioning the EPA, or litigating in court, the CAA provides both constraints and opportunities. The question is whether your state will seize them.