For nearly two decades, one scientific determination quietly sat at the center of U.S. climate policy. It did not ban fuels. It did not mandate electric vehicles. But it gave the federal government something powerful: the legal authority to regulate greenhouse gases as a threat to public health.

That foundation is now gone.

Earlier this month, the U.S. Environmental Protection Agency formally repealed its 2009 “endangerment finding,” the ruling that concluded carbon dioxide and other greenhouse gases endanger human health and welfare. The move follows through on a long-standing goal of President Donald Trump, who has argued for years that climate regulations hurt the economy, raise consumer costs, and prop up what he calls an exaggerated threat.

The decision has sent shockwaves through environmental circles, energized conservative policy groups, and reopened a legal and political fight many thought had been settled.

At its core, the endangerment finding was what allowed the federal government to regulate emissions under the Clean Air Act. After a 2007 Supreme Court ruling determined greenhouse gases qualified as pollutants, EPA scientists reviewed mountains of research and concluded in 2009 that those emissions posed real risks to people and the environment. From there flowed vehicle emissions standards, power plant regulations, methane rules for oil and gas operations, and even limits tied to aviation.

Without that finding, the federal government no longer has a clear mandate to directly regulate climate pollution unless Congress passes new laws. And in today’s polarized political environment, major climate legislation remains a steep uphill climb.

Supporters of the repeal say it removes what they view as regulatory overreach. The Trump administration argues climate rules have saddled industries with trillions of dollars in compliance costs while delivering only marginal environmental benefits. EPA leadership under Lee Zeldin has framed the finding as an improper expansion of authority that was never meant to cover global pollutants like carbon dioxide.

But critics see something far bigger at stake.

For Democrats and climate advocates, the repeal isn’t just a policy shift, it’s a dismantling of the federal government’s primary climate toolbox. They argue that nearly every meaningful emissions rule of the last 15 years traces back to that single scientific conclusion. Remove it, and the regulatory structure collapses.

And while climate change itself doesn’t always rank as a top voting issue, Democratic leaders are increasingly tying the rollback to broader narratives about political influence and fossil fuel interests. They point to Trump’s open embrace of expanded oil and gas production, industry donations, and the steady installation of energy-sector veterans across regulatory agencies as evidence of a coordinated effort to unwind environmental protections.

Beyond the politics, the science remains unchanged.

Over the last 15 years, research connecting climate change to human health impacts has exploded. Thousands of peer-reviewed studies now document rising heat-related deaths, increased respiratory illness from wildfire smoke, worsening air quality, stronger storms, and the spread of disease in warming regions.

In the U.S. alone, heat-related deaths have more than doubled since the late 1990s, climbing from just over 1,000 annually to more than 2,300 in recent years. Global studies estimate that tens of thousands of deaths each year are already linked to human-caused warming. And that’s just from heat. Add in floods, hurricanes, wildfires, air pollution, and waterborne disease, and the health toll rises sharply.

Public health researchers increasingly treat climate change not as a future risk, but as a present-day driver of illness and mortality.

Which makes the repeal feel, to many scientists, like policy deliberately stepping away from overwhelming evidence.

Legally, the fight is far from over.

States led by California, along with major environmental groups, have already pledged lawsuits challenging the EPA’s authority to reverse the finding. The heart of their argument is simple: the Supreme Court already ruled greenhouse gases qualify as pollutants under the Clean Air Act, and courts previously upheld EPA’s science-based determination that they endanger public welfare.

The administration is countering with a narrower reading of the law, claiming the Clean Air Act was designed to address local pollution like smog and acid rain, not globally mixed gases like carbon dioxide. It’s also leaning on legal theories that limit how broadly federal agencies can act without explicit congressional approval.

Most legal experts expect the issue to wind through courts for years, possibly landing back before the Supreme Court.

In the meantime, the practical impact could be enormous.

Vehicle emissions standards, power plant carbon limits, methane leak rules, and other climate regulations now face a far weaker legal footing. Some may be rolled back quickly. Others could simply stop being enforced or rewritten.

Ironically, even without federal climate rules, U.S. emissions are still expected to decline over time, though more slowly. That’s largely because renewable energy has become cheaper than coal in much of the country, solar and wind dominate new power construction, and electric vehicle adoption continues to grow. Roughly 90 percent of new power projects seeking grid connection today are renewable or battery storage.

Market forces are doing some of the work regulation once pushed.

But analysts warn that without federal policy pressure, the pace of emissions cuts will likely fall short of what scientists say is needed to avoid the worst climate impacts. The transition may continue, just not fast enough.

And there’s a longer-term consequence too.

By removing the endangerment finding, future administrations lose the easiest legal lever for climate action. Any serious attempt to regulate emissions again may require new laws from Congress, a far more difficult political task than executive action through existing environmental statutes.

In short, the science still points in the same direction. The climate is warming. Health risks are rising. Clean energy is growing.

What’s changed is the federal government’s willingness to treat climate pollution as a public health threat that demands regulation.

For now, climate policy in the U.S. is shifting from a national framework toward a patchwork of state rules, market trends, and court battles. Whether that’s enough to meaningfully curb emissions, or simply slows the response to a growing global problem, is the question that will shape energy, infrastructure, and public health for decades to come.

One thing is certain: the quiet legal finding that once anchored America’s climate response is no longer doing the holding. And the ripple effects are just beginning.