California is suing the Trump administration over its decision to roll back the endangerment finding, the U.S. government’s longstanding scientific conclusion that planet-heating pollution seriously threatens Americans, state officials announced Thursday.

Atty. Gen. Rob Bonta, Gov. Gavin Newsom and the California Air Resources Board are co-leading a coalition of 25 attorneys general, the governor of Pennsylvania, and 10 cities and counties in a petition challenging the Environmental Protection Agency, filed in the U.S. Court of Appeals for the D.C. Circuit.

The 2009 endangerment finding was a long-awaited, foundational piece of the nation’s effort to address climate change, and it underpinned much of U.S. climate policy — including the EPA’s ability to regulate greenhouse gas emissions from vehicles.

EPA Administrator Lee Zeldin touted the February repeal as “the single largest act of deregulation in the history of the United States of America.”

The coalition has argued that rescinding the endangerment finding is a violation of settled law, including clear Supreme Court precedent, as well as broad scientific consensus over the effects of greenhouse gas emissions on human health and welfare. Its rollback will disrupt the regulatory landscape and result in significant increases in greenhouse gas emissions, which drive climate change, they said.

The lawsuit will ask the court to vacate the EPA’s repeal and restore greenhouse gas emission standards for vehicles. A formal complaint is pending the judge’s acceptance of the petition.

“With the unlawful rescission of the Endangerment Finding, President Trump and his EPA have abandoned their most important mission: protecting the health and welfare of the American people,” Bonta said in a statement. “The science doesn’t lie: Climate change and [greenhouse gas] emissions are harming public health and causing devastating and ever-worsening disasters. Our communities have felt the impact of destructive wildfires, watched families run from burning homes, inhaling toxic fumes, and we’ve seen entire communities wash away in severe floods. The President can’t keep his head in the sand — climate change is real and decades of settled science warned us this was coming.”

Much of the EPA’s argument for repealing the endangerment finding hinged on whether greenhouse gases qualify as “air pollutants” under the Clean Air Act, making them subject to federal regulations. A landmark 2007 Supreme Court case, Massachusetts vs. EPA, determined that they are.

In its decision, the agency said it “carefully considered and reevaluated the legal foundation” for the finding and concluded that the Clean Air Act does not provide statutory authority for the agency to prescribe motor vehicle and emissions standards, and therefore has no legal basis for the endangerment finding or its resulting regulations.

In a statement Thursday, EPA officials reaffirmed that conclusion and said the lawsuit is not about the legality or the merits of their argument, but rather political motivation.

“EPA is bound by the laws established by Congress, including under the [Clean Air Act],” the agency said. “Congress never intended to give EPA authority to impose [greenhouse gas] regulations for cars and trucks.”

The coalition alleges that the repeal of the endangerment finding violates the Clean Air Act as well as the Administrative Procedure Act by resting on the “flawed assertion” that it lacks legal authority to regulate greenhouse gas emissions, and “ignores overwhelming and longstanding scientific evidence” about the role of greenhouse gas emissions on human health and welfare.

It also argues that the elimination of existing and future greenhouse gas emission standards for vehicles violates EPA’s legal obligations and fundamental responsibility to protect the public from environmental harm.

EPA’s repeal will not only disrupt 15 years of regulatory progress, but will also threaten American investment in future technologies and U.S. leadership in the transportation sector and efforts to address climate change, the coalition said.

“This is what corruption looks like. Donald Trump is breaking the laws that protect Americans from climate pollution — all to enrich his Big Oil and his wealthy polluting allies,” Newsom said in a statement. “Workers, families, and communities would pay the price, left choking on dirty air. No one is above the law in this country. Not even the president.”

The EPA also cast doubt on the climate science behind the endangerment finding, despite the fact that independent researchers around the world have long concluded that greenhouse gases released by the burning of gasoline, diesel and other fossil fuels are warming the planet and contributing to worsening climate impacts. Understanding of how carbon dioxide warms the atmosphere goes back more than a century.

Among its justifications, the agency’s ruling says that reducing greenhouse gas emissions from new and existing vehicles in the U.S. would have only “de minimis impacts” on global temperature and sea level rise. But many experts say reducing those emissions is critical for curbing climate change, as the transportation sector is the largest source of greenhouse gas emissions in the U.S. In California, it accounts for about half of the state’s emissions.

EPA’s proposal to repeal the endangerment finding received more than half a million public comments, including from environmentalists, scientists, civil rights groups, public health organizations and former EPA officials opposed to the plan. Support for the plan generally came from industry and regulatory reform groups who said the vehicle standards that rest on the endangerment finding are costly and unduly burdensome.

Bonta co-leads the lawsuit alongside the attorneys general of Massachusetts, New York and Connecticut. They are joined by attorneys general of Arizona, Colorado, Delaware, Hawaii, Illinois, Maine, Maryland, Michigan, Minnesota, New Jersey, New Mexico, Nevada, North Carolina, Oregon, Rhode Island, Vermont, Virginia, Washington, Wisconsin and the District of Columbia.

The coalition also includes Pennsylvania Gov. Josh Shapiro; the cities of Boston, Chicago, Cleveland, Columbus, Denver, Los Angeles, New York, San Francisco, Santa Clara and Harris County, Texas.

This marks California’s 63rd lawsuit against the Trump administration since the president returned to office last year.

A strange lack of stellar orbits around the core of the Small Magellanic Cloud (SMC) mystified astronomers for decades. Not only that, but the SMC has a strange, irregular shape, and sports a tidal. Now, a team of observers led by graduate student Himansch Rathore at the University of Arizona, has tracked down the reason why the stars don’t orbit. It’s because the SMC crashed directly through its neighbor, the Large Magellanic Cloud (LMC), in the distant past. That huge collision disrupted stellar motions and sent them on wildly different trajectories. It also disturbed the clouds of gas within the SMC and created a tail of gas stretching out across space.

The team’s work provides unique insights into how galaxies change over time. “We are seeing a galaxy transforming in live action,” said Rathore. “The SMC gives us a unique, front-row view of something very transformative of a process that is critical to how galaxies evolve.”

A Closer Look at the SMC

The Small Magellanic Cloud is one member of a trio of interacting galaxies that includes our own Milky Way, in addition to the Large Magellanic Cloud. The SMC lies about 200,000 light-years away, while the LMC is about 158,000 light-years distant. Both have sites of active star formation. The SMC is categorized as a dwarf irregular galaxy and has a mass equivalent to about 7 billion solar masses. Not all that mass is in stars, however. Most of the SMC’s mass is in giant gas clouds that eventually become sites of starbirth. That happens as the clouds cool and contract. If conditions are right, the process creates hot, young stars that astronomers can study as they seek to understand the process of star formation.

This visible light mosaic shows the LMC and SMC in context with the plane of our own galaxy, the Milky Way. Dusty filaments create dark traces across the bright central plane of the Milky Way, visible across the top of the image. Below it, separated by about 21 degrees, lie the LMC and SMC, the closest major galaxies to our own. The LMC and SMC orbit each other as well as our own Milky Way galaxy. Credit: Axel Mellinger, Central Michigan University (via NASA Goddard Scientific Vizualization Studio).

Astronomers have measured the motion of the stars that exist in the SMC, using the data from the Hubble Space Telescope and the Gaia mission. That’s when they found that the SMC’s stars don’t orbit around that galaxy’s center the way stars in most other galaxies do. That lack of orbital activity was puzzling, until Rathore’s team considered the effects of a collision on the SMC and LMC. A few hundred million years ago, the SMC crashed directly through the LMC’s disk. The LMC’s gravity disrupted the SMC’s internal structure and sent its stars into random, disordered motion. In addition, the gas in the LMC applied a tremendous amount of pressure to the SMC’s gas and destroyed its gas rotation.

Since the LMC, SMC, and Milky Way Galaxy are interacting with each other, astronomers want to understand how that interaction affects all three galaxies. Astronomers found a bridge of gas between the LMC and SMC, likely pulled from one of the galaxies by during tidal interactions between the two galaxies. That bridge is busily forming stars in the shocked gas.

Large and Small Magellanic Clouds from GAIA data. Image credit: ESA/Gaia/DPAC – CC BY-SA 3.0 IGO.

Solving the Puzzle of Disrupted Stellar Orbits

The crash between the LMC and SMC did a lot of damage to both, according to Gurtina Besla, a senior author on a paper about the finding. “The SMC went through a catastrophic crash that injected a lot of energy into the system. It is not a ‘normal’ galaxy by any means,” Besla said. To understand that crash, the team turned to computer simulations. First, they matched the known properties of the SMC and the LMC – their gas content, total star mass, and positions relative to the Milky Way. They paired the simulations with theoretical calculations of how collision affected the SMC’s gas as it plowed through the LMC’s dense gas environment. They also developed new methods for reading the scrambled star motions in a post-collision galaxy, tools that can now be used to properly interpret what telescopes actually measure in the SMC.

That matters because the SMC is small, gas-rich and low in heavy elements, which are properties that made it a standard yardstick for the kinds of galaxies that existed early in the Universe. A galaxy still reeling from a collision may not be a clean reference point, Besla said. However, it can give information about the effects collisions and interactions have had on galaxies throughout time.

This plot shows the simulated gas distribution of the Magellanic System resulting from the tidal encounter between the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) as they orbit our home Milky Way Galaxy. The solid line shows the calculated path of the LMC and the dotted line is the path of the SMC. Plot by G. Besla, Milky Way background image by Axel Mellinger (used with permission)

Other Effects of a Collision

Another study published by the team in 2025 showed that the collision also left a physical mark on the LMC that could help scientists probe dark matter. The LMC has a bar-shaped structure at its center, and that bar is tilted out of the plane of the galaxy because of the collision. Rathore, the 2025 study’s lead author, said the degree of the tilt is tied to how much dark matter the SMC contains, giving researchers a new way to measure a substance that has never been directly detected, only inferred from its gravitational effects.

“We are used to thinking of astronomy as a snapshot in time,” Rathore said. “But these two galaxies have come very close together, gone right through one another, and transformed into something different.”

In addition, the interaction between the LMC, SMC, and the Milky Way affects our galaxy’s shape. It appears that the LMC is causing a warp in the shape of the Milky Way’s stellar disk. It’s also pulling on the core of our galaxy, disturbing the halo, and accelerating its velocity through space. The SMC also contributes to this warping and pulling, and helps create the Magellanic Stream. That’s a trail of gas and stars that’s helping to populate the Milky Way.

For More Information

A Galactic Transformation — Understanding the SMC’s Structural and Kinematic Disequilibrium

A Galaxy Next Door is Transforming, and Astronomers can see it Happening