By Simon Jessop

LONDON, Feb 11 (Reuters) - The Trump administration's decision to overturn an Obama-era legal analysis underpinning greenhouse gas rules will sow confusion and add costs for businesses and investors alike, shareholder advocates and portfolio managers say.

U.S. President Donald Trump, who has called climate change a "hoax," plans on Thursday to formally rescind the 2009 scientific findings that tied carbon dioxide to health dangers – data that has guided pollution standards for more than 15 years.

The Republican administration's most sweeping climate change policy rollback yet, the change follows a string of regulatory cuts and other moves intended to unfetter fossil fuel development and stymie the rollout of clean energy.

Asset managers and shareholder activists say the move will leave companies in limbo, wondering whether they will have to course-correct under a future administration. Little will likely change for large multinational companies that will have to follow tougher emissions standards around the world.

"This rollback creates profound uncertainty for companies that have already invested billions in emissions reduction," said Marcela Pinilla, director of sustainable investing at Zevin Asset Management.

"We're interrupting a trajectory toward a low-carbon economy just as companies have committed substantial capital to that transition ... Those reversing course face stranded asset risk if policies change again."

STOP-START PLANNING

Beth Williamson, head of sustainable equity research at Calamos Investments, said the move "adds another layer of regulatory uncertainty for carbon‑intensive industries" and can move risk elsewhere.

Such "stop-start" planning also pushes volatility into the supply chain, affecting upstream providers in semiconductors, power electronics, and industrial equipment, said Williamson, who's also an associate portfolio manager.

Andrea Ranger, director of shareholder advocacy at Trillium Asset Management, said the repeal could make it harder for investors to pick winners in the transition and creates uncertainty for firms with major capital expenditure plans.

"Because if the next administration comes in and says 'yep, we're going to do this again,' it's the whiplash effect."

The reversal would add extra operational costs that most company boards are unwilling to bear, added Jonathan Pragel, executive director at Calvert Research and Management, part of Morgan Stanley Investment Management.

"The cost of eliminating this infrastructure, and then needing to rebuild it if there is kind of another change in the reporting regime, that's a really expensive proposition."

Commitments by U.S. companies to get to net-zero emissions across their business by 2050 grew 9% in 2025, data from the non-profit Net Zero Tracker showed, with 304 firms in the Forbes Global 2000 index doing so, up from 279 in the prior year.

INVESTOR PRESSURE

While automakers may be given the freedom from federal reporting requirements, their investors and other nations will continue to demand it, not least regulators in the European Union and elsewhere.

"Investors will keep making clear that managing climate risk is essential to protecting both shareholders and the bottom line," said Giovanna Eichner, shareholder advocate at Green Century Capital Management.

"Losing this finding weakens accountability, but not investor resolve. Climate risk still threatens shareholder value and company profits alike."

Since German automaker BMW BMWG.DE is headquartered in the European Union, it will still have to follow disclosure and emissions requirements there, regardless of what the U.S. does, a spokesperson said. "Therefore, the changed U.S. regulation might not have a big influence on us as a global player."

Fellow global automakers Ford F.N, General Motors GM.N, Stellantis STLAM.MI, Mercedes MBGn.DE and Volkswagen VOWG.DE did not immediately return a request for comment.

Rachel Delacour, CEO of sustainability data management platform Sweep, said: "We know from the companies we work with, that those who are pulling ahead are integrating ESG data into how they run their business, not just how they report on it. That’s the competitive advantage."

LEGAL CHALLENGES

The repeal is also vulnerable to legal challenge after a federal court ruled in January that the Department of Energy violated the law when it formed a climate science advisory group which produced a report meant to support the repeal attempt.

For Mark Wade, head of sustainability research and stewardship at Allianz Global Investors, the boards of many large companies with international investors that want the data would not want to lose them.

"These U.S companies are now so big they need non-U.S. investors. If you start to remove that incremental buyer of risk, that's a problem for (share price) valuations, Wade said.

Despite the Trump administration's pull back on climate, many U.S. companies continue to do the work on adapting their businesses to a low-carbon future, even if they are quieter about it.

While the planned EPA repeal is "very unhelpful," many large U.S. companies are still looking to profit from the energy transition: "If you find the next nuclear fusion or hydrogen solution, you're the next billionaire," Wade said.

(Additional reporting by Nick Carey, Nora Eckert and Kalea Hall; Editing by Dawn Kopecki and Nick Zieminski)

((simon.jessop@thomsonreuters.com; +44 (0) 207 542 5052; Reuters Messaging: Reuters Messaging: simon.jessop.thomsonreuters.com@reuters.net))