Updated Nov. 24 with Michael Bloomberg formally announcing his candidacy for president.

One of the Trump administration's favorite environmental talking points is that the United States has reduced carbon emissions more than any other country.

It's not an achievement that Trump can take any credit for. But his latest potential challenger, Michael Bloomberg, arguably can take some.

As market forces and regulatory controls were driving coal from its perch as the dominant electricity fuel in the U.S., they got a big assist from the Sierra Club's multimillion-dollar Beyond Coal campaign, launched in 2012 and bankrolled by Bloomberg.

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That's just one of the ways that Bloomberg, the billionaire former mayor of New York, has put time and a portion of his fortune into action on climate change over the past decade.

In the face of federal inaction under Trump and his withdrawal from the Paris climate accord, Bloomberg helped organize the America's Pledge effort by cities, states and businesses to keep the U.S. commitment to cut carbon emissions. He chaired the international Financial Stability Board's task force that in 2017 developed voluntary guidelines for climate-related financial disclosures by businesses. Bloomberg Philanthropies has stepped in to cover the $10 million funding gap at the UN created when Trump revoked most financial support for international climate action. And this past June, Bloomberg announced a $500 million investment in a new Sierra Club campaign, Beyond Carbon, whose aim is not only to shut down all the remaining coal plants in the United States but to block construction of new natural gas plants as well.

So climate voters are naturally rallying behind the idea of a Bloomberg presidential candidacy, right?

Not quite.

Many ardent advocates of climate action are dubious of whether the already crowded Democratic presidential slate needs another contender in the race to unseat Trump.

Their concerns go beyond the general worry among Democrats that Bloomberg's entry will prolong an internecine battle among the party's hopefuls. They wonder if the moderate, unabashedly pro-business Bloomberg, whose personal wealth is estimated at more than $50 billion, is the right leader for the kind of climate action they believe is needed now—a sweeping transformation of the U.S. economy and its energy mix.

"The first reaction is just a huge, 'Why?,'" said RL Miller, political director of the advocacy group Climate Hawks Vote. "Why is he doing this?

Michael Bloomberg and Bank of England Governor Mark Carney, both outspoken on the need for businesses to take climate change seriously, headed a task force that in 2017 developed voluntary guidelines for climate-related financial disclosures by businesses. Credit: Chesnot/Getty Images

"He obviously complicates things tremendously for Tom Steyer in the billionaire-funders-who-need-to-stick-to-something-and-not-run-for-president lane," Miller added archly, referring to the billionaire activist and former hedge fund manager who also has made climate change a signature issue. The presence of Bloomberg and Steyer as voices of climate activism in the 2020 presidential race is not helpful to the movement, in Miller's view.

"It creates the perception that only rich white people care about climate, and we have been working so hard to overcome that assumption by bringing climate justice front and center with the Green New Deal," Miller said.

Charles Komanoff, co-founder of the nonprofit Carbon Tax Center in New York, who is currently gearing up his organization to advocate for the Green New Deal, views Bloomberg as out-of-step with that drive—in part because Komanoff sees taxes on extreme wealth as the most logical way to pay for the initiatives and advance the Green New Deal's goal of economic justice.

"Mike Bloomberg, with all his vision and good will, is too discordant of what is the new reality of forcing the ultra-rich to give up huge portions of their wealth to finance the Green New Deal," Komanoff said.

What About Natural Gas as 'Bridge Fuel'?

Bloomberg's largest climate initiative, the Sierra Club's Beyond Coal campaign, is also his most controversial one—even among environmental advocates.

Through litigation and aggressive action before state regulators, Sierra Club lawyers and activists have helped spur the retirement or planned retirement of 299 U.S. coal plants since 2010, more than half of the nation's fleet. This shuttering of coal plants was a driver behind the rapid drop in U.S. carbon emissions that Trump has touted—and Bloomberg Philanthropies estimates that 40 percent of the plant closures would not have happened without the Beyond Coal campaign.

But many of the coal plants were converted to natural gas, which Bloomberg continued to embrace as a "bridge" fuel for some time after Sierra Club quit accepting money from the natural gas industry. As recently as the 2016 election, Bloomberg backed the successful re-election bid of Sen. Pat Toomey, a pro-gas Republican in the fracking hotbed and key swing state of Pennsylvania, who was being challenged by former Clinton administration environmental adviser Katie McGinty.

Critics have long noted that the proliferation of natural gas plants guarantees a market for fracked fuel and lengthens the bridge to a fossil-fuel-free future. Bloomberg and the Sierra Club's new Beyond Carbon push aims to address those concerns.

"We don't want to replace one fossil fuel with another," Bloomberg said in announcing the campaign at MIT's commencement ceremonies in June. "We want to build a clean energy economy."

In the view of some, Bloomberg and the Sierra Club also have not done enough thus far on building up the economy in places where they have helped shut down coal.

Marie Gunnoe, a West Virginia activist who won the prestigious Goldman Prize for her campaigning against mountaintop mining, felt that her native Boone County would have been a perfect platform to invest in renewable energy to supplant coal. Instead, the loss of coal severance taxes has strained local governments and forced public schools to close, reinforcing a message that she believes is false—that coal is the region's only economic alternative. "The Sierra Club and Mike Bloomberg have killed the economy of this culture without any investment whatsoever in it," she said.

Concerns about the Democratic Field

Earlier this year, Bloomberg, 77, a former Republican, indicated that he would focus on new climate initiatives instead of running for president. But in recent weeks, his aides have said he was reconsidering that due to what he perceives as the weakness of former Vice President Joe Biden's campaign.

In announcing his candidacy on Sunday, Bloomberg said, "We cannot afford four more years of President Trump's reckless and unethical actions. He represents an existential threat to our country and our values. If he wins another term in office, we may never recover from the damage," His website lists "a climate crisis that is growing worse by the day" among the problems he aims to fix.

Deval Patrick, the former governor of Massachusetts, also jumped into the race for the Democratic nomination as a late entry this month.

Bloomberg, who filed paperwork with the Federal Election Commission to run as a Democrat, ensured he will be a factor in the race one way or another by launching a $100 million digital campaign to attack Trump in Arizona, Michigan, Pennsylvania and Wisconsin. The ads, which are set to run throughout the primary season, will make him the single biggest spender in the presidential race so far.

Because of his work with the United Nations, with the C40 organization of cities dedicated to climate action, and the Financial Stability Board, Bloomberg has more experience in international climate diplomacy than any other Democratic candidate with the possible exception of Biden. And his experience in governance, as mayor of the nation's largest city from 2002 to 2013, sets him apart from fellow billionaire Steyer, who has no similar record of public service or effort to implement climate policy.

Komanoff gives Bloomberg credit for the groundwork he laid as New York mayor for initiatives like congestion pricing to reduce vehicle traffic in Manhattan—a proposal that was finally approved by the state legislature early this spring. His climate plans while mayor were widely seen as the most ambitious city-level efforts to mitigate and adapt to global warming.

And Komanoff recalls that it was seen as a bold stand in 2007 when Bloomberg came out in favor of a national carbon tax, at a time when most political leaders who favored climate action were still talking about cap-and-trade. But Komanoff, who himself is looking beyond carbon tax advocacy to campaign for a Green New Deal transformation, believes the times call for a different kind of leadership.

"Mike Bloomberg is the perfect climate candidate for 2007," Komanoff said.