PHILADELPHIA — The day Hillary Clinton announced her plans in July to combat global warming, she was caught flying a greenhouse gas-spewing private plane — and her campaign quickly promised it would make amends by paying to offset whatever her campaign emits henceforth.

On Thursday, a year to the day after her aides made the pledge, Mrs. Clinton will accept her party’s presidential nomination at the Democratic National Convention in Philadelphia, but there is no evidence her campaign has done anything to live up to its pledge on greenhouse gases.

None of her campaign finance records shows any payments to reputable offset companies, and she hasn’t detailed any other steps she has taken to go carbon-neutral. Her campaign declined to answer specific questions about her carbon behavior.

It’s an embarrassing omission for a candidate who has highly touted her climate leadership while secretary of state and has made global warming a central issue of this week’s party gathering.

“Our country is ready to tackle the challenge of climate change. Together we can make America the world’s clean-energy superpower,” Mrs. Clinton said in a video at the convention Wednesday, insisting that she can bend the world toward a low-carbon economy.

Mrs. Clinton’s failure to pay for offsets is all the more surprising because she made a similar pledge in her 2008 campaign and followed through on it. She paid $80,016.85 to Native Energy, a leading offset provider, and listed the expense as “office utilities” in her campaign reports that year.

Under the theory of offsets, an organization calculates the amount of greenhouse gas emissions it pumps into the air from activities — known as its “carbon footprint” — and then pays for someone else to prevent that much in emissions or to take them out of the atmosphere. Options include planting trees that absorb carbon dioxide and capturing methane gas emitted from landfills.

There is no evidence that Republican presidential nominee Donald Trump is offsetting his carbon emissions. But Mrs. Clinton’s opponent during the Democratic primary contests, Sen. Bernard Sanders, did.

Mr. Sanders’ campaign records show three payments this year to Native Energy, totaling $14,193.

Michael McKenna, an energy lobbyist and Republican Party strategist, said Mrs. Clinton’s failure to live up to her pledge was likely an oversight.

But he said making good on her pledge could raise even more uncomfortable questions for her. The size of her offset payments, which she would have to report on her campaign finance filings, would give a sense for just how much carbon her campaign is spewing.

“My guess is she’s got the second-largest carbon footprint of anyone on the planet,” Mr. McKenna said — adding that the president, with his heavy security and massive staff that attend to him, likely has the largest footprint.

Environmentalists have clammed up about Mrs. Clinton’s broken pledge.

The League of Conservation Voters, which endorsed Mrs. Clinton, and whose president, Gene Karpinski, will speak at the convention Thursday, didn’t respond to a request for comment. Nor did the league answer questions about the convention’s own energy decisions such as counting on nuclear power.

Still, in a statement announcing his speaking role, Mr. Karpinski praised the convention for making global warming “a top priority.”

Offsets were all the rage in the latter part of the last decade, and the 2008 Democratic convention in Denver made a major push to go carbon-neutral. It contracted with Native Energy to offset the emissions from its operations and even challenged delegates to pay for offsets for their own activities.

Now, offsets are heatedly debated within the environmental community, with some saying they are a distraction.

But Craig Ebert, president of Climate Action Reserve, a top carbon offset registry that has worked with major organizations over the years, said he does recommend offsets as part of a broader strategy.

For most organizations, he recommends first looking at ways to reduce carbon emissions. But that’s tough for campaigns, where air travel and massive motorcades, consuming lots of fossil fuels, are necessities.

“It’s not that it can’t be done, but it strikes me there’s a limited slate of options an organization can undertake, so you’re likely to face some need to offset,” he said. “That raises the question of how deep a dive do you go. You do travel, then maybe you consider the impact of hotel stays when you’re away from home. What about the vehicles in the cities you’re visiting?”

He said a campaign might wait until it nears its end before paying for its offsets, because then it would have a better sense of its overall carbon footprint.

But there are pitfalls to that approach. Two campaigns that promised to offset their emissions in 2008 ended up suspending the payments when they ran out of money.

The convention in Philadelphia has vowed to go carbon-neutral, with Exelon, the company providing energy for the Wells Fargo Arena and other Democratic offices and venues, vowing to purchase credits to offset the gases emitted during operations.

“Exelon will procure a combination of Renewable Energy Certificates and Emission-Free Energy Certificates from nuclear, solar, wind and hydro generation sources to counterbalance the convention’s carbon footprint,” said Judith Rader, a company spokeswoman. “Purchase of the certificates supports existing zero-carbon generation and helps incentivize the development of new projects.”

She said the company will conduct an audit after the convention ends to see what was used and what offsets are needed.

But even the choice of offsets is controversial within the environmental community because it relies on nuclear energy. While nuclear power doesn’t emit greenhouse gases, the waste it produces is an environmental headache.

“I would not recommend that, if I were working for a client, as the best strategy for lowering its carbon footprint,” said Mr. Ebert. “You’re trying to make an environmental statement, and there are obviously a lot of environmental questions about [nuclear].”

Energy companies, though, say nuclear will have to be part of any low-carbon economy.

“Nuclear energy accounts for more than 60 percent of the zero-carbon energy produced in the U.S. — far more than any other source. And unlike many other sources of zero-carbon energy, nuclear can supply reliable power 24 hours a day, seven days a week,” Ms. Rader said.

“It would be far more difficult and expensive to meet the nation’s targets without it. Continuing to support nuclear energy will help us meet our climate goals at the lowest cost to consumers, while also ensuring reliability of the power grid.”

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