The centerpiece of President Obama's climate agenda, the Clean Power Plan, ain't pretty.

But the more "elegant" stuff couldn't get passed by Congress, so here we are.

That was essentially one of the messages the head of the Environmental Protection Agency relayed to activists and diplomats at an international gathering in Washington Friday on last year's climate change deal in Paris.

"There are many ways you can place a price on carbon, I think regulation is one of them," EPA Administrator Gina McCarthy said, after being asked what she thought about implementing a nationwide fee, or tax, on carbon dioxide. Such a tax is not available to the agency, she explained.

EPA does not regulate every sector of the economy through its regulations, she explained, but that doesn't preclude the Congress from doing something in the future that does, like a carbon fee.

Obama tried and failed to do something like that in his first term, she said. That's why the U.S. is currently discussing regulations as the best solution to global warming over a national, market-based approach, like a cap-and-trade or tax system.

"The president spent a lot of time in his first term looking at whether or not you could do something broader through the legislature. That didn't happen," she said.

So, in Obama's second term "we just took a look at what authorities are available to us," she said. "That does not mean they're the most elegant of tools available to us."

Nevertheless, the regulations will work, she said, calling them "strong" policy drivers that send signals to the market.

In the future, however, there might be other tools available "that Congress might want to take up," she said. But that's as far as she went with the discussion, so as not to come across as endorsing the idea of a carbon tax.

Bill Nye, the TV celebrity/educator, posed the question to McCarthy on stage at the summit, prodding her on whether she would support the idea.

"So, do you think a tax would be more elegant than the regulations?" Nye asked.

McCarthy appeared to get a little miffed that he was calling her regs ugly and putting her on the spot. She said she couldn't respond to the question, but added that she thought the climate rules her agency devised are elegant in their own right.

Nye explained that some think a carbon tax across the board would be a better, more fair approach to reducing emissions. But others say it would be monumentally intrusive, he said.

"Elegance is in the eye of the beholder," Nye quipped. McCarthy said, "It is. It is."

"I actually think the Clean Power Plan is pretty elegant," she said, cutting off Nye from asking a follow-up question. She then went down the list of all the rules the agency has put out, including greenhouse gas regulations on cars and pick-ups, big rig trucks, aircraft, ozone layer-killing hydrofluorocarbons, and oil and gas facilities.

"All those to me are my elegant tools, and I'm making them available and I'm working on them," she said.