Later this month, hundreds of delegates will gather inside the U.N. to talk about climate change. President Barack Obama plans to attend the climate summit and reportedly wants work on a deal with other world leaders to “name and shame” countries that aren’t actively pursuing serious climate action.

But outside the U.N., thousands of activists will be protesting with one message: Whatever Obama accomplishes at the U.N., it won’t be enough to save his climate legacy.

The Obama administration has been tough on coal, directing the Environmental Protection Agency to severely limit the amount of CO2 that power plants are allowed to emit. But at the same time, the administration has embraced natural gas. Environmentalists say that embrace has created a chasm between Obama’s rhetoric and his climate-fighting actions.

That’s because a growing body of scientific evidence that shows gas development produces significant amounts of methane, a greenhouse gas many times more potent than carbon dioxide.

At the People’s Climate March on Sept. 21, activists say they’ll be pressuring the president to address his support of oil and gas. If he doesn’t, they say, he risks squandering his entire environmental record.

“He’s hoping that by killing coal and replacing it with natural gas, he’s coming out a winner, but the science is increasingly saying that’s not going to be the case,” said Anthony Ingraffea, an engineering professor at Cornell and a prominent hydraulic fracturing critic. “At best, his strategy means we’ll break even, but over decades. The Climate March is saying we don’t have decades.”

For many environmentalists, there seem to be two Obamas:

There’s the one who has pushed hard to bring a dialogue about climate change to the forefront of U.S. politics in a way no president before him has.

"The question is not whether we need to act [on climate change],” Obama said in a speech in June. “The question is whether we have the will to act before it's too late.”

The other Obama has come out hard for natural gas development.

"If extracted safely, it's the bridge fuel that can power our economy with less of the carbon pollution that causes climate change,” he said at his most recent State of the Union Address.

The problem, environmentalists and scientists say, is that natural gas isn’t a green option in the same way renewable energy is.

“It’s not a bridge. It’s a treadmill,” said Sandra Steingraber, an environmental science professor at Ithaca College and an anti-fracking activist. “It remains to be seen if gas is slightly better or slightly worse than coal, but either way, it’s not better enough. We’re still going off a climate cliff.”

Natural gas is mainly made up of methane, and methane can leak at nearly every step of the gas production process, from drilling to pipeline transportation to waste disposal. While there’s a lot less methane going into the air than carbon dioxide, the gas is, by U.N. estimates, up to 84 times more effective at trapping heat.

Hypotheses vary as to how much methane leaks into the atmosphere during the hydraulic fracturing process, but perhaps the most highly cited research on the matter, conducted by Robert Howarth and Ingraffea, suggests that 5 percent of the produced methane from fracking wells escapes.

According to the Environmental Defense Fund, natural gas production is better for the environment than coal only if less than 2.7 percent of the gas leaks before reaching its final destination.

The EPA is considering whether to require natural gas producers to cut methane emissions or to work on getting the industry to volunteer to work on emission cuts.

But environmentalists and scientists point out that 2.7 percent is just the baseline at which natural gas becomes a better environmental solution than coal. That means that even if leaks are reined in, it would still take decades for a complete switch from coal to natural gas to pay off.

And, critics point out, the infrastructure being built to support natural gas development — billions of dollars’ worth of pipelines, drilling rigs, refineries, and export terminals — signals that without policy changes, the industry is quickly establishing roots that will last far longer than Obama’s theoretical bridge. The White House has recently approved several multibillion dollar natural gas export facilities that will ship fracked natural gas to other countries.

“You can’t talk about exporting natural gas and say you’re tackling climate change,” said Wenonah Hauter, the founder of environmental nonprofit Food and Water Watch. “It doesn’t really add up.”

Climate activists acknowledge that Obama has spoken about climate change more than any of his predecessors. And they say they acknowledge that gridlock in Washington has made any action on climate change harder.

But without seriously tackling the methane issue, activists say, his administration will be a severe disappointment to the environmental movement.

“When I drive by these drilling operations, there are these flares 50 feet high, burning methane 24 hours a day,” said Madeline Stano, a staff attorney at the Center on Race, Poverty and the Environment, which works with low-income communities in heavily drilled Kern County, California. “If that’s his climate policy, we’re not in a good spot.”