Why is this 15-year-old suing Obama?

“Talk is cheap,” says Xiuhtezcatl Tonatiuh, in response to President Barack Obama’s rousing speech at the opening day of the United Nations summit on climate change Monday.

Xiuhtezcatl, barely old enough to drive in his home state of Colorado, is one of 21 young activists ages 8 through 19 in a lawsuit against the Obama administration that says it has not done enough to combat climate change.

Teaming up with James Hansen, the climate scientist whose 1988 congressional testimony helped make "global warming" a household term, the young plaintiffs are backed by a team of legal experts at Our Children’s Trust. The nonprofit organization has initiated climate-related lawsuits in all 50 states. On Tuesday, for instance, a North Carolina judge struck down on 13-year-old Hallie Turner’s petition for stricter emission standards.

This particular case, so far, is the group's most prominent.

In early November, three of the biggest trade groups from the fossil fuel industry requested to join the defendant’s bench with Mr. Obama, deeming the lawsuit “a direct threat” to their line of work.

To the activists, this was an auspicious sign.

“It’s good news for us that they’re doing this. They see this as a legitimate case,” Julia Olson, the lead attorney from Our Children’s Trust, told Slate last week.

But whether the kids have legal standing is another question. Analysts say it proposes a constitutional challenge regarding intergenerational equity, posing the questions, does the US government have an obligation to protect the interests of future Americans by controlling current resources, and does the Constitution grant the government this legal right?

Without a Supreme Court ruling, which experts say is unlikely, the answers remain hazy. But for Xiuhtezcatl, the necessity for action is as clear as day.

"The reason we are fighting for this is because of the world we want to grow up in, and the world we want our children and grandchildren and great-grandchildren to grow up in," he tells CNN’s John Sutter. "This is not a selfish cause. We're not politically invested, we're not financially invested.”

And the only way to achieve that ideal, clean world, he adds, is full abstinence from fossil fuels. Obama’s recent pledge to cut carbon emissions and promote clean energy, therefore, will fall short.

"Over the last seven years, we've made ambitious investments in clean energy and ambitious reductions in our carbon emissions. We've multiplied wind power threefold, and solar power more than twentyfold, helping create parts of America where these clean power sources are finally cheaper than dirtier, conventional power,” Obama said at the summit Monday.

But Xiuhtezcatl says he’s heard it all before – and that it won’t be enough.

“We've all seen that speech,” he says. “Talk is cheap. I wanna see concrete action and concrete promises from our country – and commitments for action on climate change."

The COP21 summit has brought more than 140 world leaders together in Paris to forge a global plan of action tackling climate change, not only with standards of emission but also methods of enforcement. It’s an ambitious goal, considering every similar summit prior to this one has failed.

For his part, Obama is not blind to the impact the summit and the agreements that result from it will have on the lives of young people.

"Let there be no doubt," Obama said in his remarks at the first COP21 session, "the next generation is watching what we do. ... I want our actions to show that we're listening."