US Secretary of State John Kerry delivers a speech in Lima on December 11, 2014, during the UN 20th session of the Conference of the Parties on Climate Change and the 10th session of the Conference of the Parties serving as the Meeting of the Parties to the Kyoto Protocol being held from December 1st to 12th in Lima. AFP PHOTO/CRIS BOURONCLE (Photo credit should read CRIS BOURONCLE/AFP/Getty Images)

Hopes were high. The need is urgent. But the United Nations climate talks that just wrapped up in Peru yielded disappointingly little progress toward the strong and fair international agreement we must have to preserve a livable planet.

Yet there is one silver lining: The goal of ending essentially all fossil fuel use by 2050 surfaced during the talks and gathered tremendous momentum.

A critical negotiating document included as one option a plan for ending virtually all man-made carbon dioxide emissions by mid-century. That requires giving fossil fuels the heave-ho.

That powerful "zero emissions by 2050" idea is gathering a huge head of steam. Dozens of governments jumped on board. Actor Leonardo DiCaprio pushed it on social media. And a group of Catholic bishops just add their voices to the growing chorus, calling on government officials around the globe to end fossil fuel use.

But one crucial person hasn't weighed in: U.S. Secretary of State John Kerry.

Sec. Kerry has rightly noted that the world is "on a course leading to tragedy" because of our failure to take meaningful action. A disturbing new report from U.N. experts warns that global warming will create food shortages, cause mass wildlife extinctions, and inflict terrible suffering on some of the world's poorest people.

The Obama administration, however, has hung back far too much in the struggle to craft an international agreement.

President Obama has certainly taken stronger domestic action in recent years against climate change. But even the EPA's much hyped "Clean Power Plan" does far too little to cut power plant pollution.

And instead of leading the way at the international level, the United States seems bent on pushing other countries out in front. That's unworkable and morally outrageous given how badly our government has failed to live up to its prior climate promises.

To understand how wretchedly America has behaved on climate issues, you have to look back to 1992, when the U.N. Framework Convention on Climate Change was negotiated in Rio de Janeiro.

In the Framework Convention, signed and ratified by the United States, the world agreed to take the actions necessary to avoid dangerous climate change.

And, crucially, the United States and other countries party to the convention also agreed as a matter of fairness that the world's rich, developed nations, having caused the vast majority of emissions responsible for the problem, would take the lead in solving it.

But then came two decades of backtracking by U.S. leaders who rejected the equity principles behind the convention. The United States decided it shouldn't agree to reduce its own emissions unless all other countries -- regardless of their responsibility or ability -- are similarly bound.

That ethically appalling idea was advanced by Obama administration negotiators. They also successfully worked to push back the date for a new international agreement on emissions cuts.

But climate science shows that delay equals disaster, and climate justice demands that the United States lead the way.

To have a chance at staving off global warming's worst effects, we need carbon emissions to peak around 2020 and then decline very rapidly. And America has to take major responsibility for cutting emissions and helping other nations do the same.

Any other course will yield bitter consequences. Consider, for example, that China's rapidly rising emissions happened in large measure because we reneged on our 1997 commitment to join the Kyoto Protocol and help fund clean energy development there.

With 2014 on track to be the hottest year on record, it's clear that we can't afford to repeat such mistakes as we move toward the Paris climate summit next year.

That's why the "zero by 2050" idea is such an exciting development. We need Sec. Kerry to endorse it. And we need a fair plan to end fossil fuel use that reflects the developed world's disproportionate responsibility for the dangerous state of our warming world.

This is not only what we need to preserve a safe climate, but it will also provide relief to communities around the globe who are fighting fracking and other new and dangerous fossil fuel extraction methods that poison our air and water and make us sick.

As the bishops' recent statement rightly explains, the world must "adopt a fair and legally binding global agreement" that moves the world quickly away from fossil fuels and toward "100 percent renewables with sustainable energy access for all."

Now there's something we can all get behind. Sec. Kerry should heed those wise words. Only urgent and equitable action will give us a shot at protecting our planet from climate chaos.