On Earth Day, 175 world leaders met at the United Nations to sign the Paris Agreement, a historic pact to curb the carbon emissions behind climate change. That's the largest one-day signing of an international agreement in history, topping the 119 countries that signed the U.N. Convention on the Law of the Sea in 1982. Several speakers kicked off the ceremony by describing what's at stake, culminating with an urgent, cogent plea from new Academy Award-winner Leonardo DiCaprio.

"Yes, we have achieved the Paris Agreement. More countries have come together to sign this agreement today than for any other cause in the history of humankind, and that is reason for hope," DiCaprio said, praising the first international treaty that commits both developed and developing nations to cut greenhouse gas emissions. "But unfortunately the evidence shows us that it will not be enough. Our planet cannot be saved unless we leave fossil fuels in the ground where they belong.

"An upheaval, a massive change, is required right now," he added.

The Paris Agreement certainly is reason for hope, representing the pinnacle of U.N. climate talks that date back more than 20 years. It's our best chance so far to rein in climatic chaos that has already begun, from droughts and storms to famine and disease that threaten both human and ecological health. Yet as DiCaprio and other speakers noted, Friday's signing ceremony is a beginning, not an end.

"You know that climate change is happening faster than even the most pessimistic of scientists warned us decades ago. It has become a runaway freight train bringing with it an impending disaster for all living things," DiCaprio said. "Think about the shame each of us will carry when our children and grandchildren look back and realize we had the means of stopping this devastation, but simply lacked the political will."

By signing the Paris Agreement, countries are signifying their intent to ratify it. A few have already done so, but the accord won't become international law until it's ratified by at least 55 countries representing 55 percent of global greenhouse gas emissions. And even then, its success will hinge on how it's implemented in various countries around the world. And therein, DiCaprio said Friday, lies the rub:

"We can congratulate each other today, but it will mean absolutely nothing if you return to your countries and fail to push beyond the promises of this historic agreement. Now is the time for bold, unprecedented action. My friends, look at the delegates around you. It's time to ask yourselves which side of history you will be on."

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.