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Despite the extensive media coverage of the “landmark” Paris Agreement over the past year, the international effort to curb carbon emissions and adapt to the changing climate is little more than a promise on the lips of world leaders until it becomes legally binding. To make it international law, 55 countries representing 55 percent of global carbon emissions have to ratify it and, in doing so, , which detail how each country plans to limit Earth’s temperature rise to 2°C above pre-industrial level (and ideally keep it below 1.5°C).

This weekend, the ratification tally got a significant boost when the world’s two largest carbon emitters—the United States and China—ratified the treaty. Before the ceremony, 24 countries representing 1.1 percent of carbon emissions were on board; afterwards, the tally jumped to 26 countries representing a whopping 39 percent of carbon emissions.

“History will judge today’s efforts as pivotal,” President Obama after the ratification ceremony in Hangzhou, China. “As the world’s two largest economies and two largest emitters, our entrance into this agreement continues the momentum of Paris, and should give the rest of the world confidence—whether developed or developing countries—that a low-carbon future is where the world is heading.”

Each country has its own process for ratification, and many require legislative support—which means support for efforts that can be politically divisive, such as investing in clean energy, preserving forests, and contributing money to poorer nations. (President Obama ratified the treaty through an executive order to bypass the politically gridlocked U.S. Congress.)

So perhaps it’s not too surprising that the first countries to ratify the Paris Agreement were the ones that contribute little to the climate problem. Combined, the first 24 nations fully on-board with the treaty emit only 1.1 percent of global carbon each year, so reducing their emissions took less (if any) effort. 

But it’s not just ease that brought these countries to the table; many of them also have the most to lose. Of the first 24 ratifying countries, 17 are small Indo-Pacific or Caribbean island nations already facing existential threats from rising seas, as , , and . And as , some of their economies () and food (reef fish) are at risk, too. After this weekend, they no longer stand alone in demanding international cooperation in limiting the extent of climate change.

With the world’s largest carbon emitters on board, for other high-emission governments to ratify—and soon. President Obama and President Xi Jinping of China ratified the treaty just as the world’s major economic powers, which are responsible for 80 percent of carbon emissions, gathered in Hangzhou for a . Talk about dropping a major hint. (Compare the carbon emissions of the top 20 emitters with the 26 countries that ratified the agreement in the chart below.)

When the Paris Agreement was written in December, world leaders planned for it to take effect in 2020, but it’s looking like . That's a good thing: The sooner the treaty is ratified, the better. Once it’s enforceable law, world leaders can start to dig into the more difficult questions, such as how the treaty will be enforced and how the work of transforming our fossil fuel-based economy will actually take place.

Sooner is better for the island nations, too. With some luck, an early ratification could mean the difference between survival and utter devastation for those countries already facing existential threats from climate change. For them, time is running out.