French Foreign Affairs Minister Laurent Fabius, center, and Christiana Figueres, left, executive secretary of the U.N. Framework Convention on Climate Change, react during the final plenary session at the World Climate Change Conference 2015 (COP21) on Dec. 12. (Stephane Mahe/Reuters)

REPRESENTATIVES OF nearly 200 countries on Saturday reached a climate change agreement that was two weeks, and decades, in the making. The Paris agreement is a landmark in the world’s response to manmade climate change, with every nation that matters acknowledging the problem and pledging to respond. But the hard work lies ahead.

Opponents, including many in Congress, can no longer claim that action is pointless because big emitters such as China will never cooperate. What negotiators in Paris achieved may prove analogous to the General Agreement on Tariffs and Trade, which began modestly and over decades steadily ratcheted up ambition among nations to liberalize international trade.

At its core lie pledges from 187 countries to cut their emissions or projected emissions of greenhouse gases, the drivers of global warming. Though the most significant piece of the deal, these pledges were the least contentious point of negotiation. The commitments were decided in world capitals sometimes months in advance. This is what gives them force: They reflect established emissions-cutting policies that countries such as the United States, China and European Union nations are already implementing.

Taken together, the pledges will get the world a bit short of halfway to the medium-term emissions reductions necessary to limit global temperature increase to 2 degrees Celsius. That’s serious progress. But it is still far from where the world needs to get, as negotiators acknowledged. After days of wrangling, they agreed that the world should limit warming to “well below 2 degrees.” This is a fine ambition, but it may turn out to be little more than wishful thinking.

To contain warming even to 2 degrees, countries must continually increase their emissions-cutting commitments. As much as anything else, the Paris agreement is about setting ground rules for future pledges. Nations will be expected to report on their emissions levels regularly. By 2023, and every five years thereafter, negotiators will gather to “take stock” of how countries are doing — and how much more they need to contribute.

The Paris agreement also enshrines the principle that countries should be held to their promises. The principle remains controversial, particularly whether nations will submit to in-country reviews. Negotiators agreed that countries will undergo “technical expert review,” leaving some limited but still-to-be-worked-out “flexibility” for developing countries, if they can show they need it. There’s little good reason for “flexibility” on transparency, and it’s important that major emitters such as China not be able to wriggle out of accountability. But technology might simplify this issue: Measuring various countries’ emissions will probably soon be done by satellite.

Important elements of the rules are still to be worked out. But the basic structure of the world’s response to global warming is taking shape. Now, the increasingly isolated critics of climate action will have to explain not only why they reject science but also why they would harm U.S. standing in the world by seeking to slow the progress so many countries are making. To save the earth from terrible injury, the United States should instead be leading, as it has so often done in the past.