It is understandably agonizing for other countries to contemplate in any way condoning the idea of the United States weakening its climate commitment under the Paris Agreement. At the G7 summit later this week, other leaders are hoping President Donald Trump will heed the rising calls to keep the U.S in Paris.

But they worry: At what cost?

The unfortunate reality is that – even with a weaker target – we’re all better off with the U.S. in than out.

Getting countries on the same page in Paris was a monumental achievement, in diplomatic terms and, more importantly, in planetary terms. Simply put, the Paris Agreement gives us our best shot ever at avoiding calamitous global warming.

It does that by instilling greater confidence among countries that each is willing to do its part – and is committed to doing more in the years ahead. The agreement is hardly a guarantee. Achieving its goals will take sustained and growing effort at every level. But that effort is easier to muster knowing others are contributing their fair share.

The threat of the United States lowering its target – by all indications, the only scenario under which it would stay in – puts that basic bargain at risk.

As a matter of law (see our analysis), Paris does not preclude a party from lowering its target. The agreement is a mix of binding commitments (these are largely procedural) and normative expectations. Its exhortations to parties to progressively strengthen their efforts – and to adjust their existing targets only upward – fall into the latter category.

That said, it’s hard to conclude that a decision by the world’s largest economy to weaken its target would be in keeping with the spirit of the Paris Agreement.

Why, then, should other countries go along?

The blunt – and undiplomatic – response is that it’s not up to them. The better response is that the agreement’s objectives are ultimately best served by keeping the U.S. on board.

For the moment, the extraordinary momentum built in Paris is still going strong. Both China and India now appear on track to meet their Paris targets ahead of schedule. U.S. emissions, meanwhile, reached a 25-year low last year, and growing state, local, and business efforts will help make up for some of Trump’s rollback of federal measures. This is especially true where market forces are already driving down emissions from power generation.

But this is just the start of the wholesale transformation of energy and transportation systems that will be required over the coming decades to even come close to the Paris goal of keeping warming below 2 degrees Celsius.

Economic competitiveness can work in our favor, as countries and companies vie for their share of the growing clean energy market. Indeed, enhanced U.S. competitiveness is one of the benefits cited by the major companies that signed recent full-page ads sponsored by C2ES and other efforts urging the president to stay in Paris.

But competitiveness is double-edged: There are in fact costs to reducing emissions. And some governments will invariably be less willing to force their industries to absorb them if their U.S. competitors are not.

The risk is not that other countries will quit Paris if the U.S. does – why expose themselves to certain international censure? The real risk is that they will be less diligent in meeting their existing targets, which after all are not binding, and will be less ambitious than they otherwise would be in the next round of targets due in 2020.

There’s no denying there would be similar risks if the U.S. were to stay in but weaken its target. Some countries might be tempted to follow the U.S. example and relax their own goals. The Paris framework would be frayed. But we face a far greater unraveling if the U.S. withdraws outright.

The United States is not only the world’s largest economy but also, cumulatively, its largest greenhouse gas emitter. While China is now the top annual emitter, as far as the atmosphere and other nations are concerned, no country bears greater responsibility for climate change than the United States. To now detach itself from the global compact it helped forge would leave a leadership vacuum no other nation could fill.

Technically, it’s true, the United States could later rejoin. But that wouldn’t be something we could count on. Politically, Paris could be irrevocably damaged in U.S. eyes. And without U.S. diplomats at the table, the Paris implementing rules now being negotiated wouldn’t reflect U.S. concerns.

It is unfair to ask the rest of the world to, once again, accommodate the vagaries of U.S. climate politics. The bitter reality, though, is that having pledged to “cancel” the Paris Agreement, the only politically viable pathway for the president to now stay in is to show he has secured a “better deal.”

The Paris Agreement was designed to be durable. No one expected the first major test of its resilience would come so soon. Other countries cannot be expected to welcome or endorse a weaker U.S. target. But by simply acknowledging rather than rejecting it, they can help keep an unfortunate setback from becoming an utter disaster, and can help preserve the fundamental framework that so many worked so hard to create.