‘What will it take to get an agreement in Paris?”

That’s the question on the minds of environmental activists, U.S. State Department officials, and foreign bureaucrats as we approach the latest round of United Nations climate-change negotiations in France’s capital city.

It was also the question asked of Sam Kutesa, the president of the United Nations General Assembly, at an event on April 17, 2015, according to an e-mail obtained through a Freedom of Information Act request.

The author of the e-mail — subject line: “Best answer of night” — was President Obama’s chief climate-change negotiator, Todd Stern. Stern was writing to a colleague to recount how President Kutesa responded to this all-important question.

His answer? One word: “Money.”

To be more specific: lots of money, transferred from technologically advanced countries to developing nations. Known as “climate finance,” this transfer of wealth is supposed to help the developing world make the transition away from the high-emission energy sources on which their emerging economies increasingly rely. And as President Kutesa suggested, money is the key to producing a new climate-change agreement in Paris.

This is the dirty little secret of international climate negotiations, obvious to anyone who devotes a moment of concentrated thought to the technological and economic realities facing developing nations. But the secret is rarely acknowledged outside of private e-mail exchanges.

There are two reasons for this.

First, there is an immense gap between the amount of money that politicians from developed countries have promised to contribute to climate financing and the amount that politicians from developing nations have requested.

The boldest pledge ahead of the Paris talks came from President Obama, when he declared last year that the United States would send $3 billion of American taxpayer dollars to developing countries to help them address the threat of climate change.

This is not a trivial sum of money, especially considering that the United States already spends tens of billions of dollars every year in foreign aid. But it is chump change compared with the total estimated costs required to retrofit the world’s emerging economies to bring them into compliance with the kind of emissions standards necessary to have any meaningful effect on global atmospheric conditions.

India provides a helpful point of reference. In a document outlining their intentions and expectations for the Paris talks, India’s negotiators explained that they would need the rest of the world to pay them between $1 trillion and $2.5 trillion by 2030 to help them adapt. to new emission targets.

And that’s just one country. Looking across the entire globe, the economic cost of reducing greenhouse-gas emissions to the extent recommended by the U.N.’s Intergovernmental Panel on Climate Change would be “beyond astronomical,” as Bill Gates recently put it.

But it’s not just the exorbitant price tag that makes climate finance a taboo topic. The second reason the Obama administration is loath to publicly acknowledge the central role that money will play in the Paris negotiations is that doing so shifts the focus away from the president and toward Congress.

For months, White House and State Department officials have insisted that President Obama has the legal authority to unilaterally commit the United States to a new climate agreement and that therefore he will not submit any deal reached in Paris to the Senate for its advice and consent.

Setting aside the dubious legal theories concocted to justify this position (theories that flatly contradict the understanding of the U.N. Framework Convention on Climate Change that has been universally accepted since its ratification in 1992) there’s one problem with President Obama’s gambit: Only Congress, not the executive, has the power to appropriate money.

Imprudent as it may be, the president is free to promise the world as much money as he pleases in the course of international negotiations. But, without the support of Congress, he is powerless to fulfill those promises.

To ensure that the foreign governments attending the climate talks in Paris are not confused about this point, members of both chambers and from both parties ought to assert with one voice that Congress will not send a dime of taxpayer money to the implementation of any agreement to which the Senate has not provided its advice and consent.

That goes for the billions of dollars that President Obama has pledged to send to the “Green Climate Fund.” And it goes for any other funds that the Paris agreement would expect the United States to give to developing countries for clean-energy adaptation.

To that end, we will soon introduce a concurrent resolution, with more than 25 Senate co-sponsors and more than 40 supporters in the House, expressing the sense of Congress that an agreement of the cost and character contemplated by the Obama administration in Paris must be submitted to the Senate for its advice and consent.

The purpose of this resolution would not be to oppose the president’s plans on the merits, although that’s a debate that we should have. Instead, it would simply clarify that if American taxpayer money is all that it will take to get an agreement in Paris, then it’s also going to require the advice and consent of the Senate.