You wouldn’t think Al Gore and Donald Trump would have much to talk about, given their political divisions. Yet that’s exactly why the former vice president recently got in touch with President Trump: to urge him not to withdraw from the Paris Agreement.

“Paris Agreement” is shorthand for a United Nations pact that the Obama administration joined last year. The agreement requires the countries that join it to submit plans for how they’ll cut greenhouse gas emissions. Not surprisingly, Mr. Gore heavily favors what he calls “a bold and historic agreement.”

You might think Mr. Gore’s plea couldn’t possibly succeed, but Mr. Trump’s decision on this issue isn’t exactly a slam-dunk. It isn’t just Mr. Gore and other environmentalists who think the United States should stay in the agreement. Senior Adviser to the President Jared Kushner, for example, thinks the U.S. should stay. So does Ivanka Trump and Secretary of State Rex Tillerson, as well as multinational corporations such as Starbucks and Exxon Mobil.

Some Republican lawmakers also say the United States should remain a party to the deal (albeit with a weakened pledge to cut emissions). The emissions targets aren’t legally binding, after all, so why not keep a seat at the table? No point inflicting diplomatic damage unnecessarily, they argue.

With all due respect, they’re mistaken. Mr. Trump should withdraw from the Paris Agreement. Doing so would underscore a simple truth that needs to be said loud and clear: The agreement is a sham deal, and no amount of pretense or diplomatic wrangling will change that.

To understand why, set aside any preconceived positions, pro or con, on the issue of climate change and look at the deal itself. It’s an extremely costly and ineffective way to address the issue — as even some of its supporters seem to realize.

The stated goal of the Paris Agreement is to limit global warming to 2 degrees Celsius above pre-industrial levels. The more than 170 countries that have signed on are supposed to do this by reducing their carbon-dioxide emissions and relying more on renewable sources of energy.

The stated goals for the United States, as submitted by the Obama administration, aimed to cut greenhouse gases by 26 to 28 percent below 2005 levels by 2025. That’s pretty ambitious. And it won’t be cheap.

As economist Nicolas Loris and U.N. expert Brett Schaefer recently noted, “The U.S. regulations alone would increase energy costs for U.S. families and businesses, causing an overall average shortfall of nearly 400,000 jobs and total income loss of more than $20,000 for a family of four by the year 2035.”

The cost to the global economy, of course, will be much worse: trillions of dollars over the next 80 years. And here’s the real kicker: Even if every country meets their stated goals (which is very unlikely), it will bring only the most minuscule reduction in warming — so little, quite frankly, that it will hardly be noticeable.

Don’t just take my word for it. Consider what former Secretary of State John Kerry has said: “The fact is that even if every American citizen biked to work, carpooled to school, used only solar panels to power their homes, if we each planted a dozen trees, if we somehow eliminated all of our domestic greenhouse gas emissions, guess what — that still wouldn’t be enough to offset the carbon pollution coming from the rest of the world.”

To call this a flawed deal is the understatement of the year. It’s all pain, no gain. No wonder President Obama didn’t submit the deal to the Senate for approval (as he should have). He must have known lawmakers would reject it.

We still can. If President Trump wants to deliver the kind of economic growth he promised on the campaign trail, it’s time to jettison this bad deal.