President Donald Trump’s announcement on Thursday that the United States will withdraw from the Paris climate agreement may prove to be the biggest miscalculation of his fledgling presidency. It appears to be driven not by an outright denial of climate change but — like his view of NATO, immigration, trade and so many other subjects — by an overheated nationalistic view that foreign countries are taking advantage of the U.S. It is applying the “America First” ideology to an issue where it is least applicable, where mutual cooperation is essential and countries share a common interest — ensuring the planet remains fit for human habitation.

After so much buildup prior to the announcement, how depressing that the president demonstrated so little understanding of the climate accord, of the threat posed by global warming, of the economic opportunities offered by green energy and of the astronomical cost of abandoning the agreement. There are at least two possible reactions by the rest of the world — either other major greenhouse gas polluters will similarly abandon the agreement and by doing so speed up the timetable toward disaster, or the U.S. will be treated as a kind of climate pariah (which is hardly a stretch given that only Syria and Nicaragua occupy the same status of refusing the Paris accord at the moment).

Perhaps the president believes there is no economic cost to this decision. If so, he is sadly mistaken. Mr. Trump may be capable of seeing the loss of jobs in coal mines or the paper, cement and steel industries that comes from reducing carbon dioxide emissions, but he fails to appreciate how sea level rise, worsening weather, loss of arable land and drinking water supplies, mass refugee migrations and other effects will dwarf the cost of the much-maligned $100 billion “green fund” to help less developed countries meet their energy needs without expanding their carbon footprints. Does he seriously believe other countries will simply submit to his offer to renegotiate the accords to the advantage of the U.S. merely to accommodate a presidential whim? It was a miracle that India and China have come as far as they have in the first place.

But what truly makes this a miscalculation is that the vast majority of Americans recognize the danger posed by climate change, and they want something to be done about it. They don’t support backing out of the Paris agreement. Even in Pittsburgh, a city twice mentioned by President Trump during his Thursday speech as a place he favors “over Paris,” a majority of residents (68 percent, according to a Yale opinion poll conducted last year) say global warming is happening. About half of those polled in Pittsburgh believe it’s already hurting people in the U.S. That doesn’t sound like a community that sees a climate treaty as a “self-inflicted major economic wound.”

Even the far right can’t be especially satisfied with Mr. Trump’s position given that he has offered to either renegotiate the agreement or pursue an “entirely new transaction on terms that are fair to the United States.” In other words, the president admits that climate change is real, that it’s a threat and that extraordinary action is required; he just doesn’t want to pay so much for the solution. It is the philosophy of a real estate developer, not a statesman, scientist or even someone who cares about the country’s future.

The decision could prove especially adverse for Maryland, a coastal state on the cusp of another hurricane season. It has already been estimated that Maryland is in danger of a one-foot sea level rise by 2050 and as much as three feet by 2100, which means flooding, severe weather and a loss of safe drinking water in low-lying regions is a near certainty. Waterfront towns such as Ocean City would be profoundly impacted, perhaps even made uninhabitable.

Mr. Trump says “We will see if we can make a deal that’s fair, and if we can, that’s great. If we can’t, that’s fine.” But the Paris accord wasn’t just some “transaction,” it is a matter of life and death.