WASHINGTON — It is not unfair to point out that President Trump, on many important subjects, is just an ignoramus.

A vivid illustration of this unfortunate fact came this week in London, when it was revealed that Prince Charles, a knowledgeable environmentalist, had tried to educate the president on climate change — and utterly failed.

“I believe that there’s a change in weather, and I think it changes both ways,” Trump told “Good Morning Britain” host Piers Morgan in an interview broadcast Wednesday. “Don’t forget it used to be called global warming. That wasn’t working. Then it was called climate change. Now it’s actually called extreme weather, because with extreme weather, you can’t miss.”

Good lord, it’s breathtaking that anyone could pack so much ignorance into so few words.

The correct answer for what human-generated emissions of carbon dioxide and other greenhouse gases are doing to the planet is, of course, all of the above. There is indeed global warming — the past five years have been the hottest since record-keeping began, and so much sea ice has melted that shipping lanes are being charted across the Arctic Ocean. There is indeed climate change — this March, temperatures in northern Alaska were 30 to 40 degrees above normal, or what used to be normal. There is indeed extreme weather — scientists have long predicted that deadly weather anomalies, such as the widespread outbreak of tornadoes last month, would become more common as the temperature continues to rise.

Trump said his meeting with Charles was supposed to last 15 minutes but went an hour and a half. One wonders how much of that time Charles must have spent gritting his teeth.

The president did say he admired the prince’s passion and shared his hope for a “good climate as opposed to a disaster.” But Trump also said the United States has “among the cleanest climates,” so it’s unclear that he understands what the word “climate” means. He seems to be talking about smog or litter.

For the record, this country is the world’s second-biggest emitter of carbon dioxide, and our emissions grew last year by an estimated 3.4 percent. But the gas is colorless and odorless, and it has no respect for national boundaries — qualities that perhaps put it beyond the grasp of Trump’s understanding.

Who bears the cost of tariffs is another topic about which Trump has views that are both unshakably settled and spectacularly wrong. China is paying the tariffs he imposed, Trump claims. Companies in Mexico will pay the tariffs he threatens, Trump promises. Yet that simply is not how tariffs work.

Tariffs are taxes, paid by the U.S. firms that import Chinese, Mexican and other foreign products. Those companies pass along those costs to American consumers, in the form of higher prices for foreign-made merchandise. In other words, the money that Trump claims is flowing into the Treasury doesn’t come from Beijing or Mexico City. It comes out of your pocket and mine.

Trump has a right to argue that trade wars and protectionism are good for the U.S. economy and will somehow Make America Great Again (though a lot of his supporters wear knockoff MAGA hats that were made in China). But it’s comical to make such an argument based on a misunderstanding of what a tariff even is.

The president is often wrong but never in doubt, a know-it-all on subjects about which he knows nothing. He is not, for example, any kind of expert on horse racing. Yet when Maximum Security was disqualified in last month’s Kentucky Derby, Trump immediately shot out an authoritative-sounding tweet:

“The Kentuky [sic] Derby decision was not a good one. It was a rough and tumble race on a wet and sloppy track, actually, a beautiful thing to watch. Only in these days of political correctness could such an overturn occur. The best horse did NOT win the Kentucky Derby — not even close!”

Political correctness? About a horse? What’s wrong with the man?

Even more dangerous than Trump’s ignorance is the near-impossibility of changing his mind about certain things. It’s one thing to stick to one’s guns. It’s another thing to stubbornly resist fact and reason — especially when the stakes are so high.

Trump is apparently convinced that acknowledging Russia’s interference in the 2016 election — and taking action to prevent a recurrence — diminishes his victory. Some aides are reportedly not even raising the subject, perhaps out of fear of losing political standing with the president.

Only someone without a clue would fail to realize that he could be the victim of such meddling in 2020. I’m just saying.