Not a word has been said in the presidential debates about what may be the most urgent and consequential issue in the world: climate change.

President Obama understands and accepts the scientific consensus that the burning of fossil fuels is trapping heat in the atmosphere, with potentially catastrophic long-term effects. Mitt Romney’s view, as on many issues, is pure quicksilver — impossible to pin down — but when he was governor of Massachusetts, climate change activists considered him enlightened and effective.

Yet neither has mentioned the subject in the debates. Instead, they have argued over who is more eager to extract ever-larger quantities of oil, natural gas and coal from beneath our purple mountains’ majesties and fruited plains.

“We have increased oil production to the highest levels in 16 years,” Obama said in Tuesday’s debate. “Natural gas production is the highest it’s been in decades. We have seen increases in coal production and coal employment.”

Romney scoffed that Obama “has not been Mr. Oil, or Mr. Gas, or Mr. Coal,” and promised that he, if elected, would be all three. “I’ll do it by more drilling, more permits and licenses,” he said, adding later that this means “bringing in a pipeline of oil from Canada, taking advantage of the oil and coal we have here, drilling offshore in Alaska, drilling offshore in Virginia, where the people want it.”

If this is a contest to see who can pretend to be more ignorant of the environmental freight train that’s barreling down the tracks toward us, Romney wins narrowly.

Obama does acknowledge that his administration has invested in alternative energy technologies, such as wind and solar, that do not emit carbon dioxide and thus do not contribute to atmospheric warming. But he never really says why, except to say he will not “cede those jobs of the future” to other nations such as China and Germany.

Romney, on the other hand, claims to pledge heart and soul to an idea that he, as a successful businessman, must know is ridiculous: “North American energy independence.” The notion seems to be that all the oil and natural gas we need can be produced in the United States, Canada and Mexico, and that achieving this continental “independence” will magically cause energy prices to fall.

This is silly. At current production levels, relying solely on good old “North American” oil would leave us more than 30 percent short of what we now consume, and no amount of drilling and despoiling could close that gap. Moreover, the price of oil is a global price — a barrel costs the same whether it’s extracted in North Dakota or the North Sea.

Natural gas is harder to transport over long distances, which means the price is more local. But we’re already moving faster than prudence would advise — through the technology of hydraulic fracturing, or “fracking” — to pump huge quantities of natural gas, and the price is already quite low.

As for coal, Romney was once more of an environmentalist than Obama; as the president noted Tuesday, Romney once stood in front of the Salem Harbor coal-fired plant in Massachusetts and said, “I will not create jobs or hold jobs that kill people, and that plant — that plant kills people.” Now, however, Romney says he is ardently pro-coal and claims that Obama isn’t.

But Obama has long been a champion of so-called “clean coal” technology, which many environmentalists believe is an oxymoron. From the point of view of limiting carbon emissions, burning more coal is the worst thing you could do.

Why does it matter that nobody is talking about climate change? Because if you accept that climate scientists are right about the warming of the atmosphere — as Obama does, and Romney basically seems to as well — then you understand that some big decisions will have to be made. You also understand that while there are some measures the United States could take unilaterally, carbon dioxide can never been controlled without the cooperation of other big emitters such as China, India and Brazil. You understand that this is an issue with complicated implications for global prosperity and security.

A presidential campaign offers an opportunity to educate and engage the American people in the decisions that climate change will force us to make. Unfortunately, Obama and Romney have chosen to see this more as an opportunity to pretend that the light at the end of the tunnel is not an approaching train.