Every year, the heads of the key U.S. intelligence agencies take part in a ritual called the worldwide threat brief, testifying before Congress at an unclassified level about the biggest perils facing the nation. In general, we know what to expect: warnings on China, Russia, Iran, North Korea, cybercrime, terrorism and other usual suspects.

But this year things turned out very differently, in two ways. First, the debate was hijacked — appropriately — by the uproar over the inadvertent release to a journalist, through the communications app Signal, of details concerning strikes against Houthi terrorists. The director of national intelligence, Tulsi Gabbard, and CIA Director John Ratcliffe spent much of their time parrying tough questioning from Democratic senators.

The second difference was something that was not brought up as a threat to the U.S., although it has featured prominently for more than a decade in the annual report: climate change. Its absence came as something of a surprise, even if President Donald Trump has long denied the validity of climate-related dangers and has been slashing jobs at the National Weather Service and National Oceanic and Atmospheric Administration.

It was a shift for the Republicans because the DNI during Trump’s first term, Dan Coats, did include climate in the worldwide threat reports. But this year’s document dropped any mention of the deterioration of the environment. Evidently, none of the agencies represented — DNI, CIA, FBI, National Security Agency, Defense Intelligence Agency — deemed the direct threat sufficient. During questioning by Sen. Angus King of Maine, an independent, Gabbard indicated that she had not directed the various agencies to drop climate change.

Has the intelligence community simply stopped thinking about the environment as a security challenge? Does this matter? Is climate change really a national security threat?

I believe it is, as do many senior national security analysts and officials from both parties. There are four key dangers we should track closely.

First is instability in vulnerable regions of the globe stemming from major droughts, massive storms and melting ice packs. When I was the military commander of NATO, we watched the worst drought in nearly a millennium destroy crops across the Middle East from 2009-2012. It helped spark the Arab Spring risings and catalyzed the Syrian civil war, which has caused millions of deaths and tens of millions of displacements. The military-trained meteorologists on my staffs at NATO and at U.S. European Command were closely monitoring the effect of climate-caused devastation throughout the Levant.

Another example of increased instability can be found at the top of the world. The Trump administration is foolish for threatening to occupy Greenland militarily, but right about the island’s immensely important geographic position, astride the increasingly open shipping routes at the entrance to the Arctic Ocean. As global warming causes melting ice, we will see a geopolitical thunderdome emerge in the High North. The chances of conflict between Russia, which has the largest footprint in the Arctic, and the seven NATO nations on the other side — Canada, U.S., Iceland, Norway, Sweden, Finland and Denmark (through Greenland) — is rising as assuredly as the temperatures.

Then there are rising sea levels. Naval Station Norfolk — the largest naval base in the world, with more than 100,000 civilian and military employees — is already at high risk of flooding, as are the many other critical facilities in the Virginia Tidewater region. Even if new dikes, tidal gates and barriers, pump stations and piping can mitigate rising oceans, the cost of safeguarding the bases may become prohibitive. Other key East Coast ports like Miami and Charleston, S.C., are also in danger.

Finally, there are the opportunity costs of using active-duty military and reserve forces to respond to climate-related catastrophes in the U.S. Every time state and National Guard units are called out to respond to wildfires in the Western U.S. or hurricanes in the Southeast, resources marked for deployment must be used for domestic transport and their warfighting training is disrupted. My friend Adm. Thad Allen did a great job leading the 2005 Hurricane Katrina joint task force — but it required more than 65,000 active-duty and Guard troops.

Finally, there is the global strategic instability that could emerge as the nations clustered around the equator — including many of the world’s largest and fastest growing populations in Africa, Latin America and Asia — come to believe the advanced economies have done little to mitigate the global warming they mostly caused. This kind of anger and resentment could lead to breakdowns in global order and massive conflict.

My co-author Elliot Ackerman and I are finishing the final novel in a trilogy about cataclysmic 21st-century conflicts: “2084: A Novel of the Climate War.” It envisions global combat stemming out of inequalities driven by climate change. Today, the premise is fictitious. But if the U.S. ignores the very real threats of rising temperatures, it may be more suited for the nonfiction aisles.

James Stavridis is a Bloomberg Opinion columnist, a retired U.S. Navy admiral, former supreme allied commander of NATO, and vice chairman of global affairs at the Carlyle Group. This column does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.