IN A SPEECH to the United Nations recently, President Obama said that climate change is a more serious threat than terrorism.

So, why aren't more Americans terrified?

A parade of panicked politicians talk about the Islamic militant group ISIS as if it were a combination of Hitler's Germany and Stalin's Soviet Union, faulting the president for not taking drastic action sooner. But climate change? What's the hurry?

A majority of Americans (67 percent, in an August Pew/USA Today poll) believe that ISIS is, like al Qaeda, a "major threat" to our national security. Yet, fewer than half (48 percent), almost none of them Republicans, see climate change the same way.

An evaluation of the actual danger posed by ISIS is a subject for a later time, but at least right now, the group - while it is vicious and barbaric and wants to hurt us - has limited capability to make that happen on American soil.

Yet, climate change already has invaded the homeland, reducing to ruins the homes of thousands of Americans, turning them into refugees of a sort. In his U.N. speech, Obama listed just a few of the terrors we already have experienced: "Along our eastern coast, the city of Miami now floods at high tide. In our west, wildfire season now stretches most of the year. In our heartland, farms have been parched by the worst drought in generations, and drenched by the wettest spring in our history. A hurricane left parts of this great city [New York] dark and underwater."

Earth's rising temperature represents a greater menace than any terrorist "sleeper cell."

Extreme weather events are not the only danger: As food and water shortages increase and already vulnerable people suffer dislocation and loss, the stability of governments and even civil societies are jeopardized.

In an ironic twist, climate change could itself have been a factor in the rise of ISIS. To be sure, the repressive regime of Bashar Assad, in Syria, is the major reason for the civil war that has fed the group's rise. However, some experts point also to the social upheaval tied to a widespread drought caused by climate change, the worst since the beginning of agricultural civilization, according to an expert quoted by the Center for Climate and Security, a think-tank advised by retired senior military and security officials. The drought destroyed the livelihood of millions of farmers and herders who migrated to the cities, and whose desperation is a contributing factor to rising militancy.

Still, while most Americans worry more about terrorism than the environment, activists sense a change. You wouldn't know it from watching mainstream media, but the message about the need for action is getting through to more ordinary Americans than in the past: The estimated 300,000 to 400,000 people who participated in the largest "climate justice" march in history on September 21 aren't putting away their signs and going home for good. The numbers of people engaging in civil disobedience is expanding. A movement to persuade institutions to divest from fossil fuels is still tiny, but growing.

We should do what we can to support them: In the not-so-long term, it won't matter what harm ISIS can or can't do if a rising planet temperature has the effect most scientists predict: global chaos.