In a speech to the United Nations recently, President Barack 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 State 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) believe the Islamic State is, like al-Qaida, a “major threat” to our national security. Yet, fewer than half (48 percent), almost none of them Republicans, see climate change the same way.

The Islamic State — while it is vicious and barbaric and wants to hurt us — has limited capability to do that on American soil.

Yet, climate change already has invaded the homeland, destroying the homes of thousands of Americans. In his U.N. speech, Obama listed what 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 the Middle East, 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. The estimated 300,000 to 400,000 people who participated in the largest “climate justice” march in history on Sept. 21 aren’t putting away their signs and going home for good.

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