In the 20th century, the greatest threats to civilization arose out of ecstatic emotions, especially when they united thousands of people. The last century's true believers rallied, wept and sang about superhuman faith, overwhelming feeling, single decisions that changed their lives and the world. They quivered to think of their heroes, who had "the power to raise up broken hearts and despairing souls." They believed that History was calling.

Decade after decade, the consequences were disastrous: rabid nationalism, Fascism, Nazism, Communism, anti-Communism and all the other isms in whose name millions died. (That praise of heroes who lead noble causes comes from Mein Kampf.)

The 21st century is different. Today, it's the small emotions—undramatic, common, seemingly inconsequential—that most threaten humanity. Enjoyment of those comfort foods you grew up with. Anxiety to get along with the neighbors. Guilt that makes you want to keep the children happy. Thousands of little daily impulses like that are the reasons people in the United States might, for instance, drive the neighbor's SUV to pick up the kids at school, then let the car idle, and then drive somewhere for a cheeseburger.

Today, at the end of the Garrison Institute's conference on behavioral research and climate change, I heard an announcement that quantifies the impact on climate of humdrum daily American life. According to research prepared by the Natural Resources Defense Council (pdf downloadable at the link), the United States could reduce its emissions of greenhouse gases by 1 billion tons per year with minor changes in the way people travel, eat, work and spend their time at home. (Full disclosure: I have written for OnEarth, the NRDC's magazine, as have many writers I admire, but I have no connection with this work.)

One billion tons would be almost 15 percent of total annual greenhouse gas emissions from the United States—removed by actions like letting cars and buses idle only half as much as they do now, eating chicken instead of red meat once a week, inflating everybody's tires, and cutting by one quarter the amounts of food Americans waste.

The work assumes 100 percent compliance with its recommendations. So it's a goal, not a plan. And even a cut of 15 percent by 2020, as imagined here, is not enough (80 percent is more in line with what looks necessary to avoid disaster), which means this problem can't be solved by individual virtue all by itself. Hard political struggles are still waiting.

But it's impressive to see how much "trivial" choices and small habits affect the fate of the world.