THE BICYCLE IS A MASTERPIECE of physics. It harnesses human muscle power directly to that old-time marvel--the wheel--and yields a vehicle more energy efficient than any other devised, ever, by anyone. A human on a bicycle is more efficient (in calories expended per pound and per mile) than a train, truck, airplane, boat, automobile, motorcycle, skateboard, canoe, or jet pack. Cycling is more efficient than walking, which takes three times as many calories per mile. Pound for pound, a person riding a bike can go farther on a calorie of food than a gazelle can running, a salmon swimming, or an eagle flying.

Oh, and the bicycle is hugely democratic: It is equally available to all. That's why on the highways, byways, and bikeways in most of the world, the bicycle is the most ubiquitous transport vehicle. Bicycles outnumber automobiles almost two to one worldwide, and their production outpaces cars by three to one. Rush-hour traffic in China is dominated by these human-powered vehicles. Even in the wealthy cities of Europe and Japan, a large share of the populace gets around by bike. Only here is it treated as little more than a plaything. About 50 million U.S. adults (and 40 million children) ride their bikes at least once each year, but only about 2 million are regular bike commuters.

A bike is a blessing for your wallet, health, and legs, but bicycles are wonders because of what they don't do to the world. At zero pounds of carbon dioxide emissions, versus a car's one pound per mile, a bike does not alter the global climate. A cyclist fuels up on carbohydrates, not fossil fuels and imported oil. Bicycles don't require paving over landscapes at the expense of croplands, government coffers, and livable neighborhoods. And bicycles are not the leading killer of Americans and Canadians 2 to 24 years old or, worldwide, of men 15 to 44 years old. That distinction is reserved for the automobile.

Bicycles are not for everyone, and they're not for every trip. Cars do many things that bicycles cannot easily do: carry heavy loads uphill, protect riders from the elements, and cover long distances quickly. But a surprising number of car trips could easily be made by bike. Nearly half of all trips in the United States are three miles or less; more than a quarter are less than a mile. While advertising sells cars and trucks as tools for the open road, most often they help us inhabit a small daily realm--"Errandsville"--defined by home, store, job, and school. Many of these trips are easily bikable--or walkable--even on roads designed without bicycles or pedestrians in mind. A bicyclist can cover a mile in 4 minutes, while a pedestrian requires 20.

Short car trips are, naturally, the easiest to replace with a bike trip (or even walking). Mile for mile, they are also the most polluting. Engines running cold produce four times the carbon monoxide and twice the volatile organic compounds of engines running hot. And smog-forming (and carcinogenic) VOCs continue to evaporate from an engine until it cools off, whether the engine's been running for five minutes or five hours.

British author H. G. Wells summed up cycling's promise best more than a half century ago: "When I see an adult on a bicycle, I do not despair for the future of the human race."

This article is excerpted from Seven Wonders for a Cool Planet, by Eric Sorensen and the Sightline Institute (Sierra Club Books, May 2008).