Water in most parts of the U.S. is free, safe, and spiked with fluoride—from the tap. No bottles needed. The biggest bottled-water brand, Aquafina, is filtered tap water. Dasani, the second-biggest seller, comes from Brampton, Ontario, where factories also make cars and paint. It’s not like those PET bottles hold anything special.

So giving up bottled water is easy. It’s the lowest-hanging fruit of the green movement. See, there’s expensive green, like buying organic food or a Prius. Then there’s the conservation type of green—driving less, turning down the heat. With bottled water, no one has to give up much of anything. In fact, if times are tight and the bank’s foreclosing on your balloon mortgage, why buy something when you can get it for free and feel self-righteous?

Other industries watch and strategize. Green is catnip to the presidential candidates. They’ll pump the bellows for the next year. Companies are jumping in like penguins off a melting ice floe. Wal-Mart wants green products on its shelves. Tractormaker Caterpillar crows that it’s included in Dow Jones’ Sustainable World Index. Green tech is now the third-most-popular sector for venture capital, according to accounting firm KPMG.

Trend watchers such as 4.5 Productions are setting up businesses to tell other businesses to get into green stuff. “We soon came to realize that there are many companies that would like to introduce more eco-friendly or socially conscious aspects but didn’t really know where to start,” says Britt Allanson Bivens, a director at the New York-based company.

In the midst of any fad, the forces driving it seem real and powerful. Carbs make you fat? My word, you don’t want to be Dunkin’ Donuts or Macaroni Grill. Gas prices spike? Yikes! Everyone’s going to buy hybrids the size of golf carts. Chinese products kill your kids and pets? No one will ever buy “Made in China” again!

But now Dunkin’ Donuts plans to triple its outlets by 2020, S.U.V.’s have held steady at nearly a quarter of the new-car market, and last time anybody looked, the trade deficit with China was still growing.

Likewise, green will waver. Most of us get global warming. We’ve seen An Inconvenient Truth, or at least Happy Feet. We know we’re causing a mess. But global warming has a major marketing problem: It doesn’t have an immediate, tangible effect on our daily lives, and the only way we can do something about it is to give things up. Americans don’t like to give things up.

So green is “a trend driven more by economic survival—to make ends meet, to boost profits, and to assuage guilt—than by intellectual pursuit,” says a ­report by trend watcher Celente. “When money flows, the trend to buy big and spend large [pushes] enviro-care from the public’s mind.”

Right. So let’s meet in 20 years at what will then be your tropical beach house in Saskatoon. What the heck—pack lots of bottled water.