Have you ever wondered how much water it takes to make a Starbucks grande latte? I hadn't until I met Jason Clay.

A common mistake in business and sustainability is to optimize for a single outcome—sell more organic cotton, say, wild-caught fish, or fair trade tea—without understanding the overall impact of products on water, energy, soil, land use, even poverty alleviation. The need for clear thinking about such matters is urgent because population and, more important, consumption are growing fast.

Jason is a Missouri farm boy who earned a Ph.D. in anthropology from Cornell, wrote a definitive book on agriculture and the environment and is now senior vice president for market transformation at the World Wildlife Fund. (I wrote this column about him last year for fortune.com.) He's one of those people who is always bursting with both facts and ideas, so I was pleased to run into him today in Atlanta, where we had both been invited to speak to senior executives of Coca Cola Enterprises, the big bottler of Coke products and a FORTUNE 500 company in its own right. CCE is doing great work on sustainability, but that's another story.

Jason's presentation was mind-expanding, as usual, but my favorite part came when he analyzed the "embedded water" in a Starbucks latte. There's a terrific video about this at the WWF website; view it by clicking on the coffee cup.

Here's the breakdown, by liters, of the water needed to make that latte:

• 0.1 for the water itself
• 2.5 to make the plastic lid
• 5.5 to make the paper cup and sleeve
• 7.5 to grow the sugar
• 49.5 to feed the cows that make the milk
• 143 to grow the coffee

That adds up to more than 200 liters of water to make a latte.

Now, this doesn't mean we should stop drinking lattes. The water to grow coffee, after all, comes in the form of tropical rainstorms, which are abundant. And a bowl of Rice Krispies with milk has a much bigger water footprint: According to Jason, roughly 58 percent of all the water on the planet used by people for any purpose -- farming, manufacturing, cooling nuclear power plants, swimming pools, showers -- is used to grow rice. His point is that we, collectively, need to better understand the full environmental impacts of all that we consume. Then we need to make and grow things more efficiently, and consume less of them.

That not as simple as it may sound. One common mistake in the world of business and sustainability is to optimize for a single outcome -- sell more organic cotton, say, or wild-caught fish, or fair trade tea -- without understanding the overall impact of products on water, energy, soil, land use, even poverty alleviation. Favoring organics might, for example, limit the development of genetically modified foods that require less water and fewer fertilizers. Clay's open to the idea of GMOs as tools to grow more calories on less land. "Let's be a little more neutral on the technology," he says, "and a little more focused on the results."

The need for clear thinking about such matters is urgent because population and, more important, consumption are growing fast.

"We're beginning to wake up to the fact that we live in a finite world," Clay says. "Business as usual is not going to set things right."

"The average cat in Europe has a larger environmental footprint than the average African over a lifetime because of the fish it eats," he says. [I'm going ask Jason for the data to back up that claim next time we meet.]

So what's he doing to provoke change? He's working with big companies like Coca-Cola, Mars, Procter & Gamble and Wal-Mart, urging them to take a thorough, science-driven approach to their supply chains, so they use less water, produce fewer greenhouse gases, make less waste and protect forests. That's because these companies have scale and clout.

"Working with 300 to 500 companies is easier than working with 6.7 billion consumers," he says.

Of course, consumers should be urged to reduce, reuse and recycle, but Clay argues that it's unrealistic to expect even committed and well-informed consumers to drink their coffee black or switch from Rice Krispies to oatmeal.

"Consumers shouldn't be asked to make those choices," Clay says. "We think they ought to have only good choices on the shelves."