We are living, of course, in a carbon era, the evidence of which is not exactly subtle; witness the energy-rating tag on the Bosch dishwasher, the darkening image of "California organic," and, gusting across restaurant tables and grocery aisles, the spirited campaign, winning ground daily, against bottled water.

The age's imperatives have supplanted, for the moment at least, the obsession with carbs (so 2005!), and for that matter, fats - except for trans fat - and even organic.

"Possessing an excessive carbon footprint," as Michael Specter put it in last week's New Yorker, "is rapidly becoming the modern equivalent of wearing the scarlet letter."

In this climate, the most-visible contributors of the greenhouse gases that trigger global warming are feeling heat: Ford has Diane Keaton shilling for its new hybrid. Sara Lee has gone solar in New Mexico.

Most ambitiously, Britain's vast Tesco supermarkets chain is vowing to label groceries with their carbon count, a tally compiled from miles traveled, the fossil fuel used to process and package, and so on.

In this last matter, the theory is that for shoppers for whom green is important, the choices will be clear - the carbon number as easy to compare as price or calories.

Not that calories are so transparent outside the supermarket aisle: Chain restaurants are battling to keep them off menu boards. And if you're feeling ambivalent about that - if I may continue to digress - flip though a copy of

Eat This Not That

, the head-snapping new little picture book by David Zinczenko, editor of Men's Health.

A few samples: Applebee's Grilled Cajun Lime Tilapia (310 calories) vs. Applebee's Fiesta Lime Chicken (1,285 calories).

Auntie Anne's Jalapeno Pretzel With Marinara Sauce (280 calories) vs. Auntie Anne's Whole Wheat Pretzel With Cheese Sauce (450 calories). McDonald's Quarter Pounder (410 calories) vs. McDonald's Premium Grilled Chicken Club (570 calories).

Headline: Quarter Pounder skinnier than Chicken Club!

Still, Specter writes, a calorie is a calorie. A carbon count can be a lot trickier: Sea-freight emissions are less than a sixtieth of those associated with airplanes. Harvest methods differ. Yields. Growing seasons. Fertilizers.

He doesn't minimize the catastrophic effects of a dramatically warming climate - wild weather, melting ice caps, disrupted fisheries and farming.

But he worries about oversimplifying - for instance, slighting New Zealand apples grown more efficiently in sunnier climes in favor of local New York-grown apples; and letting "food miles" alone determine greenness.

"You can feel very good about the organic potatoes you buy from a farm near your home," Specter quotes a British scientist as saying. "But half the emissions - and half the footprint - from those potatoes could come from the energy you use to cook them. If you leave the lid off, boil them at high heat, and then mash your potatoes, from a carbon standpoint you might as well drive to McDonald's and spend your money buying an order of French fries."

Of course you don't merely go there to buy French fries. McDonald's doesn't keep local farmland green. And I'm not ready, quite, to put my money on the Fry-o-Lator.

Locavore amateurs may get the science wrong on occasion. Eco-moms might want to start with double-glazing their windows.

But armies have always marched on their stomachs. And if shoppers can somehow zero in on carbon, well, look what happened to trans fats.

Contact columnist Rick Nichols at 215-854-2715 or rnichols@phillynews.com. Read his recent work at http://go.philly.com/ricknichols