Of course, you could go the other way: If the kosher-only omnivore replaced all the pork with chicken, their greenhouse emissions would drop to 1,216 pounds (552 kilograms) per annum. But if the "pork difference" were split equally between beef and chicken, the kosher-only meat diet would yield 1,460 pounds (662 kilograms) of emissions—about 6 percent more than the nonkosher diet.

What about shrimp? It takes 243 gallons (920 liters) of diesel fuel to trawl about 1.1 tons (one metric ton) of the shellfish, according to Dalhousie University in Halifax, Nova Scotia, making shrimp one of the most energy-intensive wild seafood harvests, with a footprint of 5,395 pounds (2,447 kilograms) of carbon dioxide per metric ton even before processing and transportation are taken into account. And shrimp farming (which provides well over one million metric tons of shrimp annually, about 25 percent of all shrimp consumed) has been linked to the destruction of almost half of the world's mangroves: coastal forests that absorb carbon dioxide and provide essential habitat for wild fish species. Crab, meanwhile, was among the least energy-intensive species to catch in the Dalhousie study, whereas the fuel needed to collect a ton of lobster swung wildly—ranging from 5.3 gallons (20 liters) per metric ton in Iceland to about 38 gallons (144 liters) in Maine to 271 gallons (1,025 liters) in Norway.

Just as with livestock, the ultimate green boost from kosher law's taboo on shrimp and other shellfish depends on what you eat in its place. Assuming that the kosher consumer replaces the average American's four pounds of shrimp a year (and its 9.79 pounds, or 4.4 kilograms, of carbon dioxide emissions) with another fish, Canadian North Atlantic herring is a good choice: it takes around 5.28 gallons (20 liters) of fuel to purse seine (net using two trawlers) a metric ton of these small fish, according to Dalhousie, releasing about 117 pounds (53 kilograms) of carbon dioxide—meaning four pounds of herring have a carbon footprint of a mere 0.21 pound (0.09 kilogram). Wild U.S. or Canadian salmon take an average of just over six gallons (23 liters) of fuel per metric ton to catch, releasing about 133 pounds (60 kilograms) of carbon dioxide. So eating four pounds of salmon a year would account for 0.24 pound (0.1 kilogram) of carbon dioxide. Both of these are obviously just a fraction of the 9.79 pounds of carbon dioxide for the shrimp eater.

Tuna are energy hogs by comparison, needing about 460 gallons (1,740 liters)—twice the fuel of trawling for shrimp—to harvest the same single metric ton of tuna. That adds up to a massive 10,212 pounds (4,632 kilograms) of carbon dioxide per catch. So eating four pounds a year would have a footprint of 18.5 pounds (8.4 kilograms) of carbon dioxide, almost twice the shrimp eater's footprint.

Kosher rules do remove some overfished wild species from your plate—such as sharks, which are in serious decline worldwide, according to the Monterey Aquarium's Seafood Watch program. On the other hand, some popular fish that are kosher, such as bluefin tuna and Chilean sea bass are also in peril.

Kosher rules also forbid mixing meat and dairy foods: No cheeseburgers, please. "The idea repeated three times in the Bible is, 'you shouldn't boil a kid in its mother's milk'," says Stern. This has evolved over the centuries into complex rules and practices to keep the two apart in the kitchen and the stomach as well as in the cooking pot. That means two sets of dishes. Doubling one's kitchenware would seem to run counter to the "less is more" mantra of contemporary environmentalism but, as Stern notes, because both sets are never used simultaneously, the useful life of each is likely extended over time.

Even though keeping kosher is not inherently more or less ecofriendly than a conventional diet, Stern notes that the small but growing kosher organic meat offerings, along with the overall boom in organic foods, make it easier to suffuse keeping kosher with her green values. And, of course, there are benefits that can't be counted by the numbers. "For me, keeping kosher is a spiritual commitment," Stern says. "It imbues the mundane with the sublime."