Most people observe holidays that align with the solar calendar. Some of the most significant of those coincide with the shortest days of the year, around the winter solstice Dec. 21.

Those solstice-related holidays often include a festive meal. An internet search of "traditional holiday dinners" returns recipes and sumptuous photos of perfect standing rib roasts, glazed hams and turkeys.

Tradition meets the future if you plan your holiday meal with climate change in mind.

Not all foods have the same environmental impact or cost. Production of equal amounts of some foods emits many more greenhouse gases and other environmental threats than others.

The environmentally costliest choice for you and your guests this holiday dinner? Beef or lamb.

The best choice, assuming you want or expect a meat main course? Turkey.

A study published in the peer-reviewed Proceedings of the National Academy of Sciences two months ago examined the environmental costs of bringing different types of meat, dairy products and eggs to the table.

That study and many before it reveal that producing beef or even dairy products has a higher environmental impact than other sources, including pork or poultry. And, of course, plant-based diets carry even lower environmental costs.

Cows and other ruminants, including sheep and goats, display a remarkable evolutionary adaptation. They eat grass and other plants that mammals such as pigs — or humans, for that matter — can't digest. Cows can't digest those plants, either, but microbes they house in their complex digestive tracts can.

Microbes housed in the rumen of cows digest grasses and other plants eaten by the animals and release sugars and other compounds that the cows can digest. The microbes, naturally, take their cut in the process, reducing the efficiency of converting the eaten plants into meat and dairy products.

Even worse, from an environmental perspective, the microbes work deep in the rumen of cows in the absence of oxygen. Hence, they release methane, not carbon dioxide, as we do when we digest food. Methane and carbon dioxide are both greenhouse gases, but methane is 25 times more potent in retaining heat in the atmosphere than carbon dioxide.

A recent study in the journal Nature concluded that limiting consumption of beef to just one meal a week and otherwise adapting a "flexitarian" diet of plant-based proteins such as beans and nuts, and modest amounts of poultry, fish and eggs, could reduce greenhouse-gas emissions by more than 50 percent by 2050.

In addition to cows emitting five times more greenhouse gases than other livestock, production of a standardized unit of beef requires 28 times more land and 11 times more irrigation water, and releases six times more nitrogen-based pollutants, than is the case with other livestock.

Some human-evolutionary biologists suggest that nearly universal winter-solstice holidays once provided a social adaptation to regulate and delay consumption of precious, preserved protein: Wait until the darkest days of the year to eat your smoked, dried, salted protein.

If so, then 2050 solstice holidays might favor a similar social adaptation. Wait until the solstice to eat your annual portion of ruminant protein.

Happy solstice holiday, and bon appétit!

Steve Rissing is a biology professor at Ohio State University.

steverissing@hotmail.com