First it was slow. Then local, then organic. Now it is firmly grass-fed.

As a rare geophysicist studying diet’s environmental consequences, I am asked daily by my colleagues – a bit bemused by my new field yet quantitatively astute and environmentally concerned – about the latest claim made about impacts of food production on the physical environment.

In this role, I get to keep a sensitive finger on the envirofood pulse. Unambiguously, grass-fed beef is all the rage now. Even the New York Times Op-Ed page featured a recent piece extolling the virtues of grazing cattle.

Depending on your guiding environmental objectives, grass-fed beef may indeed be the greatest thing since Guns n’ Roses or the environmental equivalent of entrusting former Secretary of Defense Donald Rumsfeld with military preparedness.

Yet on reasonably balancing the main geophysical dimensions of dietary choices, grass-feeding loses most of its touted allure, relegating its role in a rational food production system to the margins.

To be sure, the flesh of a healthy, thriving animal is clearly nutritionally superior to the biochemically compromised, microbiologically teeming ecosystem that is the bulk of the nation’s meat supply.

Grass-feeding is also biogeochemically sensible. Unlike mammals, bacteria in the ruminants’ digestive system can decompose cellulose, the sugar-based rigid structure into which most of the solar energy the biosphere absorbs is converted.

This decomposition converts the otherwise unavailable energy locked in the cellulose structure into metabolically readily available glucose. If it weren’t for the absorption of glucose liberated by bacteria mediated cellulose breakdown inside ruminant bodies, most of this energy would have bypassed the animal kingdom altogether.

These demonstrated virtues, however, pale in comparison to overwhelming environmental liabilities.

To begin with, there is greenhouse gas emissions, the argument most often invoked to promote grass-feeding. Yet grass-fed meat is more, not less, greenhouse-gas intensive.

In this, simple chemistry is the Draconian ring master, dictating that every decomposing carbon-containing molecule ends up as methane if the decomposition is anaerobic, as it is in the largely oxygen free rumen, and as carbon dioxide if the decomposition occurs in the presence of oxygen, as befalls most cellulose not digested by ruminants.

Since grazing animals eat mostly cellulose-rich roughage while their feedlot counterparts eat mostly simple sugars whose digestion requires no rumination, the grazing animals emit two to four times as much methane, a greenhouse gas roughly 30 times more powerful than carbon dioxide.

This, and the faster weight gain by feedlot animals, result in significantly higher greenhouse gas emissions per pound of meat by grass fed animals than by feedlot ones.

This is true, to variable a degree, for organic and non-organic, large- and small-scale grazing operations in the U.S. and overseas.

Then there is land. Upward of a quarter of the entire U.S. surface area is pasture or grazeland.

Grazing animals produce at most a quarter of the calories per acre typical plant based production systems do. While these facts are well established, they are often dismissed as irrelevant to the grass feeding question on the (partly correct) grounds that much grazing occurs on land that would otherwise produce no human destined calories.

But do we need more calories?

In recent decades, the U.S. has been consistently producing 3,800 kcal per person per year, almost twice the average person’s needs.

Given biodiversity declines due to dwindling, fragmented, wilderness, allocating all this land to inefficiently producing needless calories is foolhardy.

Even if you irrationally consider those extra calories indispensable, because of corn’s unrivaled caloric yield it makes more sense to produce them as a corn derivative on a fraction of the land, and still have some left for species protection.

Grazing cattle also compromise river systems in the fragile arid and semi arid environments in which they are disproportionately ubiquitous, and accelerate soil erosion.

Because they eat much more dry matter then feedlot animals, they also pressure dwindling local water supplies exactly where they are most vulnerable.

While some of those adverse impacts can be minimized by adequate management, most rigidly reflect cattle biology and north American geography.

Grass-feeding produces unnecessary low-quality calories at ostentatious environmental costs while displacing threatened wildlife.

While some grass-feeding may be reasonable on marginal lands near population centers in the rainy eastern U.S., the logical number of grazing cattle in the western U.S. is zero.

What we need is not grass fed cattle, but quantitative sophistication that readily distinguishes elixirs like grass feeding from actual environmental solutions.

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Cows feed on grass as they roam the hills near Pleasanton, California March 23, 2007. REUTERS/Mike Blake