WASHINGTON •I think it's safe to say we all want to save the planet and a good chunk of what's written about food systems is about how to do it.

Estimates vary, but something in the neighbourhood of 30 per cent of total greenhouse-gas emissions are pegged to food.

If you're looking for advice on how to make climate-friendly food choices, you'll find plenty of it.

Unfortunately, most of it is either wrong or self-evident. (Though you will occasionally stumble on a thoroughly researched, tightly reasoned piece of advice about, for example, why you ought to eat more oats.)

In the "wrong" category is the advice to buy local or organic. Sometimes those are better climate choices and sometimes they're not, and it's all but impossible to know which is which.

In the "self-evident" category is waste and how you should try to generate less of it, both in food and packaging.

What's left is one piece of advice that is about as true as food advice gets: Eat less meat.

Beef, particularly, is a carbon Sasquatch, mostly because the digestive systems of cattle (and all ruminant animals) release methane, a powerful greenhouse gas.

Unless you've just crawled out from under a Hummer, you've heard the eat-less-meat advice and you've probably even heard that beef is the biggest offender.

There's plenty of evidence bolstering that position and the latest came in the form of a detailed analysis of the impact of various foods and production systems from the University of Minnesota.

The lead author, PhD student Michael Clark, looked at categories of foods, organic versus conventional production, and several measures of environmental impact.

In fact, he worked with me to do a detailed analysis of the greenhouse-gas implications of a weekly five-ounce steak. We're not taking any radical step like going vegan here, just eliminating one weekly beef meal.

We looked just at greenhouse gases, even though there are many other factors at play.

There's water use, land use and pollution, all of which affect the environmental impact of what we eat. There's also animal welfare, impact on rural communities and worker conditions, all of which matter in other ways.

We're looking at one thing: the greenhouse gases you're responsible for every time you decide what's for dinner.

Here's the upshot: If you trade in steak for beans once a week for a year, you will keep the equivalent of 331kg of carbon dioxide out of the atmosphere.

Giving up beef once a week in favour of beans, over the course of a year, is the equivalent of not burning 38 gallons of gas, or of trading in 12 incandescent bulbs for LED. That 331kg is equal to about 5 per cent of the average household's electricity use. If you plant a tree, it will remove that 331kg in, oh, 83 years.

Let's face it, though. Beans might be a tough sell. What if you trade in your steak for chicken or pork? That's a win, too, although not as big a win.

The impact of beef (334kg for that weekly steak) dwarfs that of beans (3kg for a weekly serving), but it's also larger than that of chicken (64kg) and pork (68kg).

But, whatever you do, don't trade your beef for lamb; your impact will more than double.

Realistically, if you cut back on beef, you'll probably replace it with a combination of foods - pork and beans, perhaps - and the heavier you go on the legumes and grains, the better you do.

But this isn't the end of the story. Clark's analysis didn't include the potential that grazing cattle have to rebuild soils and actually lock carbon away (called sequestration) in the process.

While carbon-negative beef is possible, it might not be common. And its benefits may run out after a while. Most soil scientists I've spoken to say they believe the soil's ability to lock carbon away is finite.

Once you hit that ceiling, you're stuck grazing cattle to keep it locked away, but with no additional carbon sequestered to compensate for the methane the cattle produce.

Jason Rowntree, an associate professor at Michigan State University, cautions against trying to figure out just how long that would take. "Nature is complex," he wrote in an e-mail message. "We try to linearise nature's responses and that leads to placing ceilings on what can happen with management."

But he adds: "It is naive to believe that with changing landscapes, the carbon sequestered is permanent. It is stored in soil as energy and it will be cashed out as well."

Grass feeding does more than sequester carbon, he points out. It can restore degraded land, something that can have benefits for future uses and generations.

It also does what has made cattle a mainstay of human civilisation. It converts grass to milk and beef by letting cattle do what cattle do best: graze.

Should you eat less meat? Yeah. And waste less food. Use less packaging. Eat oats! But, of course, it's never quite that simple and we'll all do better if the people who raise our food have a reliable way to tell us about it and the people who buy it pay attention.

Also, change your light bulbs already.

WASHINGTON POST