I will admit that one of my biggest pet peeves is when people quote facts (in tandem with either a physical or implied pointing finger) while lacking a solid grasp of what the numbers underlying those facts actually mean.

As a numbers guy who has been crunching environmental impacts for years (five of which were fueled by a vegetarian diet), this peeve applies particularly to environmentalists who have been working at cross purposes to the movement by creating the perception that sustainability is as a whole impossible for the average American. It tends to produce this result (my handy formula):

(1 weak fact + 1 pointing finger) x # repetitions = anti-green sentiment (squared)

Never has this been more apparent in the environmental footprint terrain of meat consumption. Glenn Beck, Sean Hannity, even South Park reference the preachy accusations of vegans who tell meat-eaters to drop their favorite grilled meat in favor of substitutes like tofu, tempeh and soy milk.

Now those fingers have a new study to point at which makes the controversial claim that livestock industries are responsible for over 50 percent of the world's total greenhouse gas emissions. So before you turn vegan and then run out to purchase the latest model Escalade, let me straighten out a few things...

1. The current accepted figure by the UNEP is that meat production is responsible for 18 percent of GHG (greenhouse gas) emissions, this from the seminal UN's FAO report Livestock's Long Shadow.

2. The new report authored by former World Bank advisors Jeff Anhang and Robert Goodland is not yet peer-reviewed. It combines all livestock categories -- both dairy and meat. It also includes cattle respiration CO2 which normally was considered carbon-neutral as the cattle eats grass which sequesters CO2. It also uses different metrics for calculating pervasive methane impacts. I'm not saying the report isn't solid, but it needs to be put into context so that we can know if we are comparing apples to apples.

3. Deforestation is widely thought to be responsible for 20-25 percent of GHG emissions, much of which is attributed to soy production. Some deforestation is certainly attributed to the beef industry, but accurate numbers that allocate which forests have been cleared for which use (i.e. beef, timber, mining, ethanol, agriculture, etc) are not available.

4. In the most rapidly deforested state in Brazil, Mato Grosso, as much as 70 percent of newly cleared lands are used to grow soybeans, some of which is fed to cows, some exported as food products, and some used to produce biodiesel. Read more on National Geographic.

5. There is a big, big difference between total global GHG emissions and U.S. GHG emissions. This is important. Americans are the biggest contributors to climate change NOT because they eat meat (though that is a significant contributor -- about 6 percent of total carbon impact according to my calcs) but because they use HUGE quantities of fossil fuels to run their cars (28 percent) and their homes (30 percent). 

Every time I hear an American talking about how eliminating meat will save the planet, I have to remind them that though being vegetarian has both health and environmental benefits, it is no carbon panacea. 

The other thing I have to point out is that there is a big difference in types of meat and how the animals were raised. Grass-fed beef for instance is much less methane-intensive. An EPA report explains:

"Before the 1970s, methane emissions from manure were minimal because the majority of livestock farms in the U.S. were small operations where animals deposited manure in pastures and corrals."

There's also this thing called moderation. 

Meat (that isn't pumped with hormones and antibiotics) is healthy in modest quantities. Unfortunately most Americans are gorgers. According to the American Meat Institute (PDF) the typical American consumes about 65 pounds of beef per year (that works out to about 5 quarter-pounders a week). 

All of this is to stay that tracking environmental impacts is tricky business, and encouraging others to do so is even tricker, especially if they are BBQ-lovin' meat eaters. Trust me, I've tried many a time.

A good start is to really, really know your numbers. Unfortunately, the numbers are still incredibly difficult to get a hold of and often quickly outdated. To give you an example, the World Resources Institute created this slick graphic which summarized all the sources of global GHG emissions back in 2000:

Livestock was only put at about 5 percent and deforestation at 18 percent. In 2005 the updated their findings and knocked down deforestation to 11 percent only to have two reports the following year disprove both numbers -- Livestock at 18 percent and Deforestation at 20-25 percent. I've yet to see a new comprehensive graphic incorporating the latest data but you can see how complicated a task this is.

I certainly would put reducing meat consumption in the top five actions you can take to significantly reduce your climate impact, but I would advise any preaching vegans out there to make sure and tackle their larger impacts first -- transportation and electricity -- if they want to have a significant carbon footprint reduction to hang their hats on.

Check out Karl's handy chart for a quick breakdown of a typical American's carbon footprint.