Yahoo! , Oracle, eBay, and Cisco Systems have all left giant imprints on the tech world.

But on April 22, these monster corporations will take deliberate steps to shrink their footprints — their carbon ones, to be precise. On that day, which is appropriately enough Earth Day, chefs at these corporate cafes and others around the country that are all overseen by Bon Appetit Management Company, will serve low carbon meals to educate diners about the role food plays in climate change.

Of all the activities done by humans, studies have found that food and all the energy it takes to make it is one of the largest contributors to global warming. One third of all greenhouse gas emissions are caused by food production, processing, transportation, packaging, preparation, and waste.

All in all, 400 cafes in 28 states will take up the challenge. They include those at DreamWorks, the de Young Museum, Monterey Bay Aquarium, and the Massachusetts Institute of Technology.

That translates into reducing the use of imported foods such as rice and bananas. It also means cutting out beef, and even much of the cheese normally served. After all, cows (whether raised for dairy or for meat) emit a huge amount of –errr — methane gas, to be polite.  And that gas is said to be even more potent than carbon dioxide at trapping heat against the Earth.

On “Low Carbon Diet Day,” diners will see more turkey burgers, portobello burgers, pizza with white bean sauce, salad bars stocked with veggies only from North America, and agua frescas made from regionally grown fruit (tropical ones are definite no-no’s). Every food station in each cafe will have to offer one low-carbon meal option and post a sign explaining the principles involved.

If you can’t make it to one of the public cafes that day or aren’t lucky enough to get a coveted invitation to dine at one of the private Silicon Valley tech cafeterias, there’s another way you can learn more about the carbon footprint of food. Just click here to find a calculator that will compute the total carbon emissions of your average breakfast, lunch, or dinner. You might never look at that plate of bacon and eggs quite the same way again.