Food that we eat is turned into the energy that keeps us alive. Food that we compost is turned into nutrients that keep plants and animals alive.
But food that we throw away is largely turned into methane that contributes to global warming.
This article from basmati.com explains the difference well:
A compost pile [...] is a train station full of living organisms; worms, snails, bugs and bacteria all munch away at the decomposing material. Moisture and air is allowed in and out, creating an aerobic environment that generates heat and releases carbon dioxide instead of toxic methane gas. The resulting compost can be used to fertilize young plants, start seeds, or offer an inviting place for worms to continue the process.
In contrast, many landfills (such as those in the U.S.) now utilize a "dry tomb" system (emphasis added):
The landfills are lined with geo-textiles and clay, creating a shield of sorts between the waste and the earth surrounding it. Landfill managers then fill with waste, compact it, cover each day’s material with earth, and finally “cap” the pile with another layer of clay and geo-textiles. Fluid run-off is collected and treated, lowering the moisture level of the “tombs.” Organic materials decompose, but in a very different, anaerobic environment that does not favor natural biological activity. Methane and other gases build up in this setting.
The article notes that since 1996, the U.S. Environmental Protection Agency requires large landfills to monitor methane emissions. Methane above specific concentrations must be disposed of -- either by flaring, or captured for use as an energy source. This practice is also seen in other countries around the world.
In summary, food that's sent to a landfill has a few options after it is decomposed into methane gas:
- Released into the atmosphere, where it is 34 times more potent than carbon dioxide as a greenhouse gas
- Flared (burned), causing CO2 emissions and a waste of energy
- Captured for use directly as biogas for heating or electricity generation -- the carbon content is still released to the atmosphere, but some of the energy is captured in the process
While waste-to-energy systems are far better than the first two options, if the main feedstock is wasted food, it is probably still worse than composting: burning methane puts carbon dioxide directly in the atmosphere, while composting puts at least a portion of it into the soil. In addition, food that's composted for use at home doesn't required energy expenditure for transport to a landfill, and offsets a portion of the demand for fertilizer.
And ultimately, in modern agricultural systems, the energy to produce food comes from oil, meaning that food waste is like pouring oil out on the ground. From Scientific American: Waste Land: Does the Large Amount of Food Discarded in the U.S. Take a Toll on the Environment?
[R]esearchers at the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) concluded in a 2009 study that each year a quarter of U.S. water consumption and over 300 million barrels of oil (four percent of U.S. oil consumption) go into producing and distributing food that ultimately ends up in landfills.
What should we do instead?
First, limit food waste as much as possible:
Second, compost to the extent practical:
Finally, advocate! Exercise your rights to vote and participate in civic discourse to encourage your local municipality to either begin compost collection, or at a minimum to capture and convert landfill gas, if it's not being done already.