"The goal is saving the planet. No ambition less than that is worth fighting for," Tristram Stuart, an author and campaigner who has focused his life on preventing food waste, told me over the phone. "We need to flip farming from a business that eats nature and shits cash, and instead make it regenerative. And we can do that."

To talk to Stuart is to immediately feel guilty about the bag of oranges or the container of spinach you threw away last week. Since his childhood growing up on a Sussex farm, Stuart has been appalled by food waste, and what's more, he’s actually motivated to do something about it.

"I remember, at age ten, writing to McDonalds and saying I wasn't going to visit their restaurants until they stopped using CFC packaging," Stuart said. When McDonalds wrote back to say that all of the chain's restaurants were getting rid of CFC packaging as part of the Montreal Protocol, Stuart got a little taste of how powerful campaigning could be. "At that age, obviously, I thought, wow, look at what I just did. Of course, one later learns … that's the way the entire world was going. But at the time I had the delusion that an individual could make a difference. I suppose I never lost that delusion."

As a 15-year-old farmer, he fed his pigs on food that local businesses and schools were planning to throw away. The free animal feed made his pigs fat and happy, it saved uncountable pounds of perfectly edible food from being wasted, and best yet, it was free.

And, basically, he never stopped. While living as a dumpster-diving "freegan," he wrote his first book, The Bloodless Revolution, in 2007. Two years later, he published Waste. The response was so overwhelming and immediate that his life has been completely taken over by the anti-food waste campaign ever since.

"[Waste] starts in the aisles of the supermarkets," Stuart said, describing how supermarkets stock and display food to give the illusion of a bottomless cornucopia. "We've spent two million years evolving in an environment dominated by scarcity. We are hard-wired, whenever we see surplus and abundance, to take it, fill our shopping trolleys with that stuff and take it home." Once home, we forget about much of what we've bought until we discover it in a hidden corner of the fridge, fuzzy with mold, and throw it out.

Though there's a lot of work to do in individual homes, that's not where Stuart focuses his attention. He founded Feedback, a non-profit that investigates and prevents food waste, and used his now-global reach to analyze and understand the global food supply chain.

The food supply, he says, is shaped like an hourglass. At the bottom, there are thousands of farmers. At the top, there are millions of customers. Causing a pinch in the middle are supermarkets, which are mostly consolidated into a few regional or national chains of enormous power and reach. These large corporations control their inventories with a stringent set of standards: your eggplants must have these specific dimensions, your bananas must not be more yellow than this specific shade, and so on.

"[Cosmetic standards] result in farmers all over the world growing food that doesn't get eaten," Stuart said. "Producers are growing for international markets. And they're getting screwed over: orders are cancelled, cosmetic standards are imposed arbitrarily so that they waste up to half of their crops—sometimes entire harvests." Because international food producers are usually farmers in economically depressed areas of the work, they are the people who can bear these kinds of risks the least.

Stuart launched the Feeding the 5,000 event to bring attention to the problem. By making curry, soup, and smoothies out of fruits and vegetables that are perfectly edible but may be slightly small, discolored, or ugly, Stuart and his team fed 5,000 people in London using a menu entirely made of food that would otherwise been thrown away. The event started a large debate in England, and within a few days supermarkets and government groups announced new policies to reduce food waste.

Feeding the 5,000 was conceived as a one-off event, but it was so successful that it became a template for social change instead. "We got invitations all around the world to recreate the event," Stuart said. "And over time we developed sort of a format for launching a national movement on food waste." People from different kinds of organizations immediately identify with food waste, and it showed them how they could all work together to help the environment, or feed the poor, by fixing this common problem.

Read More: An EU Project Aims to Turn Food Waste into Graphene

Stuart seems to always have a dozen things in the air at once. As Feedback grew, he launched Toast, a line of beer brewed with leftover bread that was headed for the trash. More than anything, wasting bread seems to touch a raw nerve with him. "One of the things that really stands out is bread. Bread is eaten everywhere, and it is wasted in industrial quantities." Since wheat is a globally traded commodity, wasting bread in expensive sandwich shops in the West can sometimes mean not enough bread to feed hungry people across the world.

Recently, Stuart has stepped back from the day-to-day operations of Feedback to focus on the larger-scale problem posed by global climate change. Changing how farms work, changing the global food supply chain, and ending deforestation are all tall orders, but the world's pessimistic response to climate change makes Stuart more determined.

“We're terrestrial animals, and land is the most valuable thing we have. The biggest impact by a long way that humans have on the Earth is in food production. It's more than the transportation industry and all the rest," Stuart said. "My work has been on using food as a way of communicating and inspiring people to take responsibility for changing the food system."