It's a cool morning, overcast and damp, when I step off the train in New York. Penn Station is just a few blocks north of Peter Singer's apartment in Chelsea, but the train was late, and time is short. The sidewalk swarms with walkers, the four lanes of the avenue are crammed with creeping taxis, cars, and trucks, the air reeks of combustion. I rush past a ragged man curled in a doorway, a filthy towel hiding his face from the nonstop pedestrian flow. Construction barriers constrict the path, and I note that the skinny young woman striding ahead of me keeps to the concrete edge, avoiding the steel grate that forms most of the walkway. I follow her lead, wondering if others have crashed through to the black depths below, which I imagine are hostile, damp, foul, and infested with rats.

If there are rats down there, they are just about the only animals, other than a grimy white dog, barking nonstop at a street vendor, to be found in this part of the world. Green growing things are rare. Spindly trees, their brown-edged leaves quaking in the breeze, poke through the concrete. Tall buildings ring the horizon in every direction. It seems an odd place to find a philosopher whose work has focused on the rights of animals and the ethics of farms. 

Yet amid all this pandemonium, at the center of one the most urbanized masses of human beings on the planet, Peter Singer emerges from an elevator in the modest lobby of his apartment building. Serene and affable, dark-eyed and slender, he seems quite at home here. We walk around the corner to a tiny, noisy cafe for a chat about the organic farms of the future, the secret lives of shellfish, solar-powered airships, and the pros and cons of hunting wild kangaroos.

Singer is well-known for his views on the rights of animals. It all started, he tells me, not with any personal encounter with the creatures of the animal world, but an encounter of an intellectual kind. "I was a graduate student at Oxford," he says, over a cup of steaming green tea. "This was during the late '60s, the '70s, when people were thinking about a lot of new ideas."

One new idea that intrigued him arose during a conversation with a vegetarian, who said the reason he avoided meat was "not because he believed in reincarnation," says Singer, with the hint of an eye roll. "But just because we're not justified in treating animals the way we do." That struck him as eminently reasonable, and as hard as he tried, he couldn't find any ethical argument to contradict it. He concluded that it's wrong for us to inflict pain on other living creatures when we don't have to, and it was an important yet neglected issue that needed to be taken seriously. He wrote Animal Liberation in 1975, a book that helped to inspire the animal-rights movement.

In his latest book, The Ethics of What We Eat, Singer and co-author Jim Mason explore the lives of animals who exist solely for the purpose of the human food industry. They examine not only the complex ethics of our relations to animals as food, but also the impacts of our food choices on farm workers and others who bear the costs of a wasteful system. It's a complex topic, involving government standards, business practices, economics, and the competing demands on time and money that real people must cope with every day.

But whatever demands distract us, Singer makes clear that each individual must take responsibility for their choices, and eating animals that were grown for food is a bad choice. And it's a bad choice not only ethically but environmentally.

"We all ought to be living lifestyles that try to minimize our adverse impact on the environment," he says. "And the way we raise animals is one of the worst things we do. The livestock industry is the biggest single source of greenhouse gasses. It exceeds transport." A recent UN report says cattle worldwide emit vast amounts of methane and nitrous oxide, gasses with heat-trapping potential many times more powerful than carbon dioxide.

"It's incredibly wasteful. Switching to a vegan diet would do more for the environment than changing your car to a Prius," says Singer. Raising an eyebrow, he adds that it amazes him when he attends events hosted by environmental groups and finds meat on the menu. Eating meat is simply "not compatible with being a good environmentalist."

The problems don't end with greenhouse-gas emissions. Livestock now use 30 percent of the earth's entire land surface, and one-third of the arable land is used to grow livestock feed. If those fields instead grew grains for humans, the food output would increase five to ten times. The escalating demand for animal products is a major cause of water pollution, soil erosion, and deforestation. Families who live near factory farms endure noxious odors and polluted air, and for workers, farm jobs are dangerous, low-paid, and exhausting.

Change is essential, Singer says, not only to improve the humanity of our food system, but to reduce the environmental damage and provide a growing global population with healthier diets. Switching to meat that is produced organically and humanely is an improvement, but it's not a real solution.

"Michael Pollan [author of The Omnivore's Dilemma] argues that as long as the animals are raised humanely, if they have good lives, and are killed painlessly, it's okay. They are domestic animals, and wouldn't have existed at all if not for us," says Singer. "I wouldn't eat that meat, but I can see the argument." Still, to be ethical, the meat-eater needs to know for certain that the animals' lives really meet all of those criteria.

Singer is skeptical that standards are always met, even when farmers mean well. "And now big corporations are getting into organic. It's good because it will reduce the price differential, and that's important. But these are no longer people who are doing it because they care passionately about it, they're doing it because they want to make a profit. So they need to be kept up to the mark."

Around us, the chaos of the city rages. Loud music beats from the cafe sound system, a woman pushing a stroller struggles to enter through the the heavy glass door, a man carrying a briefcase rushes in and rushes out with a latte and a cupcake. A worker asks us to move so he can repair the air conditioner unit above the front window. Singer tunes it all out and stays focused on his topic, a topic that he has been devoted to for decades now.

The most environmentally-friendly food system, according to Singer, would eliminate the animals and go fully organic. "Producing all of our food organically might require a little more land, perhaps 10 percent more," he says. But cutting back on beef, chicken and egg production would free up so much space, it wouldn't be an issue. Concerns that organic food costs more, benefiting the affluent and shortchanging the poor, also would vanish.

"Organic grains and beans and lentils can provide a very cheap and healthy balanced diet," Singer says. "A lot of people spend more on food than they need to, because they go to places like McDonald's, where the food value is actually quite poor when you consider what you're getting for it. If you learn how to prepare nutritious, environmentally friendly meals at home, you can do it for a fraction of the cost."

The right and wrong of food choices can be a gray area, he says, but the line is drawn at suffering. Some shellfish, such as clams or oysters, don't have highly developed nervous systems and it's unlikely that they experience pain. If those animals can be harvested sustainably, Singer says, that may be ethically acceptable. But in general, he sees problems with harvesting wild populations.

"Every so often, someone will suggest we should eat kangaroos instead of cattle," he says. They are native animals and they don't have the hard hooves that cause land degradation and erosion. But they couldn't produce as much meat per acre as cattle. And it's unlikely that animals who are hunted die quickly and painlessly. The U.S. might have better luck switching to buffalo, which are big and meaty, but to replace the current beef demand would require huge areas of land.

However he looks at it, from an environmental standpoint or an ethical one, he finds our consumption of animal products is simply unsustainable. So how can we change? "Europe has gone the right way in starting to have standards for animal raising in advance of the ones in the U.S. and Australia," he says. "I'd like to see more of that. I'd like to see abusive, unsustainable forms of farming prohibited."

He thinks Europeans are also on the right track in objecting to transporting food from Africa via air freight. "I'm a little torn," he admits. "I do think it provides economic benefits for Africa. But air freighting is so fossil-fuel intensive. We have to find some way that's better than that." Somebody needs to invent a more rapid fuel-efficient means of mass transport. Grinning, he wonders if a solar-powered helium-filled airship, maybe, would work? But that's sometime in the future.

He believes we can create a sustainable system for the future that can support 10 billion people or more. "A hundred years from now we shouldn't need to produce animals at all," he says. We should be able to grow meat in a lab for those who want it. We could produce fruits, vegetables, and grains more efficiently. He's hopeful that we can solve the problems of using genetically modified foods, and this technology will prove to be a boon, reducing the need for fertilizers and pesticide.

He also believes the affluent -- and that means all of us -- have an obligation to do something right now about global poverty. That will be the topic of his next book, coming out in 2009.

We settle up at the cafe and head out to the wide New York sidewalk. As Singer strolls with me back to Eighth Avenue, he points out a new bike path, talks about Australians' anxiety over climate change, and suggests I should stop by the arty Chelsea Hotel, where Thomas Wolfe, Bob Dylan, and other famous bohemians stayed. We part at a busy street corner and Singer ambles off, merging into the chaotic human flow. Luxury cars inch along in the jammed traffic, a ragged woman begs for handouts on the sidewalk, an impossibly tall construction crane swings steel beams aloft, a jackhammer pounds nonstop against the pavement. And I can't help but wonder if Singer's simple, sensible, hopeful message can penetrate the din.

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