“Which is the most ugliest?” a boy of around seven says to his friend as they breathe on the glass cabinet of a bird display in Dublin’s Natural History Museum. “The one that looks like you,” the other boy shoots back. They laugh, elbow each other and pick the cutest specimen. They pay no attention to the tall man being photographed nearby beside the skeleton of a giant Irish deer.

He’s Philip Lymbery, and his fascination with birds began when he was around the same age as the boys. Born in the English market town of Leighton Buzzard, he remembers watching a sparrow picking up and dropping a tiny pebble on his grandad’s lawn. He raced outside to touch the pebble to see if it was wet, curious about whether sparrows had saliva like humans.

There is one simple idea at the heart of Dead Zone: we have to stop feeding food that people can eat, like cereals and soya, to factory-farmed animals

Lymbery is in Dublin to talk about Dead Zone, Where the Wild Things Were, his second book in a planned trilogy. This suited soft-spoken man in his early 50s looks more like an English Silvio Berlusconi than an eco-warrior ranting at us all to go vegan.

As chief executive of the English charity Compassion in World Farming he has spent decades calling out cruelty in farming practices like the destruction of newly hatched male chicks by dropping them into mincers in the egg industry. But with his first book Farmageddon and now Dead Zone, he explores what he calls “green deserts” or plantations of single crops drenched in chemicals in cleared rain forest or jungle cultivated at a frightening cost to the planet and its wildlife.

So a litre of milk in the supermarket is connected to elephant habitat in Sumatra, cleared for palm plantations, the palm kernel used in feed for intensive dairy herds, he writes. Closer to home he is lyrical about the commoner animals, like those sparrows on the lawn, whose numbers are falling as hedges and habitat disappear.

Lymbery set out to write Dead Zone with journalist Isabel Oakeshott, his co-author on the bestselling Farmageddon. “But then Call Me Dave, [the book that Oakenshott co-wrote about former British prime minister David Cameron] really took over her life,” he explains as we talk back at his hotel room. Call me Dave contained the infamous “pig-gate” allegation about Cameron and a dead pig’s head.

One of the things I face from the agri-industry is they try to argue black is white

The training wheels were off and it took a few months to find his voice. “I wasn’t a writer and I learned a lot working with Isabel, the value of colour, of not using so many facts, of story-telling and all those sorts of things. Up until [then] I’d been writing for political briefings and those sorts of things which are very fact-driven and objective.”

There is one simple idea at the heart of Dead Zone: we have to stop feeding food that people can eat, like cereals and soya, to factory-farmed animals. There is a better “have-it-all” alternative, he says. And we, as consumers, can influence that through our food choices.

“Writing the book has changed me. It’s changed my life and how I look at the world. It’s changed my sense of urgency and optimism,” he says. We are “the last generation who can leave a planet worth having”.

That stark realisation dawned when he looked out of the window of a tiny plane in Brazil at a mind-blowing sight. “To see that massive amount of countryside devoted to one thing – soya – where it had previously been forest, was breathtakingly devastating.” When the view was blotted out momentarily as the plane flew into cloud, “it was like the interval in a particularly intense and quite harrowing play”.

Although Lymbery was horrified he also felt “massively vindicated” looking down on the sea of soya. His critics accuse him of doom-mongering, over-egging the devastation in an effort to turn the clock back to a bygone pastoral idyll. “One of the things I face from the agri-industry is they try to argue black is white. They try to say that these things are not happening and if they are my account of them is a gross exaggeration. And I just looked at something which is so huge . . . and thought this is an entire landscape that has gone purely to feed factory-farmed animals.”

Most of the food value of the crops that go into our food is wasted in conversion to milk, meat and eggs.

Lymbery and other campaigners are often met with a “get over yourself” response from industry. Vast feedlot farms are necessary to feed a growing world population, they say. Leaving animals to graze in fields is a luxury that a burgeoning world population can no longer afford. Lymbery calls that out as a big fat lie and explains how unwinding a bloated system back to recognisable farms with animals in fields can both feed the world and protect it for the future.

“The industrial model is hugely inefficient, incredibly wasteful,” he says. “Most of the food value of the crops that go into our food is wasted in conversion to milk, meat and eggs.” So 40 per cent “of our entire global harvest goes towards feeding industrially reared animals”, he says, “of which 26 per cent is lost in conversion [from plant to animal protein]. That’s 26 per cent of the entire global harvest. That’s enough food to feed more than four billion extra people, if it was fed directly to people or if the arable land was used for a more useful purpose.”

So-called cheap meat is something we pay for three times

How did our food system get so skewed and who are the vested interests? Lymbery believes it’s not the individual farmers. Over the past century gross farm incomes have been increasing but net incomes have been going down, he says. “So where’s all the money going? Back out again to the feed companies, to the fertiliser companies, to the pesticide companies, to the drug companies for the antibiotics. Half the world’s antibiotics are fed to farm animals . . . In other words the money isn’t in farming. The money is in supplying the tools of industrial agriculture to the farmer. And that’s the hard one to crack.”

It’s easy to feel helpless in the face of these numbers. “The good news is that we can all help to choose another way, and we can do that three times a day through our food choices, using pasture-fed free range or organic meat, milk or eggs.”

Doesn’t that mean forcing people who can’t afford it to pay more for food? “So-called cheap meat is something we pay for three times,” he says. “Once at the checkout, the second in vast tax euros in [farm] subsidies here in Europe and the US. The third way we pay is through the cleanup costs of the environment and our health.”

So forget phrases like “farm fresh” or “country fresh” or “natural”, when you’re choosing your food. “Those generally tend to be descriptions put on stuff that often comes from factory farms.

The United Nations is warning that we have 60 harvests left in our soil

“One of my biggest bugbears is that factory farming is something is done for the community as a favour. You know ‘knocking out the £2 chicken, that’s good for us isn’t it?’ And it’s done because there are people out there who are poor. What kind of society are we in where we expect people on low incomes to have to feed their children on poor quality factory farmed food? How can that ever be right?

“I have a lot of time for what I call renegade farmers. There’s a hundred real pasture fed livestock producers in Britain right now. They’ve organised themselves into a new organisation, the Pasture Fed Livestock Association, and their pledge is to rear their animals without feeding them a single grain.”

The working title for the last book in the trilogy is Sixty Harvests Left. It’s the most chilling of all Lymbery’s statistics. “The United Nations is warning that we have 60 harvests left in our soil.

“We are heading towards a genuine food crisis not because there is a lack of food, because at the moment there isn’t. Now we produce enough food for 16 billion people. We live in a world of food abundance . . . but we are going towards a world where we will have undermined the ability of future generations to produce enough. So while we wait for governments to wake up (and let’s hope it’s not much longer), we can be creating the market, the environment, encouraging those that are doing the right thing by buying pasture fed, free range or organic and choosing some meat-free meals on a regular basis.”

To this reader the book is a slam-dunk of factory farming. But here’s the elephant in the room. In the era of fake news and short attention spans can a book asking us to consider Sumatran elephants or Brazilian jaguars when we buy cheap meat really change things, like Rachel Carson’s Silent Spring did in 1962? Carson’s book resulted in the banning of industrial pesticide DDT and the setting up of the US Environmental Protection Agency.

“I see clear signs in the time before Farmageddon was published to now the narrative has begun to change,” Lymbery says. “The whole idea of feeding vast amounts of edible crops to animals is now a serious discussion point. Whereas before it was discussed as the domain of those who would like to see us all turn vegetarian.”

Dead Zone, Where the Wild Things Were by Philip Lymbery is published by Bloomsbury