NEW YORK (BLOOMBERG) - Depending on how you want to read the numbers, the humble hamburger is a bigger contributor to the warming of the planet than the jumbo jet.

Animal agriculture produces 14.5 per cent of man-made greenhouse gases, and cows are responsible for the biggest chunk of that. Global aviation, in contrast, accounts for just 2 per cent of total emissions.

Awareness of meat's carbon footprint has created an opening for products that appeal to lower-carbon dieters, with plant-based alternatives Beyond Meat and Impossible Foods now pitching themselves as part of a climate-friendly lifestyle.

Beyond Meat's logo - a cow with a cape on a green circle - hints at what its website lists as one of the foremost reasons the company makes its products: to "positively impact climate change".

A one-day partnership with KFC selling Beyond nuggets, an alternative to chicken designed to have a similar taste, served the product in green buckets at an Atlanta restaurant with a newly green exterior, under a massive green-and-white billboard.

Impossible Foods chief executive officer Patrick Brown has gone so far as declaring the use of animals for food as "by far the most destructive technology on earth". His company, now serving a plant-made replacement for meat that "bleeds" like beef, has set the goal of getting rid of animal agriculture entirely by 2035.

"Every #ImpossibleBurger you eat helps give our grasslands and other ecosystems a new look," claims Impossible Foods' official Instagram account. Other marketing language says Impossible burgers "helps the planet when you eat it".

To drive the point home, Burger King's Impossible Whoppers come in a green (one-time use, disposable) wrapper.

Consumers are worried about the planet and want to be part of a solution but they don't want to fundamentally change their lives, said Ms Suzanne Shelton of Shelton Group, a communications firm focused on sustainability.

"We are willing to align our wallets with where our values are if it's easy," she said. "That's the brilliance of Beyond and Impossible. It's not a change I have to make in my buying behaviour."

Big Food is, perhaps belatedly, sensing the shift and racing to compete in the US$14 billion (S$19 billion) global alternative meat market. Nestle has launched an Awesome Burger under its Sweet Earth Brand. Conagra will update the plant-based burger sold under its Gardein brand. Kellogg's Incogmeato Burger will land in supermarkets next year.

Even meat companies are hopping on board.

Tyson Foods, which produces one of every five pounds of meat in the United States, announced new meatless chicken nuggets this summer, as well as a meat burger blended with pea-protein.

Canadian meat giant Maple Leaf Foods sells vegan "meat" products through its Greenleaf Foods subsidiary.

Smithfield Foods, the world's biggest pork producer, just announced that its own line of soy-based meat is on the way.

Plant-based proteins are unquestionably better for the planet than beef. A Beyond Burger generates 90 per cent less greenhouse gases than a beef burger, according to its third-party life cycle analysis. Impossible Foods' version, it says, emits 89 per cent less.

Neither, however, is healthier than eating an actual burger. Plant-based products are not exactly natural. Impossible Foods' primary ingredient is genetically modified soy, grown with pesticides and fertilisers that have their own impacts, some environmentalists have argued.

"There is no point in quibbling about whether Impossible Foods grows our ingredients in organically certified vermiculture in our backyard," said Impossible Foods spokesman Rachel Konrad.

With beef as a baseline, Ms Konrad can make a strong case for the lower environmental impact of Impossible Foods' products, which uses much less genetically modified soy and 80 per cent herbicide than patties made from cows, according to the company.

But alt-meat is not the only way to lower protein-related climate impact. Poultry and pork both already have significantly lower greenhouse gas emissions than beef.

"From a climate perspective, if people are eating chicken sandwiches instead of burgers, it's a huge win," said Mr Richard Waite of the World Resources Institute. (Chicken comes with its own set of environmental and social issues, however, such as water pollution and allegedly exploitative labour practices.)

While there is broad scientific agreement that unchecked meat consumption is unsustainable for the planet, vegan burgers alone are not a quick fix, even if a patty swap is one of the simplest changes a person can make in the course of daily life. And there is even reason to hope that Americans, in part from the appeal of meat-free replacements for burgers, might end up shifting to consuming less meat while producing more of it.

"I want American producers to produce more beef because they produce it more efficiently," said Mr Tim Searchinger, a senior fellow at the World Resources Institute. "I want them to export it."

About 70 per cent of the world's cattle emissions come from low and middle-income countries, where inefficient production is meeting rising consumer demand, according to an August report by the United Nations-backed International Panel on Climate Change.

American cattle grow bigger, faster meaning they produce more meat and less greenhouse gases. That growth, which is achieved through feeding cows corn and soy at the end of their lives, as well as the routine use of antibiotics and artificial growth hormones, carries environmental, public health and animal welfare tradeoffs.

One thing climate experts agree on is that major dietary changes need to happen. Globally, agricultural practices need to be overhauled so that the same amount of land currently being used can produce even more food to feed a growing population.

In July, the World Resources Institute released a report with a 22-item "menu for a sustainable food future" that explores ways to reduce food waste, increase production, restore forests and wetlands, increase the fish supply, and reduce greenhouse gas emissions from agriculture. Incorporating more meat substitutes is one of the ways to make diets greener - but still, just one.