A striking new study - but one that is bound to prove controversial - has provided a calculation of both the health benefits and the reductions in planetary greenhouse gases that might be achieved if the world shifted away from meat-based diets.
The results, while theoretical in nature, certainly make a strong case for treating the food system, and animal agriculture in particular, as a key part of the climate change issue. Namely, the researchers find that shifting diets toward eating more plant-based foods on a global scale could reduce between 6 and 10 per cent of mortality - saving millions of lives and billions of dollars - even as it also cuts out 29 to 70 per cent of greenhouse gas emissions linked to food by the year 2050.
"Dietary change could have large health and environmental benefits," says Marco Springmann, the lead author of the new study in the Proceedings of the National Academy of Sciences, and a sustainability researcher at Oxford University (all four researchers involved in the work were from Oxford).
But the study itself acknowledges that the research in some ways represents an idealised experiment, and changing food systems as dramatically as envisioned in the study would be a momentous task.
The researchers say it is "the first time, to our knowledge", that health models and emissions models have been joined together in this way.
Much recent research has highlighted how agriculture, and especially eating meat, contributes to climate change. Ruminant animals, like cattle, belch methane into the air as part of their process of digestion. Methane is a potent greenhouse gas, especially over short time frames of several decades - which is when the key decisions about mankind's steps to address climate change will be made.
In addition, if tropical zones are deforested to make way for ranching, then animal agriculture can drive climate change in another way, since the planet's forests are major storage areas for carbon that might otherwise end up in the atmosphere.
Meanwhile, the World Health Organisation has recently charged that eating processed meats can be a risk factor for cancer, and a large body of health research points to the importance of consuming adequate fruits and vegetables in your diet to stave off a number of deleterious health outcomes.
Taking all of this as a premise, the new study uses a computerised model to examine four different dietary scenarios, for regions of the world and the planet as a whole, out to the year 2050. One is a standard "business as usual" outlook for our global diet, based on projections by the Food and Agriculture Organisation of the United Nations.
The second study scenario, by contrast, assumed a nation-by-nation implementation of a healthier diet in which people, on average, get adequate calories based on eating required amounts of fruits and vegetables, and consuming less meat and sugar (and not over-eating). That diet, says Springmann, consists of a "minimum five portions of fruit and veg, and half a portion of red meat per day." It was based on expert assessments of a healthier diet and required energy intake by the World Health Organisation and Food and Agriculture Organisation.
In another scenario, the study also considered an even stronger dietary shift toward vegetarianism; and, finally, a shift of diets toward full veganism. In both of those diets, the food eaten was consistent with dietary guidance from the World Health Organisation.
The research notes that these diets, as modelled in the study, are "not intended to be realisable dietary outcomes on a global level but are designed to explore the range of possible environmental and health outcomes of progressively excluding more animal-sourced foods from human diets". It acknowledges that "large changes in the food system would be necessary to achieve" them and that, in truth, it is not expected that the world's human population will get enough fruits and vegetables, or even food as a whole, over the first half of this century. (At the moment, 795 million people don't get enough food in the world, according to the UN World Food Programme.) Just to underscore this point, the healthy-eating diet alone would require 25 per cent more fruits and vegetables consumed globally, and 56 per cent less meat. The vegetarian and vegan diets require even larger shifts.
Springmann acknowledges that the changes that would be required - not just political or industrial, but cultural - would be massive. "We first want to show, is it actually worth thinking about it," says Springmann. "And we show, yeah, it's definitely worth thinking about it, and we hope with those numbers, we encourage more research and action to see how we get there."
Certainly, the changes are striking - the healthy diet led to 5.1 million fewer global deaths per year in the model by 2050, from conditions like heart diseases, stroke, and cancer, especially in developing countries. The researchers said that more than half of the effect was from reductions in meat consumption (other factors included less over-eating).
The other diets in the model saved even more lives.
At the same time, implementing these diets greatly cut greenhouse gas emissions from the food and agriculture sector. With the healthy diet that still contained some meat, global greenhouse gas emissions from the food sector only increased 7 per cent by 2050, compared with an expectation of a 51 per cent increase under business as usual. Again, the vegetarian and vegan diets had even sharper effects on emissions.
And as the study notes, "we did not account for the beneficial impacts of dietary change on land use through avoided deforestation", meaning that the theoretical reductions in greenhouse gases could be even higher.
The North American Meat Institute didn't share the researchers' views.
"We disagree with the premise of the study," Janet Riley, senior vice-president for public affairs, said by email, noting that the institute had not yet had the opportunity to review the research in detail.
"The authors suggest that somehow consensus exists that a diet that is lower in meat is healthier and we would argue that no such consensus exists," Riley said.
Overall, the ability to cut emissions from the food sector could be significant, because of the urgent quest, embraced by the nations of the world, to ramp down greenhouse gases quickly in the next few decades to avoid warming more than 2C above pre-industrial temperatures.
And as if that's not enough, the research finds that these dietary shifts could reduce healthcare costs - in the US more than any other nation.
"In terms of healthcare benefits, because the health expenditure is so large in the US, we find that the pure healthcare savings that would be associated with dietary shifts would be the largest actually of all countries," says Springmann. By contrast, two-thirds of the actual health benefits of the dietary shifts would occur in developing nations, the research found.
Granted, any major shift of global diets would implicate huge changes in government policy and in industry - and might trigger some major resistance, not only from food producers, but also from individuals who, to put it bluntly, like to eat meat.
But Springmann says that over time, he thinks cultural change will push the world in this direction. "We already see a plateauing of meat consumption in higher-income countries, like Europe," he says.
"So I wouldn't say that the cultures now are prescribed to be the same cultures that we have in 2050."