The distinguished medical journal The Lancet has issued not one but two apocalyptic warnings about our food in under a month. One of its special commissions reported earlier this month that civilisation itself was at risk from the effects of the current food system on both human health and the Earth’s ecosystems. This week comes the next instalment from another special Lancet commission which finds that pandemics of obesity and malnutrition are interacting with climate change in a feedback loop and represent an existential threat to humans and the planet. The modern western diet has become a highly damaging thing that needs a complete overhaul if we are to avoid potential ecological catastrophe. It concluded that we need to halve global meat consumption, and more than double the volume of whole grains, pulses, nuts, fruit and vegetables we eat.

Cue howls of indignation from big food and its cheerleaders, the libertarian right. Those nanny statists have gone nuts eating their own double dose of nuts! Cue cries of distress from champions of local, low-impact agriculture who include grass-fed animals, and their meat and manure, in their sustainable mix. These self-appointed experts don’t understand farming! Cue grim food wheels with only a quarter of a rasher of bacon or a fifth of an egg a day. Those miserabilist medics want us all to go vegan!

Yet the evidence that our diets are the largest cause of climate change and biodiversity loss is now overwhelming. The global food system is responsible for up to 30% of total greenhouse gas emissions, the livestock sector on its own accounting for about half of that total or 14.5%. The modern western way of eating is also making very large numbers of people fat and sick as other parts of the world adopt it. Diet-related diseases now cause roughly 11 million deaths a year as preventable cancers, heart disease and strokes, obesity and diabetes have spread along with our way of eating. More than 800 million people are estimated to be chronically undernourished, and 2 billion suffer from micronutrient deficiencies, yet at the same time 2 billion are overweight or obese. In poorer counties you can even find obesity and stunting within the same family as calorie-heavy but nutrient-light processed industrialised foods are adopted.

In other words, something has gone horribly wrong and we don’t have much time to fix it. The so-called “reference diet”, published in the first Lancet report, has caused uproar in some quarters. This is a theoretical attempt to answer the Malthusian question: if the global population reaches 10 billion by 2050 (as is anticipated), will there be enough food to meet everyone’s basic nutritional needs without cutting down more forest, polluting more water courses and generally destroying the planet? The answer it comes up with is yes, but only if we share things out differently, and stop feeding a quarter of the world’s grain to animals.

The reference diet models each person globally having 14g of red meat a day, 29g of chicken, a fraction of an egg, 250g of dairy, a little fat or oil, very little sugar, and lots of grains and lentils, vegetables and nuts. And here’s where the trouble begins, because calculated day by day and universally these allocations make for a pretty depressing plateful. They contain some odd judgments. Why favour industrialised chicken? Why palm oil? They are plain weird if you try to interpret them as daily instructions without thinking about the diet as a whole or what’s available to people locally – hence the one-fifth of an egg, the quarter rasher of bacon. People don’t shop and eat by numbers and fractions. And the fact that nutrition epidemiology – the study of patterns in diet and disease – has got it wrong before does not help. Remember when fears about dietary cholesterol were used to condemn eggs? These instructions sound top down and, worse, appear to throw responsibility back to the individual.

More than 800 million people are estimated to be chronically undernourished, yet 2 billion are overweight or obese

But that is to miss the point of these instructions. They are calculations that give a sense of the scale of the problem and a science-based framework for political action. Contrary to some claims, the Lancet commission that undertook the modelling, supported by the philanthropic EAT Foundation, received no funding from industry. And the uncomfortable truth is that meat, and specifically the meat from intensively-fed livestock, processed in the way it generally is in the western diet, is a hotspot when it comes to greenhouse gas emissions and ill health. Well-produced meat and dairy can have a place, but it will be a small one, and it is expensive.

Our diets are out of kilter with what’s good for both us and the planet because powerful vested interests and misplaced economic incentives have driven them in that direction, and this is the thesis underlying the commission’s findings. The equivalent of $500bn in agricultural subsidies goes each year to the wrong sort of food – corn, soya, meat and dairy, as cheap raw materials for intensive livestock production and for highly processed foods. About $5tn a year goes in subsidies to the fossil fuels which industrialised agriculture uses so profligately. Big food has spent hundreds of millions advertising unhealthy food and lobbying to block the sort of measures that might help shift consumption.

Although individuals can make a difference by modifying their diets, and send clear demands for action to politicians , we cannot redraw the food system on our own. That will require not just governments, but global agreement.

The Lancet says the problem is so big and so urgent that we need an intergovernmental convention to agree a way forward, in the way that the Intergovernmental Panel on Climate Change or the World Health Organization’s Framework Convention on Tobacco Control have. Expect plenty of hysterical, dark-money opposition to that. The Lancet, founded in the 19th century, caused uproar back then by publishing the unwelcomenews that food was routinely adulterated. It has once again kickstarted a vital debate about how we eat. If it all sounds too gloomy, it’s worth remembering that the modern western diet is a recent invention. The separation of livestock from the land only took off in the 1950s, thanks to cheap energy to keep animals housed, to synthetic fertilisers increasing grain production for feed, and to mass production of antibiotics to control disease. It’s not so very long since most people ate in the way we now need to rediscover.

• Felicity Lawrence is a special correspondent for the Guardian and author of Not on the Label and Eat Your Heart Out