If everyone in the world consumed as much as those in North America, we would need five Earths. Earth has generously supported human consumption, but at heavy cost to the natural environment and life support systems we rely on for our very survival. Life on Earth is endangered, but mounting evidence is pointing to a simple but radical means of reversing this predicament: how we use our planet, and what we eat.
Most of us live in cities, which cover about 1% of the land used by humanity. We see the human impact there, but by far our greatest impact on planet Earth is rarely seen: two-thirds of the land we use is for food production. But 85% of this land is for livestock: grazing lands and feed crops. Changing how we use this 85% is key to reversing the environmental crises we face.
A 2016 study of United States land use found that a 100% plant-based diet with the same energy, protein and nutrient yield reduces land required by 88% . Globally, the same diet could reduce human-inhabited land use by 76% . To underline this point, humanity’s land area footprint could shrink to just one-quarter of current use by changing to plant-based food.
Many of us have only a vague understanding of the scale of human-bred livestock production, but it is immense. Nitrogen fertilizer use has effectively doubled the carrying capacity of our planet, placing enormous stress on ecosystems that evolved to support far less consumption [181, 182]. Our population of under 8 billion people requires 82 billion  animals (2016 data) at any one time kept for meat, eggs, and milk, their combined weight more than twice that of humanity[181, 153]. Indeed, 605 of mammals now on planet Earth are livestock .
Here lies the answer to those of us who argue that our main problem is that of the human population: it's not human mouths to feed that are the problem, but livestock mouths to feed.
The 2006 United Nations Food and Agriculture Organization report Livestock's Long Shadow  laid out the many impacts of global livestock production, concluding that such production was one of the most significant contributors to global environmental problems at every scale from local to global. A further United Nations Environment Programme (UNEP) report in 2010 found that reducing environmental impact would only be possible with substantial global diet change away from animal products .
Lack of adequate water is considered by some as one of the greatest threats to civilisation, and has already led to millions of people leaving their homelands.
Overuse of above ground flows has caused famine and conflict. Overuse of ground water has resulted in sinking water tables and agricultural insecurity. Climate change multiplies this threat. Water scarcity has already created millions of climate refugees, heightened international tension and even resulted in armed conflict. By 2030, half the world's population will be living in areas of high water stress , and the world will be facing a 40 percent global water deficit .
Reducing livestock numbers, therefore, provides a powerful solution to relieve stress on current water supplies and ensure water for the future. The Stockholm International Water Institute reports that by 2050 there will only be enough water to feed our population if we slash our global consumption of meat by 75% . To many of us this may be unthinkable, or an immense challenge, but business-as-usual is likely to leave us with a flood of refugees and conflict.
Closely linked to water availability, food for a growing world is facing a ‘perfect storm’ of nitrogen overuse, land degradation, water availability, land availability and climate change. Producing the expected amount of food by 2050 while protecting the remaining forests is simply not possible without a major change in diet. Our world cannot support both the projected human population and the growth in livestock population.
Many of us do not appreciate that half the world’s crops are fed to livestock, not people. If all grain and oilseed crops were devoted to feeding people rather than livestock, global food production would sustain an extra 4 billion people – our projected population by 2100 [25, 165].
A 100% global plant-based global diet would provide ample food for the projected population of 11 billion people, while dramatically reducing the burden on ecosystems that we need to support life [25, 165].
The world is in the grips of the sixth great extinction. The cause – human impact. Biodiversity, the web of life that we depend on for food, water and health is being destroyed.
In 2010 the Netherlands Environmental Assessment Agency published a major report on loss of wildlife, finding that if grazing lands are 're-wilded', this releases two-thirds of the human-used land to return to native habitat. This was found to be the least expensive and most effective means we have of halting species population loss and extinctions [184, 11].
A 2019 study by UK and German researchers also found that diet change towards a plant-based diet was the most powerful tool to halt biodiversity loss, particularly in tropical forests, the most biodiverse ecosystems on Earth .
Livestock agriculture is by far the greatest threat to forests, the bulk of forest loss driven by conversion to grazing pasture and livestock feed. Countries that import livestock feed, even if they have minimised forest loss within their own borders (such as China) are driving forest destruction in developing countries. And fires used to stop forest regrowth and remove dead grass for better pasture are a major source of harmful black carbon, smoke and other pollutants.
If humanity were to adopt a 100% plant-based diet, deforestation would all but cease  and natural and assisted regeneration could take over.
Our ravenous appetites have stripped the oceans of fish; trawling has ploughed the continental shelves and destroyed ecosystems; nitrogen and other pollutants have caused hundreds of dead zones devoid of oxygen and life; and atmospheric pollution is causing the oceans to turn acid and warm, the death knell for coral reefs.
It’s easy to see how diet can affect overfishing and ecosystem destruction, but diet can also have a major impact on ocean acidity, warming and dead zones.
Carbon dioxide causes ocean acidification, and together with other greenhouse gases is responsible for warming oceans. Agricultural carbon dioxide emissions come mostly from deforestation, which makes up about 20% of global carbon dioxide emissions . We know that deforestation would all but cease if livestock production ceased , therefore these emissions would immediately reduce accordingly.
But there is transformational potential in reducing areas devoted to pasture. Researchers in the United States found that if 41% of the world’s grazing pastures were “re-wilded” and the original vegetation was re-established, this would draw down 27 years of current carbon dioxide emissions . Adopting this measure, also determined to be the least-cost climate mitigation option , would be truly transformational, ‘winding back the clock’ on carbon dioxide emissions. Forest growth would lower ocean acidification since carbon dioxide is readily exchanged between the atmosphere and the oceans.
And it does not end there – the greatest source of human-caused methane emissions is from grazing livestock , and methane is a major contributor to global warming. In a 2017 study, European researchers found that if a global diet without ruminant products were adopted (no red meat or dairy), the drop in methane would slow warming by 15-20 years, buying time to move away from fossil fuels . In this way, ocean warming would also be abated, offering hope for coral and ocean ecosystems imminently threatened. Organic or ‘agroecological’ farming methods also show great promise in storing carbon in soils, offering more relief for the oceans.
Ocean dead zones can also be directly improved through diet change. Many authors are now seeing meat consumption as the most effective means of reducing nitrogen (protein) pollution, the main cause of ocean dead zones.
Synthetic nitrogen fertilizer has enabled the ‘green revolution to feed humanity’s rapid livestock population growth, however, the world is now awash in reactive nitrogen. Reactive nitrogen builds protein, but the pollution it causes has been identified as the single greatest cause of environmental pollution , creating lifeless waterways, toxic tides, and hundreds of permanent ocean dead zones. To see how livestock causes nitrogen pollution, the worst example is beef production, where just 4% of the nitrogen (protein) taken in by the animal ends up in our bodies; the rest is nitrogen waste .
The consensus view now emerging is that a 50% reduction in production and consumption of animal products – along with an accompanying transition in animal product investment and supply chains – will stem the cause of this pollution .
Methane is a potent greenhouse gas, and two-thirds of methane emissions are human-caused , the greatest source (responsible for 37% of emissions) being ruminant livestock (cattle, sheep and goats) . Ceasing livestock production entirely will have a major impact on methane emitted: global production of cattle and sheep is responsible for 19 to 48 times more greenhouse-gas emissions, based on weight of food produced, than the global production of protein-rich plant foods like legumes .
Methane lasts about 10 years in the atmosphere, and this short life gives us a transformational means of moderating and slowing global warming in coming critical decades. Cutting methane emissions by half is equivalent to cutting all carbon dioxide (CO2) emissions for a global temperature target 45 years ahead . Steep reductions in the major source of methane emissions, ruminant livestock, is doubly important now that our planet is fast approaching tipping points .
Just as importantly, a move away from red meat and dairy releases grazing pastures – more than a third of Earth’s land surface – to revegetate; hydrating and cooling local climates, increasing productivity and resilience and re-absorbing decades of emissions from the atmosphere. Allowing grazing lands to revegetate and regenerate by removing livestock is by far the lowest cost and most scalable climate mitigation option . All credible climate mitigation solutions now recognise that preserving and re-growing forests is critical to a habitable climate, and this in turn relies on re-purposing land now largely used for grazing pasture [189, 190].
Published research on livestock's environmental impact is growing rapidly. Many publications now address livestock’s impact on climate, biodiversity, nitrogen pollution, forest destruction, and water and air pollution, among others. Livestock's impact is also being linked to overstepping planetary boundaries, to the UN Strategic Development Goals and to climate change projections.
Authors are now explicitly modeling impact scenarios based on diet, most recently the 100% plant-based or vegan diet. To quote from a 2018 article by Oxford University authors "a diet excluding animal products has transformative potential, reducing land use by 76%, agricultural greenhouse emissions by 50%, acidification by 50%, eutrophication by 49% and freshwater withdrawals by 19%." 
To propose a scenario of our own, imagine for a moment that the world was to completely shun animal products. More than two-thirds of the occupied land could then be freed to return to native vegetation, native habitat. This would profoundly impact the current biodiversity crisis  and as vegetation regrows, the climate crisis .
This information is starting to reach public awareness because these arguments to reduce livestock are compelling and cannot be long ignored. Thankfully, the strong growth in the number of people adopting the 100% plant-based diet is now considered a megatrend, prompting large investments to flow into meat and dairy substitutes and lab-grown meats.
Working within environmental boundaries is not just important for Earth’s ecological health, it turns out that similar changes to diet are equally important for our physical health. In 2019 the EAT-Lancet Commission on healthy diets and sustainable food found that, for human and environmental health, major changes to our food systems are needed .
The Food in the Anthropocene report found that global red meat and sugar consumption must be cut by half, while vegetables, fruit, pulses, and nuts must double. High-income and developed country populations, with heavy meat and dairy consumption, will need dramatic changes – North Americans need to eat 84% less red meat but six times more beans and lentils to achieve this. For Europeans, eating 77% less red meat and 15 times more nuts and seeds meets the guidelines.
So what do you want your personal footprint to be? We know that eliminating livestock would solve or go a long way to solving our most pressing environmental problems, while at the same time assuring us of plentiful food for our growing population . Would you choose a plentiful future for you, your family, and humanity, or a future where you know we are depleting our planet's environmental reserves and will face an inevitable 'credit crunch'?
It's empowering to know that the future is in our hands.