What would happen if everyone stopped eating meat?

Livestock account for a substantial share of agricultural greenhouse gas emissions, with ruminant methane the largest single component, and they occupy a majority of the world's agricultural land while supplying a minority of its calories. A global shift away from meat would therefore free very large areas of pasture — much of which is not suitable for crops and would revert to grassland or forest, absorbing carbon — and reduce methane emissions sharply within a decade, since atmospheric methane has a lifetime of about twelve years. The costs are concentrated rather than diffuse: several hundred million people depend on livestock for income, and in arid and mountainous regions grazing animals convert vegetation humans cannot eat into food on land that grows nothing else. Nutritional consequences would be manageable in wealthy countries and harder elsewhere, with vitamin B12, iron and zinc requiring deliberate fortification. The uncertainty is not the physical accounting but the transition.

How confident this is

This is a reasoned extrapolation rather than an observation. The mechanism is understood, but the scale of the consequences depends on assumptions that could reasonably be argued differently.

Society

• What would happen if everyone stopped having children? Schools would empty within a decade, the workforce would begin shrinking in about twenty years, and the last human would die in roughly a century.
• What would happen if antibiotics stopped working? Routine surgery, chemotherapy and childbirth would all become far more dangerous — this is already happening slowly.
• What would happen if all jobs were automated? The production problem would be solved and the distribution problem would not — nobody has demonstrated the second half.
• What would happen if half the world's population disappeared? Not abundance — the collapse of specialised supply chains, because modern systems need specific people rather than a certain number of them.
• What would happen if the world's population doubled overnight? Food is the binding constraint — global production feeds around 8 billion, and doubling demand cannot be met from existing stocks.
• What would happen if everyone lived to 150? It depends entirely on whether those are healthy years — extended lifespan without extended healthspan makes almost everything worse.
• What would happen if everyone worked a four-day week? Trials generally show output holding steady with better wellbeing — but the results come from office work, not from hospitals or factories.
• What would happen if everyone drove an electric car? Urban air quality would improve immediately; the grid would need roughly a quarter more electricity, and mining demand would rise sharply.
• What would happen if everyone spoke the same language? Trade and science would get modestly easier, and roughly seven thousand languages worth of knowledge would be lost.
• What would happen if nobody paid taxes? Everything funded collectively stops — roads, courts, defence, schools, health systems and pensions — within weeks to months.
• What would happen if all borders opened? Economic models suggest a very large rise in global output, and nobody can model the political response, which is the part that would decide it.