What would happen if the world's population doubled overnight?

The world produces enough food for its current population with a modest surplus, and global grain stocks typically cover a few months of consumption. Doubling the number of eaters overnight would exhaust those stocks in weeks, and agricultural output cannot double on a timescale shorter than years even with unlimited inputs, because it is bounded by land, water and the growing season. Water is the second constraint, and a sharper one regionally: several major aquifers and river systems are already withdrawn faster than they recharge. Housing, sanitation and transport would fail in the same period, and the resulting mortality would be concentrated where food import dependence is highest. Interestingly, physical space is not the issue at all — the entire population could be housed at the density of a mid-sized European city inside a small country — which is a reminder that carrying capacity is about flows, not floor area.

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 everyone stopped eating meat? A large drop in agricultural emissions and land use, offset by disruption to a billion livelihoods and some regions with no alternative.
• 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 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.