The Survivability Trap

The world's population, 8.2 billion in 2024, is projected to peak at about 10.3 billion in the mid-2080s and stand at 10.2 billion in 2100. Almost all of the growth comes from sub-Saharan Africa, whose share of humanity rises from 16% to 34%. Africa as a whole grows from 1.5 billion to 3.8 billion. South Asia's share holds near a fifth: India peaks at about 1.7 billion around mid-century and falls back to 1.5 billion, while Pakistan doubles to 510 million. East Asia's share halves, from 29% to 15%, as China's population falls from 1.4 billion to about 630 million; Europe and Central Asia's falls from 11% to 8%.

Together, sub-Saharan Africa and South Asia hold 37% of humanity today and 52% in 2100, crossing half in 2087. Nigeria (from 230 to 480 million), DR Congo (110 to 430 million) and Ethiopia (130 to 370 million) become among the most populous countries on Earth. This shift is driven by the timing of the demographic transition: fertility has already fallen below replacement in most of Asia, Europe and the Americas, and is falling in Africa, but from higher levels and later, so populations keep growing for decades on momentum.

The shift is not a problem in itself. It is a large and young population whose energy and talents will shape the century. The problem is where it meets climate and pollution. The regions gaining population are the hottest, among the most polluted and, today, the least resourced to adapt. The rest of this report measures what that means for the conditions of an ordinary life.

Humanity has long lived in a narrow band of climates. Today, half of all people live where the mean annual temperature is between 15 and 26 °C, and the median person lives at 21.9 °C. By the 2090s under middle-of-the-road warming (SSP2-4.5), the median person lives at 26.1 °C; under SSP3-7.0, at 27.7 °C; even under SSP1-2.6, at 24.4 °C. The rise for the median person, 2.5–5.7 °C from the 1995–2014 climate, is 1.3 to 2.6 times the rise in global mean temperature over the same period, depending on the pathway, for two reasons: land warms faster than the ocean, and population grows fastest in places that are already hot.

The whole distribution shifts. Today, a quarter of humanity lives above 26.0 °C and a tenth above 27.4 °C. By the 2090s under SSP2-4.5, a quarter lives above 28.5 °C and a tenth above 29.8 °C; under SSP3-7.0, above 29.4 °C and 30.8 °C. The right-hand chart shows the human climate distribution today, with one peak at 15–17 °C and a larger one at 25–26 °C, and its 2090s version: the cooler peak shrinks as Europe and East Asia age and decline, and the warm peak grows and moves to 27–29 °C, with a long shoulder beyond.

Temperature alone does not determine suffering; humidity, housing, work, cooling and health all matter. But mean temperature is a reliable summary of how often a place is dangerously hot. Xu and colleagues showed that for 6,000 years humans have concentrated in a narrow range of mean temperatures, and that 1 to 3 billion people could be left outside it within 50 years. The next section measures how many.

A mean annual temperature of 29 °C or more is found today in only a few inhabited places, mostly in Sudan and the Sahel (Niger, Mali, Chad, Mauritania, Burkina Faso), parts of Arabia and Yemen, and central Thailand; 0.11 billion people, 1.4% of humanity, live there. Lenton and colleagues estimate that warming of about 2.7 °C, roughly where current policies lead, would leave a third of people (22–39%) outside the broader human climate niche by the end of the century, and that limiting warming to 1.5 °C would cut exposure to unprecedented heat about five-fold.

On the gridded projections used here, the number living in unprecedented heat rises to 0.6 billion under SSP1-2.6 by the 2090s (8% of people), 1.6 billion under SSP2-4.5 (18%), 3.0 billion under SSP5-8.5 (39%) and 3.8 billion under SSP3-7.0 (32%). Climate models disagree on the details: for SSP2-4.5 the range is 1.0–3.1 billion, for SSP3-7.0 2.3–6.2 billion. Under every pathway, the countries most affected are the same. By the 2090s under SSP2-4.5, nearly everyone in Mali, Burkina Faso, Niger and Chad, nearly nine in ten in Sudan and South Sudan, three quarters in Thailand and Benin, and half of Nigeria's 480 million would live in unprecedented heat.

India's exposure depends heavily on the pathway: 2% of its people live in unprecedented heat today, 9% by the 2090s under SSP1-2.6, 23% under SSP2-4.5, 42% under SSP3-7.0 and 86% under SSP5-8.5. Because India is so large, these percentages translate into hundreds of millions of people. The difference between the low and high pathways, for India and for the world, is the difference between a challenge that can be adapted to and a humanitarian emergency on a scale without precedent.

Mean temperature summarises the climate; dangerous days describe life within it. A heat index above 41 °C is the US National Weather Service's "danger" category, in which heat cramps and exhaustion are likely and heatstroke possible with prolonged exposure or physical activity. Outdoor work, sleep without cooling, pregnancy, infancy, old age and chronic illness all become hazardous. Today, three quarters of humanity experiences less than one such day a year; the most exposed tenth, mainly in Pakistan, northern India and the Gulf, experiences nine or more.

By the 2090s, the threshold of the worst-off quarter of humanity rises to 27 days a year under SSP2-4.5, 68 under SSP3-7.0 and 105 under SSP5-8.5. The threshold of the worst-off tenth rises to 74, 115 and 154 days. People living with three months or more of dangerous heat each year go from about a million today to 0.5 billion under SSP2-4.5 and 2.2 billion under SSP3-7.0 and SSP5-8.5. In Pakistan, the 2090s average under SSP2-4.5 is 83 days, in Bangladesh 66, Cambodia 58, Thailand 56 and India 50, up from 7 today.

Days with a heat index above 35 °C, conditions in which work and exercise are already hazardous, become the norm: the median person, who has almost none today, would have 46 such days a year under SSP2-4.5 and 93 under SSP3-7.0; the worst-off quarter, 141 and 178. Physiological survival limits, where even a healthy person resting in shade cannot cool down, are crossed only in limited places even at high warming. The burden of the century is not mainly sudden death but chronic, prolonged stress: lost work, lost sleep, lost learning, illness and early death, concentrated on people with the least protection.

Air pollution behaves differently from carbon dioxide. Fine particles stay in the air for days, not centuries, so pollution falls quickly when emissions fall. China proved it: its population-weighted PM2.5 fell by about two-fifths after 2014, pulling the world average down from a peak of 35.9 µg/m³ in 2015 to 29.9 in 2024. But the improvement is uneven. South Asia's average, 46 µg/m³, is down from a peak of 57 but still the highest of any region; the Middle East and North Africa average 41, much of it desert dust; sub-Saharan Africa 32 and rising slowly. North America breathes 7.

Because population is shifting to the most polluted regions, the average person would breathe more polluted air even if every country's pollution stayed at today's level: the world's population-weighted PM2.5 would drift from 29.9 to 31.7 µg/m³ by 2100 from composition alone. And because hot places and polluted places overlap, the number of people living in both hot conditions (mean temperature 27 °C or more, or 30+ days of dangerous heat) and PM2.5 above 35 µg/m³ rises from 0.6 billion today to 3.0 billion by the 2090s under SSP2-4.5 and 4.8 billion under SSP3-7.0, if pollution does not fall.

Heat and pollution compound each other's harm. Across 620 cities, the extra deaths on the hottest days were 5.3% where PM10 was low and 12.8% where it was high (Stafoggia and colleagues, 2023); this platform's own city study found ozone higher on hot days in 131 of 149 cities. Smoke from landscape fires, which hotter, drier conditions make more frequent, already exposes 2.18 billion people a year to at least one day of substantial pollution (Xu and colleagues, 2023). Clean air is therefore heat adaptation: every microgram removed lowers the toll of every heatwave.

Divide humanity into four equal groups by the heat of the places they live. Today, the hottest quarter is 39% South Asian, 24% sub-Saharan African and 23% East Asian (mostly Southeast Asia); India alone is 35% of it, Nigeria 9%. It lives at an average of 27.3 °C, with five days a year of dangerous heat, breathing PM2.5 of 37 µg/m³. The coolest quarter lives mostly in Europe, North America, Russia and northern China, at 10 °C.

By the 2090s under SSP2-4.5, the hottest quarter is 42% sub-Saharan African and 26% South Asian: India 25%, Nigeria 20%, Pakistan 6%, Indonesia 5%, the Philippines and Niger 4% each. It lives at an average of 29.7 °C, beyond the human climate niche, with 33 days a year of dangerous heat on average and PM2.5 of 39 µg/m³ if pollution does not fall. Under SSP3-7.0, it averages 30.8 °C and 68 days. The middle half of humanity, the second and third quarters, is by then 57% sub-Saharan African or South Asian, lives between 19 and 28.5 °C and averages 7 to 35 days of dangerous heat a year under SSP2-4.5, and 17 to 60 days under SSP3-7.0.

These are also the people with the fewest protections: the lowest incomes, the least access to cooling, electricity, clean cooking and health care, the most outdoor and informal work, and the weakest social protection. Their own emissions are among the lowest on Earth: Lenton and colleagues estimate that the person exposed to unprecedented heat by the end of the century comes from a place emitting about half the global average per person today. This is the core of the humanitarian case: the burden falls on those who did least to cause it and have least to withstand it.

Global temperature tracks cumulative carbon dioxide emissions. The IPCC finds that reaching net zero CO2 is required to stabilise warming, and that only sustained net zero across all greenhouse gases would bring a slow decline after a peak. In practice, the temperature reached when emissions stop is the temperature humanity lives with for centuries. Every pathway in this report except SSP1-2.6 is still warming in the 2090s; under SSP3-7.0 and SSP5-8.5, warming continues well into the 22nd century. The heat measured here for the 2090s is therefore a floor for the centuries that follow, not a ceiling.

Other changes are slower still. The IPCC projects that over the next 2,000 years, sea level will rise by about 2 to 3 metres if warming is limited to 1.5 °C, 2 to 6 metres at 2 °C and 19 to 22 metres at 5 °C, and keep rising beyond. Ocean warming, acidification and ice loss are irreversible on human time scales. Deltas and coasts that hold hundreds of millions of people in Bangladesh, Egypt, Nigeria, Viet Nam and elsewhere face a long, slow retreat on top of the heat.

Demography also has momentum. Africa's population is still growing in 2100, and the regions that will hold most of humanity are the ones where heat will be greatest. Without deliberate action, the conditions described here would not be a passing crisis but the lasting setting of human life for a majority of people, for generations. That is why the response must be built now: emissions cuts decide how hot the floor is, and development, clean air and protection decide how people live on it.

The atlas shows each country's population change to 2100 on the UN's central projection, the population-weighted mean annual temperature where its people live today and in the 2090s under SSP2-4.5, days of dangerous heat, and the share of its people in unprecedented heat.

The pollution layer shows national population-weighted PM2.5 in the latest year (2024). The card for each country shows all four pathways, so the difference emissions make can be read country by country.

Hover over or tap a country for its card. The sortable table lists all 156 countries with at least 1 million people; filters pick out countries whose population at least doubles, the low-income group, and countries where most people would live in unprecedented heat under SSP3-7.0.

Lower the floor: cut emissions now. The difference between SSP1-2.6 and SSP3-7.0 is 3.2 billion people in unprecedented heat and 2.2 billion with three months of dangerous heat a year. Every tenth of a degree avoided keeps millions inside the human climate niche, and because warming persists for centuries, emissions cut this decade protect people for generations. Those who will bear the heat emit least; those who emit most carry the responsibility to cut fastest and to finance the response.

Clean the air and power development. Air pollution can fall within years, as China showed, and every reduction lowers the toll of heat. Clean energy, clean cooking, cleaner transport and industry, and an end to open burning in South Asia and Africa cut pollution and emissions together. Electricity and cooling, from efficient fans and air conditioners to cool roofs, shaded streets and passive building design, turn dangerous heat from a killer into a hardship. Development itself is protection: education, income, health systems and social protection are what let people withstand conditions their parents never faced.

Prepare the places and protect the people. Heat-health action plans with early warnings, outdoor-work standards, protection for older people, children and pregnant women; water security; heat-resilient agriculture; and cities planned for the heat of 2080 rather than 1980. Mobility is part of adaptation: safe, legal pathways for people to move within and between countries, and planned support where places become too hard to live in, reduce suffering for those who move and those who stay. And global solidarity in finance, technology and humanitarian preparedness must match the scale of a crisis that will define the lives of the century's majority.

Climate: World Bank CCKP CMIP6 multi-model median climatologies at 0.25° for 1995–2014 and 2020–39, 2040–59, 2060–79 and 2080–99 under SSP1-2.6, SSP2-4.5, SSP3-7.0 and SSP5-8.5: mean annual near-surface temperature (tas), days with heat index above 41 °C (hi41) and above 35 °C (hi35); 10th and 90th model percentiles for 2080–99 under SSP2-4.5 and SSP3-7.0. Population: SSP-consistent gridded projections (Jones and O'Neill) on the same grid for 2020 and each period's midpoint. Percentiles are population-weighted over all inhabited cells; quarters are formed by ranking people by mean temperature.

Pollution: national population-weighted PM2.5 (Washington University SatPM via Our World in Data), latest year (2024), assigned to cells by country and held constant in projections. Composition: UN World Population Prospects 2024, medium variant, by World Bank region. Unprecedented heat follows Xu et al. (2020) and Lenton et al. (2023): mean annual temperature ≥ 29 °C. Dangerous heat follows the US National Weather Service heat-index categories. Hot and polluted: mean ≥ 27 °C or 30+ days above 41 °C, with PM2.5 ≥ 35 µg/m³.

Caveats: climate climatologies are model medians and differ across models (ranges reported); the SSP gridded populations differ from the UN medium projection (SSP2 has 9.2 billion people in 2090, SSP3 12.2 billion); national pollution is applied uniformly within countries and is held at today's levels, though it may fall or rise; heat-index thresholds and mean temperature are summaries of heat stress, not measures of mortality; adaptation, cooling and development will change what exposure means. Exposure is not impact, and none of these futures is fixed.