Human Exposure Across the Polycrisis Century

The UN projects that the world's population will grow from 8.2 billion in 2025 to about 10.3 billion in the 2080s and then edge down. Summed over the years, that is about 739 billion person-years between 2025 and 2100. Three scenarios bracket how many of them are spent exposed. Current trends follows the IPCC's middle pathway, SSP2-4.5, which reaches about 2.7°C by the 2080s and 2090s; lets air pollution keep falling by 2% a year, the world's population-weighted rate over 2016–2024; and holds today's risk map fixed. Drift follows SSP3-7.0 (about 3.6°C) with no progress on air quality. Sustainable combines SSP1-2.6 (about 1.8°C), a 5%-a-year cut in air pollution and peer-level resilience with conflict halved by 2050.

On current trends the ledger records 306 billion person-years in air above the WHO's interim target, 148 billion in hot climates and 259 billion in high-risk countries. Because these hazards concentrate in the same places, 180 billion person-years are spent under two or more at once, and 47 billion under all three. Under drift, the overlapping years double to 367 billion and the triple-hazard years more than double to 108 billion. Under the sustainable scenario they fall to 54 billion and 8 billion — a sevenfold difference from drift in the overlap and thirteenfold in the triple.

Exposure is also unevenly spread in time. On current trends, 54% of the century's air-pollution person-years fall before 2050, because pollution is high now and falling. Only about a quarter of the high-risk person-years fall before 2050, because the populations of today's high-risk countries are growing and accumulate most of their years later. Heat moves later still, rising with warming. The order matters for policy: the years that can be avoided soonest are air-pollution years, while risk and heat years are built up by choices made now but lived later.

Almost everyone breathes air dirtier than the WHO's guideline: in 2022 the WHO reported that 99% of the world's population lives where its air-quality limits are exceeded. The ledger therefore uses the less demanding third interim target of 15 µg/m³ of fine particles as its hazard line. In 2025, about 6.5 billion people live in countries whose population-weighted average exceeds it. Population-weighted exposure worldwide has fallen from 35.1 µg/m³ in 2016 to 29.9 in 2024, about 2% a year, led by China's clean-air programme; the satellite-based series includes natural dust, so countries near deserts start from high levels that policy alone cannot remove.

The three air pathways diverge sharply. If every country's pollution stopped falling, 630 billion person-years — 85% of the century — would be spent above the interim target. If the 2016–2024 rate of 2% a year continued everywhere, the figure would fall to 306 billion, with South Asia, Nigeria and the Gulf states still above it in the 2070s. A clean-air push of 5% a year — roughly China's pace in its most intense decade — would cut it to 106 billion. Against the guideline of 5 µg/m³, even the push leaves 298 billion person-years exposed, a reminder that the guideline is a long-run goal.

Air is the hazard where cumulative accounting makes the strongest case for speed. Most avoidable person-years are in the next three decades, because pollution is highest now and populations in the most polluted countries are large. A year of delay in a country of 1.4 billion people breathing 44 µg/m³, as India does, costs more person-years than a decade of delay in a small, cleaner country. The State of Global Air report attributes 7.9 million deaths in 2023 to air pollution, which is why these person-years are not abstractions.

Heat exposure in the ledger follows the IPCC's assessed pathways. Global warming reaches about 1.5°C in 2021–2040 on every pathway, then diverges: 1.7°C to 2.4°C in 2041–2060 and 1.8°C to 4.4°C in 2081–2100. Each country's 1991–2020 mean temperature is raised by its share of that warming, using a land-to-global ratio of 1.36 derived from the IPCC's estimate that land warms about 1.6 times as fast as the ocean. In 2025 about 0.7 billion people live in countries with a national mean of 27°C or more, mostly in West Africa, the Sahel, the Horn and the Gulf.

Within this century the pathways differ less than might be expected for the 27°C line: 103 billion person-years under SSP1-2.6, 148 billion under SSP2-4.5 and 231 billion under SSP3-7.0, because most of the warming before 2050 is already committed and the pathways separate mainly after 2060. At the extreme the difference is large. Person-years in countries whose mean reaches 29°C — the upper edge of the human climate niche identified by Lenton and colleagues — rise from 12.5 billion under SSP1-2.6 to 51 billion under SSP2-4.5, 80 billion under SSP3-7.0 and 131 billion under SSP5-8.5. By 2100 under SSP2-4.5, Nigeria, Sudan, Côte d'Ivoire, Niger, Mali and Chad cross that line.

National averages make heat exposure lumpy. Under SSP3-7.0, India's area-weighted mean crosses 27°C in the early 2070s, moving about 1.7 billion people into the hot category in a few years; that single crossing accounts for much of the late jump in the chart. Within countries, people in the hottest plains cross such thresholds decades earlier, and people in highlands later. Lenton and colleagues, using fine-scale data, estimated that around 2.7°C of warming could leave 22–39% of people outside the human climate niche by 2080–2100. The ledger's national measure is cruder but points the same way, and shows the same sensitivity to the pathway.

The socioeconomic hazard is living in a country classed as high or very high risk by the INFORM Risk Index, which combines hazard exposure, vulnerability and lack of coping capacity. In 2026, 44 countries with 2.0 billion people are in those classes. Holding the map fixed, their population grows to 3.0 billion in 2050 and 4.4 billion in 2100, as this series' report on human exposure to 2100 showed. Summed over the century, that is 259 billion person-years — 35% of all the years humanity will live — spent in high-risk countries.

Risk is the hazard that policy can move fastest in principle, because its vulnerability and coping components are about institutions, services and incomes rather than physics. If every country reached the best quarter of its income group on vulnerability and coping capacity by 2050, recomputed with INFORM's formula, the high-risk person-years would fall to 157 billion. If conflict hazard were also halved, they would fall to 88 billion. Because the populations concerned are growing fast, every decade of delay costs more person-years than the last.

Unlike air, most of the risk ledger lies in the future: only about a quarter of the high-risk person-years fall before 2050 on the frozen map. That makes it the clearest case of exposure being built now and lived later. The institutions, health systems and peace settlements of the 2030s will determine the risk faced by children born in the 2050s and their own children, in countries whose populations will double or triple.

The three hazards are not independent. Polluted air, heat and high risk concentrate in the same belt of countries, from the Sahel and West Africa through Central and East Africa to the Middle East and South Asia. On current trends, 180 billion person-years are spent under at least two hazards and 47 billion under all three. In 2025, ten countries face all three, including Nigeria, Niger, Chad, Mali, Sudan, South Sudan and Burkina Faso. Over the century, the overlap moves from air-plus-risk, which dominates today, to heat-plus-risk, which grows as warming and population rise in the same places.

Sub-Saharan Africa accounts for 56% of the overlapping person-years on current trends, South Asia for 26% and the Middle East and North Africa for 12%. East Asia and Latin America together account for 6%, and Europe and North America for almost none. The concentration is sharper than for any single hazard, because the overlap requires two hazards in the same country and year. It is also sharper than for population: sub-Saharan Africa will hold about a quarter of humanity's person-years over the century, but more than half of its overlapping exposure.

The map in the scenario builder (section 7) shows the overlap country by country for any year and scenario. The chart below shows its composition over time on current trends: the share of humanity under no hazard rises from 20% to 45% as air pollution falls, but the share under two or three hazards falls only slowly, from 26% in 2025 to 17% in 2100, because heat and risk replace air as the drivers. The overlap is where adaptation, clean air and peace pay off together.

Comparing each lever with current trends, one at a time, shows how much of the overlap each can remove by 2100. Bringing every country to its income peers' resilience by 2050 and halving conflict avoids 71 billion overlapping person-years; resilience alone avoids 39 billion. Cutting air pollution by 5% a year instead of 2% avoids 61 billion. Moving from SSP2-4.5 to SSP1-2.6 avoids 11 billion within the century. Combining all three levers avoids 126 billion, leaving 54 billion.

The downside levers are as instructive. If air pollution stopped falling, the overlap would grow by 130 billion person-years, the largest single risk in the ledger. Slipping from SSP2-4.5 to SSP3-7.0 would add 32 billion, and drift on both — high emissions and stalled clean air — would add 187 billion, doubling the overlap. The asymmetry reflects timing: air and risk are large now and in the next few decades, while the warming pathways separate mainly after 2060, when much of the century's person-years have already been lived.

This is not an argument for ranking mitigation lower. The ledger stops in 2100, while warming's consequences continue for centuries; mitigation's largest effect within the century is at the extreme, where person-years at or above 29°C rise from 12.5 billion to 131 billion across pathways; and the thresholds of the Earth system, discussed in this series' S-curves report, are not captured here. The point is that clean air, resilience and peace are not lesser climate-era goals: measured in years of human exposure this century, they are among the largest available.

The builder recomputes the ledger for any combination of the three levers. The warming pathway sets heat; the clean-air pace sets the annual change in every country's fine-particle pollution; the resilience path sets whether today's risk map is frozen, moved to the income-peer benchmark by 2050, or moved there with conflict halved. The tiles give the cumulative person-years for each hazard and their overlap, and the chart shows how the world's population divides by number of hazards in each year.

The map shows each country's number of hazards in the year chosen with the slider, or its cumulative overlapping person-years. Selecting a country shows its own strip of the century: a band for each year from 2025 to 2100, coloured by how many hazards apply. Strips make the timing visible — a country can leave the air hazard in the 2040s and enter the heat hazard in the 2060s, so that its overlap persists even as one hazard ends.

The presets reproduce the three scenarios of section 1. The results are scenarios built on national averages, constant rates and the stated thresholds; small changes in a threshold can move large countries between categories. Use the builder to compare choices, not to forecast any country's future.

Person-years sum each country's population in each year from 2025 to 2100 (UN WPP 2024 medium variant via Our World in Data, interpolated from five-year values) over the years in which it meets a hazard condition. Air: population-weighted PM2.5 (SatPM via Our World in Data, latest year 2024, including natural dust) changing at a constant rate, against the WHO 2021 interim target 3 (15 µg/m³); the guideline (5 µg/m³) is cited for comparison. Heat: the 1991–2020 area-weighted national mean temperature (ERA5 via Our World in Data) plus 1.36 × (global warming − 0.85°C).

Global warming follows IPCC AR6 best estimates — 1.5–1.6°C in 2021–2040, 1.7°C (SSP1-2.6), 2.0°C (SSP2-4.5), 2.1°C (SSP3-7.0) and 2.4°C (SSP5-8.5) in 2041–2060, and 1.8, 2.7, 3.6 and 4.4°C in 2081–2100 — interpolated from 1.38°C in 2025 and extended to 2100 at the 2050–2090 slope. The factor 1.36 converts a land-to-ocean warming ratio of about 1.6 into a land-to-global ratio. Risk: the INFORM Risk Mid-2026 class, frozen or moved linearly by 2050 to the income-peer benchmark for vulnerability and coping, with conflict hazard halved in the peace path, recomputed with INFORM's formula.

National averages hide within-country variation and create threshold effects; area-weighted temperatures misstate heat where population and land are distributed unevenly; the 15 territories without temperature data are counted as not hot; and constant rates of change over 75 years are simplifications. The ledger ends in 2100 and omits sea-level rise, other pollutants, health effects and Earth-system thresholds. All outputs are scenarios, not forecasts. The map uses Natural Earth 1:50m boundaries via world-atlas; boundaries do not imply endorsement. Related reports: human exposure to 2100, global survivability and S-curves against the clock.