■ H Heuristics Research · Digital Report
Four exposures. One population. One century.
Air and water pollution, extreme poverty, and heat stress are tracked as separate crises. They are not separate. From 2000 to 2100 and beyond, the same people stand in the overlap — and the overlap is where the risk compounds.
DATA WINDOW 2000–2100+ · BASELINE 2025 · ILLUSTRATIVE SCENARIOS BUILT ON WHO, WORLD BANK, IPCC AR6 & LANCET COUNTDOWN SERIES
The lens
The polycrisis is one problem that lands on the same bodies
The four stress fields of this century — air pollution, unsafe water, extreme poverty, and heat — are measured by different agencies, modeled in different silos, and funded from different budgets. The people they hit are not siloed.
A household breathing PM2.5 in a South Asian city is typically the same household that lacks piped water, earns under $2.15 a day, and works outdoors through heat waves. Treating the four exposures separately understates the risk each one creates, because each one raises the damage the other three can do.
The mechanism is not additive. It is compounding. Heat cuts a laborer's daily income; the lost income delays a water connection; unsafe water weakens the body before the next heat episode; polluted air inflames the lungs that heat and diarrhea already strain. Four siloed ministries see four manageable problems. The household experiences one escalating one.
That is why this report reads the polycrisis as a single exposure surface — a map of how many people stand under how many stressors at once, and how that surface changes between 2000 and 2100. The surface is the object of analysis. Everything else is downstream of it.
The exposure surface
Four curves have moved in four different directions since 2000
Start with the surface, not the silos. The share of humanity under each stressor has followed four different paths across the century's opening decades.
Extreme poverty fell fastest — from 29 percent of humanity in 2000 to under 9 percent today. Unsafe water fell steadily as rural water investment scaled. Air pollution exposure rose with urbanization and fuel use before bending downward where emission rules tightened. Heat stress moved the other way almost everywhere, climbing as the climate warmed and cities densified.
Extreme poverty
From 29 percent of humanity in 2000 to under 9 percent by 2024. The single largest reduction in human exposure this century — and the curve most reversible under climate damage.
Unsafe water
From 26 percent in 2000 to 22 percent today. Progress runs at roughly half the pace needed to reach universal safe water by 2030.
Heat stress
From about 31 percent in 2000 to 44 percent today, tracking global mean temperature. Under a high-emission pathway it reaches roughly three-quarters of humanity by 2100.
Why overlap matters
Two exposures rarely behave like two exposures
The World Bank estimates that 1.2 billion people already live with high exposure to at least one climate hazard. The number that matters more is the share of those 1.2 billion who also breathe bad air, lack safe water, or earn below the poverty line — and that share is high.
The overlap concentrates in South Asia and sub-Saharan Africa, where all four stressors peak in the same districts. A heat wave in a high-income suburb is a discomfort event. A heat wave in an unserviced settlement is a cascade: income stops, water runs short, stored food spoils, and medical bills follow.
This is the compound vulnerability multiplier at work. When two exposures overlap, the damage is more than the sum of the two, because each one consumes the reserves — income, health, social connection — that would otherwise absorb the other. Models that add exposures linearly understate the human cost of the polycrisis precisely where the polycrisis is densest.
Distribution
The burden is not distributed evenly — it is distributed inversely
The regions with the least historical responsibility for emissions carry the largest share of current exposure. That inversion is the political fact underneath every exposure statistic in this report.
| Region | PM2.5 above guideline | Unsafe water | Extreme poverty | High heat exposure | Overlap profile |
|---|---|---|---|---|---|
| South Asia | 99% | 23% | 10% | 72% | Densest overlap |
| Sub-Saharan Africa | 94% | 49% | 35% | 68% | Densest overlap |
| East Asia & Pacific | 97% | 12% | 1% | 46% | Air + heat |
| Latin America & Caribbean | 88% | 9% | 4% | 41% | Moderate |
| High-income countries | 67% | <1% | <1% | 19% | Lowest |
Note: regional shares are rounded author estimates from WHO air-quality data, the WHO/UNICEF JMP, World Bank poverty lines, and ISIMIP heat-stress grids. Overlap profiles are qualitative summaries of the four columns.
The century ahead
The exposure surface in 2100 is a policy choice, not a fate
Two scenarios bound the century. The low-pressure pathway, SSP1-2.6 with rapid poverty reduction, cuts the number of people under at least one severe exposure from 5.1 billion today to roughly 2.9 billion by 2100. The high-pressure pathway, SSP3-7.0 with slow development, pushes it toward 7.6 billion.
The gap between those two lines is the entire policy space of this report. Every tenth of a degree of avoided warming, every percentage point of poverty reduction, and every water connection moves the same number: how many people stand in the overlap.
The polycrisis is not four separate trends to monitor. It is one exposure surface to shrink. Any policy that reduces one stressor without touching the others leaves people standing in a shallower — but still occupied — overlap.
Method
How to read this atlas
Every figure in this report is an illustrative model estimate, rounded to the precision its source supports.
Historical values calibrate to WHO air-quality and WASH data, World Bank poverty lines, the Global Burden of Disease, and the Lancet Countdown. Projections to 2100 use the IPCC Shared Socioeconomic Pathways — SSP1-2.6, SSP2-4.5, and SSP3-7.0 — as bounding narratives, not forecasts. Where a chart shows a single line, it follows the SSP2-4.5 middle pathway and says so.
The core metric, severe exposure, counts a person as exposed when they cross a defined threshold in any domain: PM2.5 above the WHO guideline, no safely managed drinking water, income under $2.15 a day, or more than 30 high-heat-stress days per year. The metric deliberately counts co-exposure — a person under three stressors counts three times on the surface chart and once on the headcount chart.
This is an atlas of exposure, not of harm. Exposure measures what people stand under; harm measures what it does to them. The two are connected but not identical, and the connection itself — the compound vulnerability multiplier — is the subject of the pages that follow.
Read the four domains
Each page unpacks one stress field — the data, the mechanism, and what bends its curve across the century.