Systems in depth
One section per system: its state at evidence close, its exposures and what the operational series will watch.
Contents
- ·Edition home
- ·Executive summary
- 1People and places change together
- 2Key findings
- 3Systems and sectors at baseline
- 4Risks and capability
- 5A heatwave that meets an ageing, dense and informally housed district
- 6Readings from the other desks
- 7The 2027 agenda
- 8Systems in depth
- AMethodology
- BData appendix
- CReferences and record
- ·Complete edition, for print
8.1Common causes
One cause can change many human systems at once: falling fertility, longer lives, migration, urban growth or heat. The shared cause matters more than the sector.
| Instance | Mechanism in action | Evidence | Status |
|---|---|---|---|
| Falling fertility | global rate down from 3.31 in 1990 to 2.25 in 2024 | 1 | Reported |
| Longer lives | life expectancy up 8.4 years since 1995 | 1 | Reported |
| Urban growth | cities add 986 million residents by 2050 | 2 | Reported |
| Heat | heat deaths among over-65s up about 85 per cent in two decades | 21 | Reported |
Pathways in this mechanism
- DEM → HLT ageing
- URB → DEV growth
- CLI → HLT heat
Evidence to bind in 2027
- common cause tagging of local change
- fertility drivers on one basis
Related editions
- GLB Global Systems
- SR-02 Critical Infrastructure and Lifeline Systems
- SR-07 Food–Water–Energy Security
- SR-08 Health Security, One Health and Biosecurity
- GS-10 Cities and Urban Systems
- GS-06 Development and Humanitarian
8.2Domino effects
A change in one human system loads the next. Fewer births reach schools, then the workforce, then care and public finance.
| Instance | Mechanism in action | Evidence | Status |
|---|---|---|---|
| Births to schools | smaller birth cohorts reach schools and later the workforce | 1112 | Inferred |
| Ageing to care | people aged 80 and over reach 265 million by the mid-2030s | 1 | Reported |
| Heat to health | older people face higher heat mortality | 21 | Reported |
| Housing to health | informal settlements can be hotter than other urban areas | 21 | Reported |
Pathways in this mechanism
- DEM → HLT → FIN
- CLI → HLT
- DEV → HLT
Evidence to bind in 2027
- timing of service demand shifts
- care capacity data
Related editions
- GLB Global Systems
- SR-02 Critical Infrastructure and Lifeline Systems
- SR-07 Food–Water–Energy Security
- SR-08 Health Security, One Health and Biosecurity
- GS-10 Cities and Urban Systems
- GS-06 Development and Humanitarian
8.3Inter-systemic feedbacks
Human systems feed back on what they depend on. Cities draw people and food, rural areas lose young people, and heat pushes up the demand that strains the grid.
| Instance | Mechanism in action | Evidence | Status |
|---|---|---|---|
| Urban growth to farmland | built-up area grew almost twice as fast as population | 2 | Reported |
| Rural out-migration | ageing and out-migration press on rural areas | 2 | Reported |
| Heat to energy | cooling demand rises with heat | 16 | Inferred |
| Finance feedback | a finance gap delays adaptation and raises later losses | 8 | Inferred |
Pathways in this mechanism
- URB → FOO
- CLI → ENE
- FIN → INF
Evidence to bind in 2027
- rural to urban flows by age
- cooling demand series
Related editions
- GLB Global Systems
- SR-02 Critical Infrastructure and Lifeline Systems
- SR-07 Food–Water–Energy Security
- SR-08 Health Security, One Health and Biosecurity
- GS-10 Cities and Urban Systems
- GS-06 Development and Humanitarian
8.4Shared vulnerabilities
Different human systems share the same weak points: informal housing, heat exposure, flood-prone land and thin city finance.
| Instance | Mechanism in action | Evidence | Status |
|---|---|---|---|
| Informal housing | about one in four urban residents | 510 | Reported |
| Flood-prone land | one billion people in riverine flood-prone areas | 5 | Reported |
| Informal work | about 61 per cent of workers informal in 2016 | 9 | Reported |
| Unsafe streets | road traffic deaths about 1.16 million a year; 92 per cent in low- and middle-income countries | 22 | Reported |
Pathways in this mechanism
- DEV → HLT housing
- CLI → DEV flood
- FIN → INF city finance
Evidence to bind in 2027
- neighbourhood-scale exposure data
- informal work in cities
Related editions
- GLB Global Systems
- SR-02 Critical Infrastructure and Lifeline Systems
- SR-07 Food–Water–Energy Security
- SR-08 Health Security, One Health and Biosecurity
- GS-10 Cities and Urban Systems
- GS-06 Development and Humanitarian
8.5Thresholds and tipping
A human system can absorb stress until a threshold is crossed. Replacement fertility, service capacity and household cooling access are such thresholds.
| Instance | Mechanism in action | Evidence | Status |
|---|---|---|---|
| Replacement fertility | more than half of countries are below 2.1; a return is highly unlikely in countries already below 1.4 and past peak | 1 | Reported |
| Population peak | 63 countries or areas passed their peak before 2024 | 1 | Reported |
| Rural population peak | global rural population expected to peak in the 2040s | 2 | Reported |
| Heat thresholds | older people face higher risk above heat limits | 21 | Inferred |
Pathways in this mechanism
- DEM → DEM peak
- URB → DEV service thresholds
- CLI → HLT heat thresholds
Evidence to bind in 2027
- service-capacity thresholds at city scale
- household cooling access
Related editions
- GLB Global Systems
- SR-02 Critical Infrastructure and Lifeline Systems
- SR-07 Food–Water–Energy Security
- SR-08 Health Security, One Health and Biosecurity
- GS-10 Cities and Urban Systems
- GS-06 Development and Humanitarian
Notes
Numbers are the edition’s reference numbers, so a note reads the same in every chapter. The full list, with every source and its licence, is in Annex C. Annex C, references
- 1UN DESA Population Division (ed.), World Population Prospects 2024: Summary of Results (advance unedited version, July 2024). United Nations, New York. Observation year 2024. Accessed 2026-11-03.back
- 16WHO (ed.), news release on the 2025 report of the Lancet Countdown on health and climate change, 29 October 2025 (about 546,000 heat-related deaths a year; up 23 per cent since the 1990s; USD 261 billion cost among older adults). World Health Organization, Geneva. Observation year 2024. Accessed 2026-11-03.back
- 2UN DESA Population Division (ed.), World Urbanization Prospects 2025: Summary of Results, ten key messages (November 2025). United Nations, New York. Observation year 2025. Accessed 2026-11-03.back
- 21WHO (ed.), Climate change, heat and health, fact sheet (page dated 31 July 2026; 489,000 heat-related deaths a year in 2000 to 2019, 45 per cent in Asia; over-65 mortality up about 85 per cent from 2000 to 2004 to 2017 to 2021). World Health Organization, Geneva. Observation year 2019. Accessed 2026-11-03.back
- 11National Bureau of Statistics of China (ed.), Statistical Communique on the 2025 National Economic and Social Development, 28 February 2026 (year-end population 1,404.89 million; births 7.92 million; deaths 11.31 million; urbanisation 67.89 per cent; 65 and over 15.9 per cent). NBS, Beijing. Observation year 2025. Accessed 2026-11-03.back
- 12Japan Ministry of Health, Labour and Welfare, vital statistics for 2024 (686,061 births; total fertility rate 1.15; 1,605,298 deaths; Tokyo 0.96), read in a news summary of 5 June 2025; the ministry release was not opened. Observation year 2024. Accessed 2026-11-03.back
- 8UN-Habitat (ed.), World Cities Report 2024: Cities and Climate Action (cities receive less than 20 per cent of the finance they need for effective climate action; page read, not the full report). UN-Habitat, Nairobi. Observation year 2024. Accessed 2026-11-03.back
- 5United Nations (ed.), The Sustainable Development Goals Report 2025, Goal 11 (1.12 billion in slums or informal settlements; urban green space 19.5 per cent in 1990 and 13.9 per cent in 2020; one billion in riverine flood-prone areas, half in cities). United Nations Statistics Division, New York. Observation year 2025. Accessed 2026-11-03.back
- 10World Bank (ed.), Urban Development overview (page: more than four billion people in cities; nearly 7 in 10 by 2050; about one quarter of urban residents in slums). World Bank, Washington. Observation year 2025. Accessed 2026-11-03.back
- 9ILO (ed.), Women and men in the informal economy: a statistical picture, third edition (news release of 30 April 2018; about 2 billion workers, more than 61 per cent of the employed; Africa 85.8 per cent). International Labour Organization, Geneva. Observation year 2016. Accessed 2026-11-03.back
- 22WHO (ed.), Road traffic injuries, fact sheet (page dated 20 July 2026; about 1.16 million deaths a year; 92 per cent in low- and middle-income countries; the page states no data year). World Health Organization, Geneva. Observation year 2026. Accessed 2026-11-03.back