Custodes Futurifor the advancement of humanity

Environmental factor, barrier 3 of 9

Heat and climate change

Heat reduces learning and economic output and causes deaths.

Evidence

  • Heat and learning. In a study of 10 million students who retook the PSAT, a 1°F hotter school year reduced that year's learning by 1 percent without air conditioning. Hot school days accounted for roughly 5 percent of the racial achievement gap (Park et al., 2020).
  • Heat and output. Productivity peaks at an average annual temperature of about 13°C, and unmitigated warming was projected to cut average global incomes roughly 23% by 2100. This projection depends on adaptation assumptions and is debated (Burke, Hsiang, and Miguel, 2015).
  • Heat and mortality. WHO estimates, from models, that about 489,000 heat-related deaths occurred each year from 2000 to 2019. The elderly and people with chronic illness are at highest risk, so heat deaths remove fewer future contributors than diseases that kill children.
  • Other pathways. Crop losses, water scarcity, extreme weather, and forced migration add further costs, with hotter and often poorer regions bearing the largest.

Proposed and experimental methods

Methods that are proposed, under trial, approved in some places, or tried and then failed. Each shows a stage label and an evidence rating. A stage label shows how far a method has progressed, not whether it works. The stage labels are explained on the environmental factor page.

  • Carbon dioxide removal (Approved but not scaled, B). Methods that take carbon dioxide out of the air and store it, from tree planting to direct air capture machines. An independent assessment estimated current removal at about 2 billion tonnes of CO2 per year, almost all from conventional methods such as forestry, with novel methods removing about 1.3 million tonnes per year (less than 0.1%), against 7 to 9 billion tonnes per year by 2050 in the more sustainable scenarios it reviewed (Smith et al., 2024).
  • Heat action plans and heat warnings (In wide use, contested, C). Early warnings on forecast hot days plus outreach, cooling centers, and health system preparation. A difference-in-differences study found Montreal's plan reduced deaths on hot days, more so for older people and low-education neighborhoods (Benmarhnia et al., 2016), and a before-and-after study in Ahmedabad, India estimated 1,190 deaths avoided per year (Hess et al., 2018). But across 2,817 U.S. counties, National Weather Service heat alerts were not associated with lower deaths among adults 65 and older (rate ratio 1.005) (Weinberger et al., 2021).
  • Cool roofs (Early trial, C). Reflective roof coatings that keep homes cooler. A scoping review found that all 14 studies of cool roofs and health were models, mostly in high-income countries (Bunkley et al., 2026). In a randomized pilot of 40 rural houses in Kenya, cool roofs lowered the indoor heat index by 3.3 °C by day and 2.4 °C at night (Abong'o et al., 2026). A two-year cluster randomized trial of 1,200 people in 600 homes in Burkina Faso, with heart rate as the main outcome, has been completed but results were not found (Bunker et al., 2024).
  • Green corridors for urban heat (Theoretical, C). Turning streets into tree-lined green corridors. A health impact model of Barcelona's plan to green one in three streets estimated 178 premature deaths prevented per year, mostly through greenness itself, while average daily temperature fell only 0.05 °C, so the authors called for stronger heat measures (Iungman et al., 2025).
  • Reversed: Outdoor solar geoengineering experiments (Failed or reversed, B). Solar geoengineering would reflect sunlight to cool the planet. Harvard's SCoPEx balloon experiment, planned to release a few kilograms of particles in the stratosphere, had its 2021 test flight in Sweden canceled after an advisory committee and the Saami Council objected, and was abandoned in March 2024 (Temple, 2024). In May 2024 the city of Alameda, California ordered a University of Washington team to stop spraying sea salt aerosol for a marine cloud brightening study, citing its lease (City of Alameda, 2024).

Sources cited on this page

  1. Park, R. J., Goodman, J., Hurwitz, M., & Smith, J. (2020). Heat and learning. American Economic Journal: Economic Policy, 12(2), 306-339. AEA A Strong
  2. Burke, M., Hsiang, S. M., & Miguel, E. (2015). Global non-linear effect of temperature on economic production. Nature, 527, 235-239. DOI C Limited: for the 23% projection
  3. Ma, L., Graham, D. J., & Stettler, M. E. J. (2021). Has the ultra low emission zone in London improved air quality? Environmental Research Letters, 16, 124001. link B Moderate
  4. Abong'o, B., Kwaro, D., Bange, T., Moshi, V., Simwero, J., Otima, J., et al. (2026). Housing modifications for heat adaptation, thermal comfort and malaria vector control in rural African settlements. Nature Medicine, 32(2), 518-526. link C Limited
  5. Benmarhnia, T., Bailey, Z., Kaiser, D., Auger, N., King, N., & Kaufman, J. S. (2016). A difference-in-differences approach to assess the effect of a heat action plan on heat-related mortality, and differences in effectiveness according to sex, age, and socioeconomic status (Montreal, Quebec). Environmental Health Perspectives, 124(11), 1694-1699. link B Moderate: (C for size)
  6. Bunker, A., Compoaré, G., Sewe, M. O., Laurent, J. G. C., Zabré, P., Boudo, V., et al. (2024). The effects of cool roofs on health, environmental, and economic outcomes in rural Africa: Study protocol for a community-based cluster randomized controlled trial. Trials, 25(1), 59. link B Moderate
  7. Bunkley, N., Stowell, M., & Bullen, C. (2026). Assessing the impact of cool roofs on health and wellbeing: A scoping review. Current Environmental Health Reports, 13(1). link C Limited
  8. Hess, J. J., Lm, S., Knowlton, K., Saha, S., Dutta, P., Ganguly, P., et al. (2018). Building resilience to climate change: Pilot evaluation of the impact of India's first heat action plan on all-cause mortality. Journal of Environmental and Public Health, 2018, 7973519. link C Limited
  9. Iungman, T., Caballé, S. V., Segura-Barrero, R., Cirach, M., Mueller, N., Daher, C., et al. (2025). Co-benefits of nature-based solutions: A health impact assessment of the Barcelona Green Corridor (Eixos Verds) plan. Environment International, 196, 109313. link C Limited
  10. Smith, S. M., Geden, O., Gidden, M. J., Lamb, W. F., Nemet, G. F., Minx, J. C., et al. (Eds.). (2024). The state of carbon dioxide removal 2024 (2nd ed.). link B Moderate
  11. Temple, J. (2024, March 18). Harvard halts its long-planned atmospheric geoengineering experiment. MIT Technology Review. link B Moderate
  12. Weinberger, K. R., Wu, X., Sun, S., Spangler, K. R., Nori-Sarma, A., Schwartz, J., et al. (2021). Heat warnings, mortality, and hospital admissions among older adults in the United States. Environment International, 157, 106834. link B Moderate

Every source for this factor is listed on the environmental factor page.