City systems

A city under strain

You don’t have to work outdoors to be caught by heat. When transformers trip on a hot night, water pumps stop, clinics lose their cooling and no one sleeps. Choose what your city has in place, then run five days of a heatwave.

A teaching model. Its strain levels are illustrative, not measurements, and it is not a forecast for any real city.

What does your city have in place?

Cool roofs and shade on homesRooms stay cooler, so fewer air conditioners run flat out.
Efficient fans and air conditionersThe same cooling for less power at the peak.
Street trees, watered through the heatShade by day, and nights that cool a little more.
Back-up power at clinics and pumpsSolar and batteries keep clinics cool and water flowing in a cut.
Water stored before the heatTanks filled ahead, so taps keep running for a while without pumps.
Hospitals warned two days aheadExtra staff, fluids and cool beds before the peak.

Before the heatwave

The evidence behind the city

US$1.4 billion was the World Bank’s early estimate of what northern Viet Nam’s power cuts cost in May and June 2023, after heatwaves pushed up demand and dried hydropower reservoirs.Source: World Bank estimate, reported by AFP via The Manila Times, 2023: Vietnam’s heat-driven power crisis (opens in a new tab)
In 2016, India lost 14 TWh of electricity because thermal power plants ran short of water.Source: World Resources Institute, 2018 (opens in a new tab)
Overheating equipment and power cuts take down phone masts, networks and data centres.Source: Asia News Network, 2026: Bangladesh’s mobile signals weaken as blackouts spread (opens in a new tab)
When the grid fails, homes, shops and hospitals that can afford it switch to diesel.Source: IFC, 2019: The Dirty Footprint of the Broken Grid (20–30 million back-up generator sites) (opens in a new tab)
Heat, drought and missing upkeep kill street trees and parks just when their shade is needed. Across 164 cities, 56% of urban tree and shrub species already face temperatures beyond those in their natural range, and in hot cities such as New Delhi and Singapore every species studied is at risk.Source: Esperon-Rodriguez et al., Nature Climate Change, 2022: climate change increases global risk to urban forests (opens in a new tab)
After Ahmedabad’s heat plan prepared hospitals and warned the public, an evaluation estimated about 1,190 deaths a year were avoided.Source: Hess et al., Journal of Environmental and Public Health, 2018 (opens in a new tab)

Watch: city systems in the heat

Power grids in a heatwave · 0:41
Water for power, power for water · 0:29
The diesel fallback · 0:32
Clinics in a heatwave · 0:33
Cold chains in the heat · 0:34
When phones go quiet · 0:34
City trees under heat · 0:40
Shade follows wealth · 0:36
Cool classrooms · 0:38
Roads and rails in the heat · 0:31

The other two simulations · The energy–water loop · Cooling options · Who holds it up · Cities