The Adaptation Gap: Why Milder Cities Hurt More in a Heat Wave

I spent the last week and a half in Colorado Springs and Houston. Both ran hotter than London’s most recent heat wave. Neither bothered me.

In Houston I was either in air conditioning or in a pool, more or less permanently, watching World Cup with family and friends. Ceiling fans did the rest. At one point I was carrying a jumper around, because the AC was blasting hard enough that indoors had become the cold problem. That kind of comfort has a cost too, Americans carry one of the highest per-capita carbon footprints in the world, and cooling this aggressively is part of why. But in the moment, I had a great trip and didn’t think about the heat once.

Coming back to London, I felt something closer to dread. Not about the temperature itself, London’s heat waves don’t touch Houston’s numbers, but about the night. About needing a fan powerful enough to actually move air in a bedroom that won’t cool down. About the low hum of irritability that seems to settle over the whole city when the nights stay warm. It’s a strange inversion: the hotter place left me calm. The milder place left me bracing.

That contradiction is the starting point for this piece. How did the place with less heat end up harder to live in? And where else in the world has this problem, worse or better than we do?

Building a pain index

To answer that properly I split the question into two variables that get conflated far too often: how much heat a city actually experiences, and how well-equipped it is to handle it. I’m calling them exposure pain (drawn from climate data, how often and how severely a place experiences heat stress) and pain index (the inverse of adaptation capacity, mostly air conditioning penetration and building stock). This second measure is close to what’s usually called “vulnerability” in climate research, but I’ve broadened it deliberately: it’s meant to capture everyday discomfort, bad sleep, a fan that can’t keep up, the general misery of a hot night, not just the health and mortality risk that “vulnerability” usually implies. Score both 0–10, plot every city on both axes, and track each one from around 1980 to today, and a much more interesting map appears than “which city is hottest.”

A few things fall out of it immediately.

Five takeaways

1. Europe’s heat pain is real, rising, and worst at night. London and Paris sit in the “mild most of the year, ill-equipped” quadrant for a reason that has almost nothing to do with peak daytime temperature. The real damage happens after dark, and the FT’s reporting on this is genuinely alarming once you sit with the numbers. London matched its own record this June with five consecutive tropical nights, defined as nights that never drop below 20°C. The FT’s analysis of weather stations found hundreds of sites across at least fifteen countries logging their hottest night on record that same month. The trend isn’t new so much as it’s accelerating: Milan has gone from around three tropical nights a year in the 1970s to about 33 now, Athens logged over 100 in 2024 alone, and Vienna, a city that used to see almost none, now averages around ten a year.

The mechanism matters as much as the numbers. The Met Office’s Mark McCarthy put it bluntly, hot nights are among “the most acute ways in which we are impacted by our changing climate,” because the body relies on temperatures dropping to trigger deep sleep, and when they don’t, there’s no recovery window between one hot day and the next. A Nature Climate Change study cited in the piece found the ten hottest nights of the year are now warming faster than the ten hottest days, and that heat-stress nights, hot enough that humidity alone becomes dangerous, have gone from roughly 1-in-30 in the 1970s to 1-in-10 today. This June alone, Germany recorded around 5,500 excess deaths, France about 2,000, Belgium 1,200, and Spain at least 1,028 confirmed heat deaths.

But there’s a structural reason this keeps happening, not just a meteorological one, and it shows up right on the chart in the 1980 starting points. London and Paris don’t sit high on the pain index in 1980 because air conditioning was some idea Europeans considered and rejected. They sit there because most of the building stock that existed in 1980 already predated the technology by decades. Roughly 78% of UK homes were built before 1980, against a European average of 61%, and most of that is prewar or immediate postwar construction, thick walls, small windows, designed to hold heat in through a cold winter, never to shed it in a hot summer. Retrofitting air conditioning into a Victorian terrace or a nineteenth-century Paris apartment block is a fundamentally harder and more expensive problem than specifying it at construction. That’s before you get to somewhere like Haussmann-era Paris, where heritage rules routinely block exterior condenser units on aesthetic grounds. The chart isn’t just tracking two different policy choices, it’s tracking two cities that were pouring concrete during completely different eras of both climate and technology.

2. Some places have it meaningfully worse than Europe. Delhi shows one version of the problem: exposure has climbed into genuinely dangerous territory while AC ownership, though rising fast, hasn’t caught up. Karachi shows a different one entirely. Its 2015 heat wave killed over 1,200 people in roughly a week, and the cause wasn’t primarily a lack of fans or AC units, it was the power grid buckling under demand, leaving people with cooling equipment they couldn’t run. Between the two, you get both halves of what the pain index actually means: do you own the equipment, and does the infrastructure survive the exact moment you need it.

3. Houston and Phoenix haven’t just been coping for a few years, they’ve been coping for decades, and the same building-stock logic explains why. Their arrows on the chart are almost flat, exposure has climbed, but the pain index barely moved, because it was already low in 1980. Texas and the desert Southwest were in the middle of a massive population and construction boom starting in the 1970s, at exactly the moment air conditioning was becoming standard rather than a luxury. The median owner-occupied home in Texas today is about 28 years old, and only around 4% of Houston’s housing stock predates 1940. So while Europe’s 1980 housing stock was already decades old and built for a different climate problem entirely, Houston’s 1980 housing stock was, in a very real sense, being built alongside air conditioning itself. It wasn’t retrofitted in. It was there from the start.

4. Adaptation doesn’t need a crisis to happen, it can piggyback on something else entirely. Auckland’s arrow is one of the steepest on the chart, but the story behind it isn’t a heat emergency. New Zealand’s shift toward near-universal home cooling came from heat pumps bought for winter heating that happened to double as summer cooling. Roughly three-quarters of heat pump owners now use them that way. Nobody had to win an argument about the merits of air conditioning, the equipment showed up for a different reason and adaptation followed almost by accident. It might be the single most exportable lesson on the whole chart.

5. Rich, mildly-exposed cities like London have a cheap win sitting on the table, and mostly aren’t taking it. This one comes straight off the chart’s geometry: London and Paris don’t need Phoenix-level infrastructure to escape “mild most of the year, ill-equipped.” Given how much of their housing stock genuinely can’t be retrofitted quickly or cheaply, they need targeted investment in a much smaller set of high-risk building types instead. That’s essentially the argument in the UK Climate Change Committee’s 2026 A Well-Adapted UK report, prioritize cooling in hospitals, care homes, schools, and prisons first, with broader home AC rollout as a slower, later step. Their own cost-benefit estimate: every £1 spent on adaptation returns about £5.

There’s an even cheaper option than the CCC’s building list, and it’s one I didn’t expect to find so glaring. Across the week and a half I spent in Colorado and Texas, ceiling fans were everywhere, in bedrooms, on porches. I don’t think I’ve ever actually seen one in a UK home. They’re not a substitute for air conditioning at true extremes, the evidence on electric fans gets much weaker above about 35°C, and weaker still for older adults, who are also the group most at risk from heat. But London’s actual problem, per the FT numbers above, isn’t extreme daytime heat, it’s a run of nights that won’t drop below 20°C. That’s comfortably inside the range where a fan’s evaporative cooling effect genuinely works. To be clear, the number that follows isn’t a lived judgment or a sourced figure, and it isn’t mine either: I asked Claude, the AI I used to help draft this piece, to extrapolate from the pattern on this chart, and I’m reporting its estimate rather than my own. If ceiling fans went from essentially unseen in UK homes to standard, Claude’s estimate is that London’s pain index could realistically drop from around 8 to somewhere near 5.5–6, a real move toward the low-pain quadrant, though not a full crossing of it. That’s a rougher estimate than the other numbers on this chart, the research on fan effectiveness is genuinely mixed, but it’s about as cheap and fast a partial fix as exists anywhere in this piece.

Part of what’s held this back is cultural, not just structural. The Economist made the case this June that a lot of European resistance to AC isn’t really about cost at all, it’s closer to carbon guilt, a sense that installing air conditioning is an admission of first-world excess, particularly in northern Europe, where buying one for a handful of scorchers can feel like an environmental indulgence. The magazine’s rebuttal is that this guilt is increasingly out of date: heat already contributes to roughly 175,000 deaths a year on the continent by UN estimates, and much of Europe’s grid has quietly gone green enough that running the AC barely moves the needle. Spanish electricity produced just 86 grams of CO2 per kilowatt-hour on a June day this year, against 442 grams in the US state of Georgia, and on a sunny afternoon Spain gets roughly half its power from solar. France does better still on the back of nuclear. The laggards are real, Poland and Germany remain heavily coal- and gas-reliant, but the broader point stands: the goal should be to make cheap, clean air-conditioning available to everyone, not to treat cooling as a moral failure while heat keeps killing people at scale.

One caveat, not a takeaway

None of this shows up on the chart itself, so it’s worth saying explicitly: even where a city looks “solved” on the country-level average, coping capacity is never evenly distributed within it. New York’s own emergency management office flags the South Bronx, Upper Manhattan, and Central Brooklyn as its highest-risk heat zones, largely because of lower AC access layered onto less tree cover. A high average hides who’s still exposed. “Rich country” is not the same claim as “everyone in that country is fine.”

Methodology note: exposure-pain scores are drawn from climate normals and heat-day data; pain-index scores are inverted from documented AC/cooling-penetration figures. The 1980 positions on the chart are reconstructed from historical climate normals and are directional rather than precise, current-day positions rest on firmer ground. The ceiling-fan pain-index estimate in takeaway 5 was generated by Claude (the AI assistant used to help draft this piece) as an extrapolation from the chart pattern, not a sourced figure or a calculation of the author’s own.