For the roughly half of Greeks not going on vacation this summer, the urban heat island effect brings the tropics to them anyway, minus the greenery and plus the asphalt.

Anyone living in a densely built area already knows this feeling, even without a name for it. Drawing on a doctoral thesis definition, an urban heat island is an atmospheric effect in built-up areas where dense construction traps heat, raising air temperature compared to surrounding rural areas.

What the heat island actually feels like

For residents of the Attica basin, it’s the wave of heat, exhaust, and pollution that hits when crossing a street on foot at midday. It’s asphalt that scorches underfoot on a motorbike. It’s the blast from air conditioner units outside, the price paid, by us and the environment, for cooling indoor spaces. It’s the concrete walls and rooftops of older, poorly maintained, non-bioclimatic apartment buildings that radiate heat back at night, denying relief even after sunset.

According to the National and Kapodistrian University of Athens’ Risk and Crisis Management Committee, Athens’ urban heat island typically runs 4 to 7 degrees Celsius, averaging around 5 degrees, and can hit 10 degrees in extreme cases between the city center and surrounding areas. This effect compounds on top of climate change’s existing warming trend rather than simply adding to it.

When air conditioning backfires

A new report from the European Commission’s climate action arm, DG CLIMA, examines the toll of extreme heat on urban residents. Since the mid-1990s, Europe has warmed at twice the global rate, 0.56°C per decade versus a 0.27°C global average, with only the Arctic warming faster, at 0.75°C per decade.

Heat is the deadliest of all extreme weather events; 95% of deaths linked to extreme weather and climate conditions are heat-related. Recent estimates put this June’s heatwaves alone at 20,000 excess deaths across Europe.

DG Clima warns that air conditioning, while necessary in dangerous heat, intensifies the urban heat island effect in densely built areas. It cites Paris, where heavy AC use has been shown to raise outdoor temperatures by up to 4°C. The report urges exhausting other cooling measures, shading, insulation, ventilation, before resorting to air conditioning. Urban greenery is the most valuable ally against overheating: a study across 600 European cities found trees can lower ambient temperature by 0.8°C or more in some areas.

No single fix works everywhere

Renowned Greek scientist Matheos Santamouris, in a recent National Geographic interview, explains why there’s no universal solution. Santamouris teaches physics at the University of Athens and holds the high-performance architecture chair at Australia’s University of New South Wales; he’s considered a leading expert on the built environment.

“Every heat mitigation project in a city must be designed by specialists and tested,” he says. “Otherwise we risk poor results, huge budgets spent, and even higher city temperatures.”

In desert-adjacent cities like Dubai and Sydney, reflective surfaces help deflect solar radiation and reduce the hot-air dome over the city. Athens, though, overheats mainly due to dense construction and surrounding mountains blocking air flow. For the capital, the priority isn’t just reducing heat but improving ventilation: cooling rooftops below ambient temperature through greenery and reflective materials lets denser, cooler air sink into the streets, pulling in a sea breeze that flushes heat out of the city.

Other cities have adopted similar strategies. Singapore, warming roughly twice as fast as the global average, has become a pioneer in architectural and technological cooling innovations, investing in green space, shade, and underground cooling pipes.