White roofs and urban parks reduce heat in cities, but do not offset extreme global warming

The implementation of reflective white roofs and new urban parks can significantly reduce temperatures in cities and decrease population vulnerability to heat waves, although these measures are not sufficient to counteract the projected increase of more than 6 °C by 2100. This is demonstrated by a recent study led by the Institute of Environmental Science and Technology at the Universitat Autònoma de Barcelona (ICTA-UAB), Spain.

The research, carried out in collaboration with the Servei Meteorològic de Catalunya, used high-resolution meteorological simulations and the Pseudo Global Warming (PGW) method to project future heat waves in the Barcelona metropolitan area. The researchers evaluated three urban heat adaptation strategies: painting roofs white to increase albedo, installing irrigated green roofs, and expanding urban parks and peri-urban agriculture, while simultaneously adjusting urban forest cover in accordance with the Metropolitan Master Plan of Barcelona.

Regarding the strategies analysed, white roofs substantially increase albedo and reflect a large fraction of incoming solar radiation, cooling rooftops but potentially becoming counterproductive if applied to façades, as they could increase street-level temperatures. Green roofs act as insulation and cool the surrounding air through evapotranspiration, while also enhancing biodiversity and providing shelter for birds and insects, although their cooling capacity depends on vegetation type and irrigation requirements. Urban parks and agricultural areas provide shade and cool the environment through evapotranspiration, reduce impervious surfaces and favour rainwater infiltration. The effect of agriculture depends on the land-use transformation involved: replacing forested areas with agricultural fields may generate more negative than positive temperature impacts, whereas converting urban areas into agricultural spaces may provide benefits, even if this type of vegetation is not the most effective in reducing heat.

The study findings conclude that white roofs are the most effective daytime strategy, reducing temperatures by up to 1.75 °C in the most vulnerable areas. The creation of new urban parks and peri-urban expansion achieved moderate cooling effects of −0.26 °C and a slight increase in nighttime temperatures, while green roofs modestly reduced daytime heat (−0.37 °C) but contributed to a nighttime increase of 0.24 °C. “At night, vegetation slowly releases the heat stored during the day and limits heat loss through radiative cooling to the atmosphere,” explains Sergi Ventura, ICTA-UAB researcher and lead author of the study. In this way, the combination of white roofs and urban parks emerges as the most promising measure to mitigate heat impacts in the most sensitive areas.

Despite the benefits provided to varying degrees by the three strategies, heat vulnerability could double in densely populated and low-income areas by 2100. Under current climate conditions, adaptation strategies can reduce vulnerability by between 43% and 47%, but their effectiveness decreases over time, reaching a modest 16% under climate scenarios projected for 2100.

The researchers underline that, although no single intervention can offset extreme warming, targeting strategies in the most vulnerable neighbourhoods can significantly reduce health risks. Although focused on Barcelona, the findings may be applicable to other cities with similar climates, offering low-cost urban solutions to help protect populations from extreme heat.