Observed and projected climate change is increasing the risk of heat stress across a wide range of regions worldwide . In particular, cities, which are economic hubs with an increasing area and population, are expected to be confronted with increasing heat stress [2,3,4]. At present, global mean temperatures are approximately 1.09 °C above pre-industrial levels, and further warming is predicted . More frequent and greater temperature extremes, such as hot days and nights and heat waves, have been observed in urban areas [5,6,7]. Compared with their rural surroundings, cities are generally characterised by higher surface and air temperatures, a climate condition referred to as the urban heat island (UHI). The UHI intensity depends on spatial modification, for example, the distance from the city centre, the density of the built-up area and the type of land use (e.g., buildings, lakes, open spaces, parks) . Owing to the progress of urbanisation, it is estimated that UHIs will intensify significantly up to 2050, with the intensification depending on the climate zone and settlement size, with temperate and tropical zones, as well as medium-sized urban clusters, likely to be most affected .
Since even climate projections that include unprecedented efforts to mitigate greenhouse gas (GHG) emissions nonetheless predict further global warming and urban expansion, increased heat stress in human populations is likely [5,9,10]. The especially vulnerable social groups include children, elderly people, people living in poverty, pregnant people, those working outdoors, and people with underlying medical conditions [11,12,13]. Consequently, adaptation is necessary to offset the risk to human health and ensure liveable cities in the future [14,15].
Considering the future aggravation of heat stress, and the inequalities that exist between regions and within cities , the need for solutions and evidence of their efficiency in different contexts is apparent. Nature-based strategies such as urban greening, which is listed among several other feasible adaptation options in the IPCC special report on global warming of 1.5 °C , represent an ecosystem approach with mitigative …
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