Heat mitigation by greening the cities, a review study

Abstract

In the past decades, many studies have proposed urban greening as a method to mitigate the rising temperatures in cities and urban heat islands. The cooling effect of urban green spaces can be divided into two components; cooling intensity and cooling extension. These two components are affected by different factors. This paper provides a review of research on the impact of urban greening on temperature and on the parameters that have an effect on that. Based on this review, two categories that have an effect on cooling intensity and cooling extension were identified: internal factors and external factors. These play an important role in the effectiveness of urban green spaces. As internal factors or indexes, the following were extracted; Size index, Shape index, Vegetation species and Tree canopy coverage are the most common indexes in most of the studies. The external factors can be divided into climate and surrounding properties. A discussion of the reviewed studies reflects the impact of each index on the cooling intensity and cooling extension. The existing methods and techniques for determining these indexes have also been considered in this paper. As such, this study is helpful for designers and planners for decision-making during the design phase.

Humid heat and climate change

“…Mindful of the limits to human acclimatisation, there is a strong incentive to limit exposure to dangerous heat where possible. Because heat stress is a phenomena that has been, and will continue to be, experienced mainly in cities (Fischer et al, 2012), there is much scope to achieve this through physical modification of urban environments (Ahmadi Venhari et al, 2017; Taleghani, 2017). However, it is critical that studies exploring this prospect do so using heat-humidity indicators in place of air temperature; ‘greening’ cities through the introduction of more vegetation may well reduce the latter, but its influence on the former is unclear (Hass et al, 2016).…”

Progress in Urban Greenery Mitigation Science – Assessment Methodologies Advanced Technologies and Impact on Cities

“…Other studies showed that evolution of microclimatic conditions in such areas is much more unstable than in larger green zones and that they may act as heat islands under specific conditions, but the overall cooling potential is not necessarily much lower than that of larger parks (Cao et al 2010); moreover, small parks might also cool the neighboring streets in some conditions (Chang, Li 2014). Extensive reviews on the effect of small urban green area focused on identifying the relation between cooling potential and most relevant parameters were recently carried out (Venhari et al 2017;Saaroni et al 2018). Well-vegetated courtyard gardens were monitored in Tel Aviv, Israel in 1994(Shashua-Bar, Hoffman 2004.…”

Evaluating the Cooling Potential of Urban Green Spaces to Tackle Urban Climate Change in Lisbon

“…On the other side, evapotranspiration consists in the loss of water to the atmosphere from a plant [9]. Then, there is an energy consumption of the surrounding environment, increasing latent heat and humidity and, consequently, reducing air temperatures [10]. Thanks to these two processes urban green spaces are, in general, cooler than their surroundings and this effect is called Park Cool Island (PCI), also known as Cool Island Effect, that is, the difference between air temperature inside and urban green space and the built area around.…”