The effect of increasing vegetation cover on heat stress in urban areas affected by global warming: A case study of Rotterdam, the Netherlands

Abstract

This study assesses how current and future urban heat stress in the Netherlands can possibly be reduced by increasing the vegetation cover. Climate change is expected to increase the global atmospheric temperature, causing the probability of being exposed to heat stress to increase (McCarthy et al., 2010). In urban areas, factors such as the urban heat island (UHI) cause unfavourable conditions for the energy balance of the human body, which leads to the occurrence of heat stress (Oleson et al., 2015). Increasing the vegetation cover is believed to reduce heat stress in the public outdoor space (Lindberg et al., 2016). The aim of this study is to quantify the effect of vegetation on heat stress in urban areas of the Netherlands, in the current and future climate. By doing this, knowledge is gained of how climate change will influence heat stress in urban regions of the Netherlands, and how heat stress can be reduced by increasing the vegetation. The study aims to assess how increasing the vegetation cover affects heat stress in urban areas, by analysing various factors that influence heat stress and the effect vegetation has on these factors. Furthermore, an empirical regression model is used to calculate the physiological equivalent temperature (PET) in Rotterdam, the Netherlands, under current and future climatic conditions. Multiple numerical simulations are carried out for different tree-scenarios involving varying numbers of trees in the study area to determine the heat stress in Rotterdam. The results of the literature study show that vegetation can considerably reduce heat stress by affecting shade and evapotranspiration rates, but its impact on wind can also slightly increase heat stress. Earlier research states that the type and arrangement of vegetation in an area have a strong impact on how effectively vegetation can lessen heat stress (Dimoudi & Nikolopoulou, 2003). The results of the model runs reveal that Rotterdam currently experiences strong to extreme heat stress on summer days with a moderately high probability of occurrence. In addition, heat stress is expected to worsen and become more widespread over time. By 2050, on summer days that occur every 3 years, extreme heat stress is expected in the majority of Rotterdam. In 2085, extreme levels of heat stress can be found over the entire study area. In both the current and future climate, the potential heat stress is not equal throughout the study area, as heat stress is more severe in densely built-up areas in the city centre compared to the surrounding suburbs. On average, the level of heat stress increases when an individual leaves a building. Trees have the potential to decrease the potential exposure of citizens to severe levels of physiological stress during warm summer days. However, increasing the number of trees with 50 percent of the original number of trees will not be enough to lower the average level of heat stress in Rotterdam from ‘extreme’ to ‘strong’ or ‘moderate’ heat stress. Increasing the vegetation cover can be used as a measure to mitigate heat stress in urban areas in the Netherlands, however reducing the inevitable future heat stress caused by climate change will require more than this single mitigation strategy.