Assessing equity in heat mitigation ecosystem services of urban green space in Singapore

Urban green space provides ecosystem services such as heat mitigation, which are associated with improved resident health and welfare. While estimating the disparity in ecosystem services is essential to improve equity, little guidance is available as to which indices and indicators should be used. Here, we assess the distributional equity in heat mitigation benefits as measured by two indicators – land surface temperature reduction and wet-bulb temperature reduction – in the city-state of Singapore, and compare three inequality indices: Gini coefficient (GC), Atkinson index (AI), and Theil entropy index (TE). We include scenarios with and without migrant construction workers to test the changes in inequality indices when considering marginalized groups, and explore the influence of income subgroups and other socio-demographic factors. Results suggest that heat mitigation ecosystem services are inequitably distributed in Singapore, as shown by all inequality indices, except AI applied to wet-bulb temperature. Compared to GC, the increase in AI and TE clearly reflects the exacerbation of inequity under scenario with migrant construction workers. Lower-income subgroups typically contribute more to heat mitigation inequity than wealthier communities. The decomposability of AI and TE provided policy-relevant insights; AI helped evaluating inequity differences between heat indicators, while TE proved better at detecting inequity changes across scenarios. We discuss the strengths and limitations of each index, paving the way for improved guidance on analytical approaches to assessing environmental equity.