implementation feasibility – this intervention can be considered ‘low hanging fruit’ that produce strongly beneficial outcomes for limited financial outlays. The finding suggests that cities would be well advised to prioritize heat-health early warning systems.

Implementing heat early warning systems is shown to be a highly effective strategy. Our research, along with other studies, consistently shows a significant reduction in mortality rates and a benefit-cost ratio (BCR) exceeding 50. These findings are not unique; similar BCR values are observed in studies worldwide. For instance, research by Chiabai et al. (2018) and Ebi et al. (2004) revealed BCRs well above 50 for cities like Madrid and Philadelphia.

Our study has several limitations. The health impacts analysis examined mortality without performing estimates of health costs associated with non-fatal illness. The exposure-response curves utilized were derived from all-India mortality data; our analysis could be improved through use of locally derived temperature-mortality relationships that would better characterize heat impacts in India’s varied climatic zones. In future work, labor productivity impacts should be modeled with improved treatment of difference in heat exposure between occupations (eg. indoor vs. outdoor work). Despite these limitations, the present work contributes to advancing modeling approaches for urban heat mitigation and adaptation while pointing to strong policy conclusions about the urgency and efficacy of heat mitigation and adaptation actions that can be championed at the city level. Additional impact channels that could be analyzed would include extreme heat impacts on transport, power and water infrastructure and city services.

# **5. Conclusion**

Indian cities already experience night-time temperatures 5°C or more higher than outlying rural areas due to the urban heat island effect, with densely packed neighborhoods most affected. Combined with impacts of global climate change, cities such as Chennai, Surat and Lucknow will see 30%-50% more heatwave days and hot nights per year. The rise in heat stress will increase the number of working hours per year which are too hot to safely perform work – particularly in high physical-intensity and heat-exposed outdoor sectors such as construction, agricultural work, policing, etc. – putting as much as 4% of economic output in cities such as Lucknow and Chennai at risk by 2050 under a high carbon emissions scenario. In parallel, based on existing heatmortality relationships and accounting for a reasonable degree of additional heat-tolerance as urban Indians acclimatize to hotter conditions, heat-related excess mortality is expected to rise by approximately one-third by 2050, regularly resulting in hundreds of additional excess deaths per city during heatwaves, particularly among elderly and vulnerable populations. Cities are uniquely positioned to address the challenge of urban heat through policy instruments and investments such as tree canopy expansion, cool roofs and heat-health early warning systems. Benefit-cost ratios are particularly high in the case of heat-health early warning systems, suggesting that such measures should be prioritized for more widespread implementation in Indian and global cities.

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