_(iv) Cool roofs._ Various cool roof interventions can be adopted, ranging from low-cost to high-cost options. In this context, we focused on the most affordable option. Studies of cool roof treatments in Indian cities provide per-meter unit costs for several treatments, including lime wash—a natural mineral-based paint—and high-density polyethylene (HDPE) cool roof membranes (NRDC, n.d). We applied a unit cost for lime wash of US$ 0.75 per m2 and for HDPE cool roof membrane of US$ 2.10 per m2. Lime wash is assumed to be effective for 2 years, while the HDPE cool roof membrane is expected to last for 5 years. In the medium-ambition package, we assume that 12.5% of buildings' roofs will be treated with cool roof interventions, while in the high-ambition package, we assume 25% coverage of the buildings being covered.

# **2.4 Benefits of intervention options**

For each action, we identified expected benefits in terms of reducing adverse impacts of urban heat and adopted a suitable approach to value these benefits (see Table 2).

(i) _Urban greening_ . Past studies have estimated heat reduction benefits from tree planting considering the sensitivity of local heat stress metrics to canopy cover (Lonsdorf et al. 2021). We apply a linear regression of tree canopy coverage within the study cities against present-day daily temperatures as well as WBGT temperatures to derive our own local estimates of temperaturetree canopy coverage relationships (see Supplementary Figures – Figure SF2). Temperature reduction benefits from the assumed 10% tree canopy area increase (medium ambition scenario) and 30% increase (high ambition scenario) are derived based on this relationship.

The temperature reductions were used to calculate the decrease in the Relative Risk (RR) of mortality. This reduction in RR was then subtracted from the summed daily relative risks of each city at each timestep (which are calculated when temperatures exceed the optimal level for each city, as illustrated in Figures 5A and 5B). The adjusted RR values were then applied to population distribution maps for each city to estimate the overall impact on mortality rates. Through a similar procedure, predicted heat stress reductions due to the greening interventions were used to derive expected benefits in terms of averted labor productivity loss.

(ii) _Heat-health early warnings_ . A study of the Ahmedabad heat early warning system identified a 23% reduction of heat-related excess mortality in the years immediately following its introduction, while a study of France’s heat early warning system noted a reduction of up to 60% in heat-related excess mortality (Hess et al. 2018, Fouillet et al. 2004). We assume that mortalityreduction efficacy will vary according to the level of effort invested in developing a robust system grounded in extensive stakeholder consultation and supported by proactive communication and outreach efforts to the most vulnerable. Based on the range of mortality reduction benefits reported in the literature, we defined a ‘medium-ambition’ intervention as achieving a 20% reduction in heat-related excess mortality and a ‘high-ambition’ intervention as achieving a 40% reduction. These reduction factors were applied to the projections of heat-related mortality 11