A tale of two coastal megacities: asymmetric trends in heat and rainfall extremes

Introduction: Coastal megacities face increasing risks from hydroclimatic extremes under global warming, yet comparative assessments across contrasting climatic regimes remain limited. Lagos and Mumbai were selected as representative coastal megacities with large, highly exposed populations but differing climatic systems, enabling a comparative analysis of evolving climate risks. This study presents a high-resolution assessment of Lagos (Tropical West Africa) and Mumbai (Monsoonal South Asia) over the period 1981–2023, integrating Expert Team on Climate Change Detection and Indices (ETCCDI) extreme climate indices, non-parametric trend analysis, change-point detection, and Generalized Extreme Value (GEV) modeling. Materials and methods: Daily rainfall (CHIRPS v2.0) and temperature (ERA5-Land) datasets were analyzed alongside a Composite Vulnerability Index (CVI) incorporating population density, informal housing, drainage coverage, and healthcare accessibility. Sea-level rise was considered as an amplifying factor for coastal flood exposure. Results: Results indicate contrasting hydroclimatic trajectories: Mumbai exhibits significant increases in rainfall (+5.18 mm/year) and wet spell persistence (CWD +0.133 days/year), while Lagos shows stronger warming trends, particularly in extreme temperatures (TXx +0.047 °C/year). Change-point analysis suggests a shift in Mumbai’s rainfall regime in the early 2000s, although the linkage to broader climatic drivers is inferred rather than explicitly tested. Both cities exhibit increasing clustering of extreme events, with Lagos showing greater interannual variability. GEV-based return level estimates indicate substantially higher multi-day rainfall extremes in Mumbai (~478 mm for a 100-year event) compared to Lagos (~99.6 mm). Spatial analysis reveals a strong overlap between areas of high flood exposure and socio-environmental vulnerability in both cities. Conclusions: These findings suggest that while hydroclimatic drivers differ between Lagos and Mumbai, their impacts converge through shared patterns of urban vulnerability. This underscores the importance of integrating hazard-specific approaches with vulnerability-focused planning in rapidly urbanizing coastal megacities.