Chronic humid heat as a labour-capacity constraint: WBGT-based stress tests for tropical economies

Chronic humid heat is an increasingly important yet under-recognised manifestation of climate change that constrains labour capacity and economic functioning in tropical economies. Unlike episodic heatwaves, humid heat operates as a persistent background condition, routinely pushing workers toward physiological limits and reducing effective labour input. However, most economy-wide assessments rely on temperature-based indicators or aggregate damage functions that abstract from workability thresholds and the propagation of localised labour constraints through production networks. We quantified how year-round Wet Bulb Globe Temperature (WBGT)–induced labour-capacity losses translate into economy-wide exposure across tropical economies with contrasting sectoral structures. Using bias-adjusted ISIMIP3b climate projections, empirically validated WBGT exposure–response functions, and harmonised labour and value-added data, we analyse Singapore, Cambodia, and Vietnam. Sector-level reductions in effective labour capacity are mapped to value-added constraints and propagated through national input–output tables as a diagnostic stress test of constrained short-run adjustment. The year 2018 is treated as a baseline exposure year reflecting observed climate conditions and economic structure, rather than a climate-free counterfactual. In 2018, effective labour-capacity losses reach 9–11%, with sectoral losses approaching 17% in agriculture and construction. These constraints correspond to economy-wide gross output exposure of approximately USD 0.9 billion in Singapore, USD 2.6 billion in Cambodia, and USD 22.3 billion in Vietnam (2018 USD). Future impacts are interpreted as incremental exposure relative to this baseline, isolating the additional burden associated with rising heat stress under fixed production structures. Indirect exposure transmitted through supply-chain linkages often exceeds direct sectoral losses, particularly in service-oriented economies. Expressed relative to sectoral value added, impacts reveal substantial heterogeneity, with pronounced exposure in labour-intensive and heat-exposed sectors. By treating chronic humid heat as a routine operating constraint, the framework identifies production-network bottlenecks where protecting work capacity can yield disproportionate economy-wide resilience and supports near-term climate adaptation prioritisation in tropical economies.