Limitations to air free cooling in data centers under rising heat and humidity

Reliable data center operation depends on effective thermal management, with air free cooling (direct air-side economization) serving as a widely deployed energy-saving and water-minimizing strategy, implemented either as a primary mode or within hybrid cooling architectures that incorporate periods of direct exposure to ambient air. This study provides a global assessment of historical and projected climate-driven constraints on air free cooling operability using high-resolution hourly observations and climate model simulations. Over the past 45 years, the number of hours characterized by co-occurring high temperature and humidity conditions that limit direct air cooling has increased significantly, particularly across the tropics and the southeastern United States. Even regions with modest long-term trends experience longer daily exceedance events, highlighting intensifying operational thermal stress. A site- and market-level analysis further shows that the share of data centers exposed to conditions that limit air free cooling availability for at least one quarter of the year is rising. Projections through mid-century indicate continued expansion of these constraints under sustained warming and increasing humidity. These results define evolving climatic boundary conditions for facilities incorporating direct air free cooling and provide a quantitative basis for evaluating adaptive cooling strategies and hybrid system designs under future climate conditions.