Subsurface urban heat islands across the United States

Subsurface urban heat islands increasingly present both challenges and opportunities for cities worldwide. While these phenomena are attracting growing attention and monitoring globally, they remain poorly characterized in the United States. This short communication provides the first overview of the intensity and evolution of subsurface urban heat islands across multiple cities in the United States, based on data from scientific publications, technical reports, online repositories, and direct field measurements. The results reveal maximum ground temperatures reaching up to +69.4 °C and temperature anomaly rates of up to +2.21 °C per year at distinct locations, which represent exceptions relative to the lower-yet still marked-anomalies caused by subsurface urban heat islands. While the analyzed data should not be considered representative of the entire geography of the studied urban areas, they reveal alarmingly high ground temperatures that must be controlled and mitigated to prevent adverse impacts on the environment, transportation systems, foundation and structural systems, and public comfort and health. This work underscores the urgent need for expanded sensing initiatives, research, technological innovation, and informed policy-making to address the often-overlooked phenomenon of urban ground warming-particularly in light of projected urban population growth and the interconnected release of heat into the subsurface.