Climate change-induced thermal load as a natural form of thermal pollution in semi-enclosed seas

Abstract

Understanding the spatial and temporal variability of oceanic heat accumulation is essential for quantifying anthropogenic thermal pollution under a changing climate. This study investigated multi-decadal and seasonal thermal dynamics across seven semi-enclosed and marginal seas (Mediterranean, Marmara, Black, Baltic, Red, and Japan Seas, and the Persian Gulf) using daily ERA5 sea surface temperature (SST) data for 1990-2024. Surface heat fluxes (Q_day) were estimated to assess long-term trends, cumulative heat storage, and extreme thermal events. Mann-Kendall and Sen's slope analyses revealed a significant warming trend in the Mediterranean Sea (0.006 degrees C yr(-1), p < 0.01) and a significant cooling trend in the Sea of Marmara (-0.008 degrees C yr(-1), p < 0.05), while other basins exhibited statistically insignificant trends dominated by short-term atmospheric variability. Local Thermal Anomaly (LTA) analysis showed a significant positive residual warming trend in the Mediterranean (+0.0055 degrees C yr(-1), p = 0.015) and a negative trend in the Marmara Sea (-0.0062 degrees C yr(-1), p = 0.047), indicating basin-specific modulation of the large-scale climate signal. The Mediterranean basin also recorded the highest coastal built-up expansion rate (48.3 km(2) yr(-1)), suggesting that coastal urbanization may contribute to localized thermal loading. Seasonal diagnostics revealed strong net surface heating from April to August, followed by cooling during the rest of the year. A marked post-2000 increase in high-energy flux days (Q_day >1 & times; 10(7) J m(-2) day(-1)) was detected, particularly in the Black Sea and Sea of Japan, indicating intensification of episodic marine heat extremes. Sensitivity tests incorporating time-varying seawater density showed that heat flux estimates differed by less than 2% from the constant-density assumption, confirming the robustness of the results despite substantial inter-basin salinity contrasts. Rainfall trends were weak and statistically insignificant across all basins, suggesting that atmospheric heat fluxes and circulation dynamics primarily govern regional thermal variability. As a result, semi-enclosed seas exhibit heterogeneous responses to global climate forcing, shaped by the interplay of atmospheric dynamics, basin morphology, circulation efficiency, and coastal anthropogenic pressure. This framework demonstrates how reanalysis-based SST datasets can be operationalized to monitor regional ocean heat stress and anthropogenic thermal pollution in vulnerable marine systems.