What sea-level rise does world seaborne trade experience? Trade-weighted relative sea-level trend at 2,015 ports and the observation gap
Dünya deniz ticareti hangi deniz seviyesi yükselişini yaşıyor? 2.015 limanda ticaret ağırlıklı göreli deniz seviyesi eğilimi ve gözlem açığı
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Abstract
Global mean sea level is the average of the ocean surface; seaborne trade, by contrast, is concentrated at a small number of coastal sites, often on subsiding deltas and reclaimed land. This study matches port-level cargo volumes derived from ship tracking with tide-gauge records, satellite altimetry and GNSS vertical velocities to measure the relative sea-level trend experienced by world port trade and to establish how much of that measurement rests on observation. The 2019–2025 mean cargo volume of 2,015 ports is used as the weight. Only 42.8% of port trade lies within 100 km of a tide gauge covering 1993–2024; the share is 73% in high-income and 15% in upper-middle-income countries, and 17 of the thirty largest ports are not covered. At ports observed by tide gauges the trade-weighted relative trend is 3.91 mm/yr (95% confidence interval 3.51–4.35), 21% above the global mean (3.23 mm/yr). About two thirds of the excess comes from land subsidence and the rest from ports being located in sea areas rising faster than average; 66% of the excess comes from ten ports. A composite estimate extended with altimetry and GNSS covers 99% of trade and gives 3.77 mm/yr; however, land motion is unknown for 45% of trade, and each 1 mm/yr of subsidence at these ports raises the global trade-weighted trend by 0.46 mm/yr. The findings indicate that the main gap in port adaptation planning is not sea-level projection but the observation of land motion at port scale.
Keywords
Relative sea level; Ports; Land subsidence; Tide gauge; Satellite altimetry; GNSS; Observation gap; Seaborne trade
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