The annual local mean sea level (MSL) at 13 tide gauge stations bordering the Gulf of Thailand in Thai waters was used to investigate the apparent sea-level rise over the last 25 years (1985-2009). The annual local MSL was computed by averaging the hourly tidal data at each station for the whole year. The data at 11 stations showed a higher average annual local MSL than the mean Thailand MSL, which was set up over a century ago. The data from most stations showed rising trends of sea level, although at different rates depending on the station location. Averaging the annual local MSL by region into a single time series revealed a linear trend for the sea-level rise of about 5 mm/yr in the last 25-year time span. Land subsidence at the river mouths where the tide- gauge stations are usually located seems to play a major role in the observed higher annual local MSL. The findings are a warning that Thailand may face severe coastal recession in the near future if no measures are taken to halt the land subsidence near the coast.

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