He pointed to a coral reef in a protected coastal area of Rayong province, saying it once had been abundant coral and thousands of giant clams.
An increase in water temperature, however, has triggered coral bleaching and only 5% of the coral is still alive, said Thon, a lecturer at Kasetsart University’s Fisheries Faculty.
"There was death everywhere," he wrote, following a recent visit to the reef, which he described as a "graveyard".
"Global warming is a catastrophe," Thon told his more than 200,000 followers on Facebook. "It is already happening in Thai seas."
He urged the government to pay more attention to the impact of global warming to save Thailand's marine resources.
Many countries worldwide are making an effort to achieve net-zero carbon emissions to meet the Paris Agreement's goal of limiting global warming to well below 2° Celsius by pursuing efforts to limit it to 1.5°C.
Thailand has pledged to achieve its carbon neutrality goal by 2050 and net-zero carbon emissions by 2065.