> Unusually warm water (as little as 1 °C above usual summer means) can cause the breakdown of the symbiosis and the mass expulsion of zooxanthellae, referred to as “coral bleaching.” [1]

Coral bleaching isn't typical and is a direct indication of water temperatures exceeding the max habitable temperatures. You can look at coral, or manatees, or algal blooms to confirm that our present conditions are anomalous. Extraordinary claims require extraordinary evidence but the research is in regarding rising sea temperatures.

[1]: https://link.springer.com/article/10.1007/s12237-014-9875-5