The size of the increase over the old mark is tiny, but its timing is what climatologists have flagged. Global sea surface temperatures follow a regular annual cycle and normally reach their maximum in March or April, after the southern hemisphere summer has warmed the ocean-dominated southern half of the planet. August sits close to the seasonal trough of that cycle. Setting an all-time record at the low point of the annual rhythm implies the underlying baseline has shifted far enough that ordinary seasonal cooling no longer brings the ocean back below previous peaks. A developing El Nino is adding heat on top of that raised baseline.

Marine heat damages coral reefs through bleaching, shifts the distribution of fish and other marine species, and lowers dissolved oxygen levels in seawater. Warmer oceans also absorb more carbon dioxide, raising acidity and affecting shell-forming organisms and the fisheries that depend on them. Because many marine species have narrow temperature tolerances, fisheries and aquaculture output is directly exposed, with consequences for coastal food security. Hotter oceans additionally alter currents and rainfall patterns and can intensify storms and land-based heatwaves. Marine heatwaves also encourage toxic algal blooms, which can contaminate seafood and reach human health through it.