Scientists look at the timing and depth of heat changes to separate seasonal cycles from permanent shifts. Natural variability often follows predictable patterns, such as the El Niño-Southern Oscillation or the North Atlantic Oscillation. These cycles move heat around the ocean in specific, repeating loops. If temperatures bounce back to a baseline after a few years, scientists classify it as variability.
A permanent shift looks different. Instead of a loop, we see a steady, upward slope in the data. Oceanographers use Argo floats—autonomous robotic probes—to measure heat deep below the surface. If the warmth stays trapped in the deep layers rather than cycling back to the surface, it signals a structural change in the ocean's heat content. This
