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When most people hear the words “heat wave,” they picture sweltering city streets, triple-digit temperatures and air conditioners running nonstop. But some of the most consequential heat waves are happening out of sight — in the ocean.
Marine heat waves are periods when ocean temperatures remain unusually warm for an extended stretch, and scientists are increasingly able to measure how much human-caused climate change is influencing those conditions. On a recent episode of Tidal Flooding Talk, Climate Central meteorologist Shel Winkley explained how a warming ocean can ripple far beyond the shoreline, affecting humidity, rainfall, hurricanes and even how well people sleep at night.
Climate Central’s Climate Shift Index is designed to put numbers on climate change’s influence. For land temperatures, the index runs from negative five to positive five. A reading of four indicates a temperature that would have been rare without climate change, while a five signals conditions that would have been virtually impossible without its influence.
The ocean version can produce far more dramatic numbers.
Because ocean temperatures change more slowly than temperatures on land, unusually warm water can carry an especially strong climate signal. Winkley said some sea-surface temperatures can be calculated as 500, 800 or even 1,000 times more likely because of climate change. About 90% of the excess heat associated with human-caused warming is absorbed by the ocean, he said.
That extra heat does not remain offshore.
Warmer oceans increase evaporation, pumping more moisture into the atmosphere. That added humidity can help keep nighttime temperatures elevated because humid air cools more slowly than dry air. Winkley said summer nighttime temperatures have been warming nearly twice as fast as daytime temperatures, with warmer oceans helping feed the moisture that pushes inland.
More atmospheric moisture can also mean heavier rainfall. Warmer ocean water can provide weather systems with a larger supply of moisture, potentially increasing both rainfall totals and rainfall rates.
Then there are hurricanes.
Tropical cyclones draw energy from warm water, and Climate Central is using ocean-temperature attribution to estimate how much climate change may boost a storm’s intensity. Even an increase of only a few miles per hour can matter because wind damage rises sharply as wind speeds increase. In some cases, Winkley said, the science can help determine whether climate change helped push a hurricane across a category threshold.
The takeaway is increasingly difficult to ignore: ocean heat is not simply an issue for swimmers, fisheries or coral reefs.
What happens in the water can reshape the weather people experience hundreds of miles inland — and the warmer the ocean becomes, the more consequential those connections may become.