Escalating marine heat waves in U.S. waters

Unraveling the mysteries and consequences of intensifying oceanic temperatures

The nation’s coastlines are currently under siege from steadily increasing ocean temperatures, a phenomenon causing mounting concern among scientists. These marine heat waves, a term designating periods where ocean temperatures rank in the top 10% for a given time of year, have been rising. The impacts on the marine ecosystems are alarming, especially as we delve deeper into the Atlantic hurricane season.

“The ocean absorbs 90% of the excess heat associated with global warming. As a result, marine heat waves all over the planet are becoming warmer over time,”


What defines a marine heat wave?

A marine heat wave is usually defined when the ocean temperature exceeds the 90th percentile for a specific length of time. This entails temperatures are warmer than 90% of past readings for the same period. These marine heat waves can endure for weeks, months, or even years. NOAA’s Physical Sciences Laboratory and NOAA Coral Reef Watch actively monitor these conditions.

This graph shows how the average surface temperature of the world’s oceans has changed since 1880. This graph uses the 1971 to 2000 average as a baseline for depicting change. Choosing a different baseline period would not change the shape of the data over time. The shaded band shows the range of uncertainty in the data, based on the number of measurements collected and the precision of the methods used.
Data source: NOAA, 2021

How are these conditions tracked?

The National Weather Service’s National Data Buoy Center collects and disseminates real-time quality-controlled marine observations using 1,300 weather observing stations. Besides, daily global ocean surface temperatures are monitored using blended satellite measurements.

Why the concern for South Florida?

Water temperatures throughout the Gulf of Mexico and the Caribbean Sea have been about 1.8-5.4˚F (1-3˚C) warmer than usual. The surrounding temperatures of Southern Florida are the highest on record, tracing back to 1981. The worry arises from the potential of tropical storms gaining strength due to these exceptionally warm ocean temperatures, and the likely subsequent impact on sensitive marine ecosystems, such as shallow water corals.

Climate change and marine heat waves

Global warming is a key driver of these heat waves as the ocean absorbs most of the excess heat. Although the current event could still have occurred without climate change, its absolute temperature is higher due to global warming.

This map shows how average sea surface temperature around the world changed between 1901 and 2020. It is based on a combination of direct measurements and satellite measurements. A black “+” symbol in the middle of a square on the map means the trend shown is statistically significant. White areas did not have enough data to calculate reliable long-term trends.
Data source: IPCC, 2013; NOAA, 2021

Impacts on marine ecosystems and corals

Marine heat waves can severely stress corals and other marine ecosystems. Extended exposure to extreme temperatures triggers a breakdown in the relationship between corals and the algae living inside of them, leading to bleaching and potential coral death.

Anticipated effects on fisheries and ecosystems

Extreme oceanic heat can be devastating for marine systems. The infamous marine heat wave known as “the Blob” that reigned from 2013 to 2016 in the northeastern Pacific resulted in ecosystem disruption, fishery collapses, and increased marine entanglements.

Concluding thoughts: heated future

These trends highlight an urgent need to mitigate climate change and develop adaptive strategies for our marine ecosystems. As marine heat waves grow in intensity and frequency, we are called to not only understand these events but to forecast them more accurately. The long-term implications of these escalating marine heat waves will shape our marine life, our climate, and our future. It’s time we turned the tide on marine heat waves.


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