Climate MattersAugust 23, 2023

Record Ocean Heat Impacts: From Hurricanes to Corals


  • Global sea surface temperatures have been continuously hotter than any previous April-August period on record — and by a wide margin. 

  • Long-term warming and 2023’s exceptional ocean heat have major implications for both tropical cyclones and coral reefs.

  • Warmer oceans can fuel stronger hurricanes, increasing the risks that they could undergo rapid intensification and cost billions in damage. 

  • Ocean warming also risks coral reef bleaching, which can have devastating impacts on marine ecosystems and people who depend on them. 

  • Around Florida coasts and across the Caribbean, corals are currently experiencing record heat stress and risk of bleaching.

CM: 2023 Sea Surface Temperature (EN)
Click the downloadable graphic: 2023 Sea Surface Temperature

Record-shattering ocean heat

This summer’s relentless and record-breaking heat hasn’t been limited to land. 

Over the last four months, global sea surface temperatures have been continuously hotter than any April-July period on record — and by a wide margin. 

In a typical year, sea surface temperatures decrease from April to July (after summer ends in the Southern Hemisphere, which has a much larger ocean area). But not this year. 

Temperatures have not decreased since April; they’ve increased, charting an unprecedented path toward a record-shattering July. 

Sea surface temperatures were hotter in July 2023 than during any other month since 1850, followed by June 2023. 

According to NOAA, 48% of the global ocean is currently experiencing marine heat waves — the record largest marine heat wave area of any month since 1991. And these conditions are forecast to continue.

This exceptional ocean heat is primarily due to warming from human-caused carbon pollution and the arrival of El Niño conditions in June. See Why is it so hot this summer? below for more information. 

Click the downloadable video: Bleaching Alert May to August 2023 (EN)
Click the downloadable video: Bleaching Alert May to August 2023 (EN)

Warming oceans put people and ecosystems at risk

We all depend on the ocean. It regulates climate and affects weather on land. It hosts vast biodiversity, provides nutrition for billions, and underpins livelihoods and cultures around the world. 

But ocean warming disrupts each of these critical functions. And as the planet has warmed in recent decades, about 90% of the excess heat has gone into the ocean — mostly the surface ocean, which has warmed steadily as a result. 

Long-term warming and 2023’s exceptional ocean heat have major implications for both tropical cyclones and coral reefs.

CM: Ocean Heat Impactst 2023 (EN)
Click the downloadable graphic: Ocean Heat Impacts

Warmer oceans fuel stronger, more costly tropical cyclones

Sea surface temperatures influence the weather we experience on land. Warm water is a necessary ingredient for tropical cyclones, which include hurricanes and tropical storms. 

In response to ongoing record-warm oceans, NOAA recently increased its 2023 Atlantic hurricane season outlook to above normal despite the ongoing El Niño conditions, which typically limit Atlantic hurricane activity.  

About 80% of major hurricanes (Category 3-5) undergo rapid intensification — defined as an increase in the maximum sustained winds of a tropical cyclone of at least 30 kt (about 35 mph) in a 24 hour period. 

And ocean warming contributes to an increased fraction of tropical cyclones that undergo rapid intensification. 

Forecasting rapid intensification can be challenging, which contributes to the high human and economic toll of such storms.

Of the 56 tropical cyclones that have caused at least $1 billion in damage in the U.S. from 1980-2021, 73% underwent rapid intensification.

The six most costly hurricanes since 1980 — which caused a combined estimated $744 billion (inflation-adjusted) in damages — all had maximum rapid intensification rates between 40-70 kt in 24 hours.

CM: Stronger Storms, Higher Cost 2023 (EN)

Coral reefs: vital, valuable, vulnerable

Coral reefs are the world’s most diverse marine ecosystems. About one-quarter of all marine species depend on corals. 

Corals support fisheries that provide nutrition and livelihoods, especially in the tropics. Coral reefs also provide coastal protection during tropical storms. Each year, coral reefs in the U.S. provide $3.4 billion in economic value from fisheries, tourism, and coastal protection. 

But these valuable ecosystems are also extremely vulnerable to climate change. 

Long-term ocean warming and more frequent marine heatwaves like those currently affecting many parts of the globe cause heat-stress in corals — which can lead to coral bleaching and death. 

Set 2 - World Oceans Day
Set 2

Zooxanthellae, the single-celled algae that live within coral tissues, provide food and color to corals. But when waters reach 1-2°C above the normal range for an extended time, algae leave the coral, resulting in coral bleaching. 

Bleaching drains corals of their algae and their color, leaves corals at high risk for disease, and can ultimately lead to coral death. 

It can take reefs decades to recover from mass coral bleaching events, which can be devastating to marine ecosystems and the people who depend on them. 

Mass bleaching events were first observed in the 1980s. Since then, there have been three major global coral bleaching events, the most recent occurred in 2015-2017, the last El Niño year. 

Mass coral bleaching events are most prevalent in tropical mid-latitudes

Florida’s coral heat stress: earlier and more intense than ever

Florida is home to one of several major coral reef areas in U.S. states and territories. This year’s record-shattering sea surface temperatures are putting Florida’s corals under extreme heat stress. 

The NOAA Coral Reef Watch program’s daily Bleaching Area Alert shows extreme risk to corals along around Florida and the Caribbean expanding and accelerating through the summer and into mid-August. 

The Bleaching Area Alert is a measure of the combined intensity and duration of coral heat stress. 

There are two alert levels. Both levels have the same heat intensity threshold (sea surface temperature anomalies of 1°C or higher), combined with either:

  • Level 1: 4 to 8 degree heating weeks, indicating significant bleaching is likely

  • Level 2: 8 or more degree heating weeks, indicating severe bleaching and significant mortality is likely 

Both Southeast Florida and the Florida Keys have exceeded a Bleaching Alert Level 2 this summer. And they’ve reached these thresholds earlier and by a wider margin than any other year on record, back to 1985.  

These dangerous conditions are projected to continue in the Florida Keys and Southeast Florida for at least another month, putting Florida's corals at unprecedented heat stress. 

Future outlook

NOAA and partner local organizations will continue to monitor and rescue corals during Florida’s historic bleaching event. NOAA’s Coral Reef Conservation Program works to conserve and restore coral reef ecosystems across U.S. states and territories over the long-term. And there are ways that people can help protect corals even if they live far from the coast. 

But until ocean warming stops at the main source — carbon pollution — coral bleaching is likely to become more frequent among the world’s coral reefs. 

According to recent IPCC reports, “multiple lines of evidence indicate that the majority (70–90%) of warm water (tropical) coral reefs that exist today will disappear even if global warming is constrained to 1.5°C.” 

Why is it so hot this summer?

Unprecedented summer heat has prompted rapid analyses of the human-caused and natural factors influencing recent trends. 

An August 14 analysis from Berkeley Earth confirms that human-caused warming—the direct result of emissions of greenhouse gases, especially CO2—is the primary factor driving long-term warming trends. 

The next most important factor is the recent and strengthening El Niño, the warm ENSO phase that boosts global temperatures on top of the primary warming from human activities. 

Several other factors are also contributing to short-term warming, including: a phase of increasing solar activity, warming effects of a 15% increase in stratospheric water vapor following the January 2022 Hunga Tonga–Hunga Haʻapai volcano eruption, and the reduction of sulfur aerosols from heavy shipping due to recent pollution and health regulations. 

In the North Atlantic, sea surface temperatures for the month of July 2023 were substantially warmer than any previous July on record. Exceptional sea surface temperature anomalies in the North Atlantic have continued into August and are contributing to global records. 


How are corals doing across U.S. states and territories?

NOAA’s Coral Reef Conservation Program monitors and reports on conditions in American Samoa, Commonwealth of the Northern Mariana Islands, Florida, Guam, Hawai’i, Puerto Rico, and the U.S. Virgin Islands. Use NOAA’s Coral Reef Watch tools and interactive maps to monitor observed and forecasted coral heat stress conditions around the world. 

How does ocean warming affect different U.S. regions?

Check out our Ocean Warming Climate Matters brief for free, broadcast-ready graphics for 11 major U.S. ocean regions showing long-term sea surface temperature trends and their primary impacts on people and ecosystems in each region.


Kiho Kim, PhD (he/him)
Provost and Dean of the College, Professor of Environmental Science and Studies
Washington College, Chestertown MD
Relevant expertise: coral reef ecology, marine conservation, climate change

Daniel Gilford, PhD
Climate Scientist
Climate Central
Related expertise: tropical cyclone intensity and coastal impacts


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