Marine Heatwaves 101

In recent years, the Gulf of Maine has emerged as a hotspot of ocean warming — a place where the impacts of climate change are accelerating faster than most other parts of the global ocean. One indicator of this trend is the rise in marine heatwaves (MHWs): extreme periods of elevated sea temperature that impact ecosystems and coastal economies alike.

Below, we explore what these events are, their frequency, and how they ripple through the Gulf of Maine and the communities that depend on it.

What Are Marine Heatwaves?

You can think of MHWs as comparable to those heatwaves we experience on land, except they occur in the ocean. MHWs are prolonged periods during which ocean temperatures remain above long-term averages. Scientific perspectives on how to best quantify these periods vary somewhat, but the most widely accepted measure is a period in which daily average sea surface temperatures exceed the 90th percentile of a climatological (i.e., 30-year) average for at least five consecutive days. Temperatures falling below this threshold for two days or less do not constitute a break in the MHW event once it has been reached.

Are We Experiencing More Marine Heatwaves?

Marine heatwaves are a newly defined phenomenon in the past two decades, but when applied to long-term temperature records, these events are occurring with both higher frequency and intensity. Over the past decade or so, this increase in intensity and frequency has accelerated further, contributing to extreme ocean warming events around the world.

The Bering Sea and "The Blob" (2013 – 2016)

Between 2013 and 2016, the Northeast Pacific experienced a prolonged marine heatwave known as "The Blob," characterized by sea surface temperatures up to 3°C above average. This event disrupted marine ecosystems, leading to massive die-offs of seabirds, including nearly four million common murres in Alaska, marking the largest recorded die-off of a single seabird species.

Northeast Atlantic Heatwave (2023)

In 2023, land masses over Europe were dealing with a deadly heatwave that was so bad it got a name — the Cerberus heatwave. But the heat wasn’t limited to just land masses. The Northeast Atlantic experienced an intense marine heatwave, with sea surface temperatures peaking at up to 5°C above average west of Ireland and the United Kingdom. These temperatures were categorized as “beyond extreme” using the NOAA heatwave classification system. This prolonged event disrupted normal seasonal patterns and marine ecosystems across the region, with downstream effects on biodiversity, fisheries, and broader ocean health.

Florida's Coral Reefs (2023)

The summer of 2023 brought unprecedented marine heat to Florida's coastal waters, with temperatures exceeding 90°F (32°C). This extreme heat led to 100% coral bleaching across monitored sites in the Florida Keys, severely stressing coral ecosystems. The event prompted scientists to reassess traditional reef restoration techniques, as many transplanted corals failed to survive the prolonged heat stress. The crisis underscored the need for innovative approaches to coral conservation in the face of escalating climate challenges.

Ningaloo Reef, Western Australia (2025)

Beginning in early 2025 and still ongoing, Western Australia's Ningaloo Reef, a UNESCO World Heritage site, is experiencing its most severe marine heatwave on record. This protected site, unlike the Great Barrier Reef, had largely avoided mass bleaching events as a result of warming waters up to this year. But sea temperatures rose up to 4°C above average this year, causing widespread coral bleaching, including in deep-water corals previously considered less vulnerable to MHW events. The event also led to mass fish die-offs along the Pilbara coast, highlighting the extensive ecological impact of marine heatwaves on diverse marine life.

Here in the Gulf of Maine, sea surface temperatures have warmed almost three times faster than the rest of the world’s oceans. SSTs are routinely well-above the long-term averages, making researchers question whether marine heatwaves need a refreshed definition.

This phenomenon, sometimes called "saturation," occurs when background ocean warming causes temperatures to frequently exceed thresholds that historically would have marked extreme events. Since around 2010, changes in ocean currents and other regional dynamics have driven a rapid shift in temperature norms, leading to more frequent and longer-lasting marine heatwaves. This constant shifting of baselines in the Gulf of Maine makes it challenging for scientists to determine whether species are reacting to a genuine anomaly or are simply adjusting to a new normal.

The Impact of Marine Heatwaves

Due to the complexity of oceanographic conditions and the challenges of tracking marine species, it can be difficult to identify definitive examples of how marine heatwaves impact specific species.

One clear exception is the bleaching of coral reefs observed along the coast of Australia. Coral plays a unique role in the study of marine heatwaves because it is extremely sensitive to elevated temperatures. As an immobile species, coral cannot migrate to cooler waters during extreme events, and its bleaching response provides a clear and measurable indicator of stress caused by warming temperatures.

Most marine organisms are sensitive to temperature, but unlike coral their responses to marine heatwaves can be complex and varied. Some mobile species, such as lobster or Atlantic salmon, may move to deeper or cooler waters during short-term events. Prolonged exposure, however, could lead to permanent migration or physiological stress.

In the Gulf of Maine, researchers are still working to understand how marine heatwaves affect local species. As Adam Kemberling explains, it’s difficult to pinpoint which elements of a heatwave are most stressful to marine life.

Marine heatwaves don’t just affect marine life — they also impact the people and industries that rely on those ecosystems. Whether you are an aquaculturist dealing with increased disease risk, a fisherman who can't depend on reliable fishing locations, or any of the myriad of waterfront workers who depend on a stable, healthy Gulf of Maine, the consequences of marine heatwaves are difficult to predict and far reaching when it comes to their impacts on the dynamic Gulf of Maine system.

For example, in 2012 the Gulf of Maine experienced a significant marine heatwave that marked a dramatic shift in regional temperature patterns, triggering noticeable effects on local fisheries — particularly the lobster fishery. Warmer waters prompted lobsters to molt and migrate to shallower waters earlier in the season. This led to unusually high lobster hauls, which overwhelmed processing infrastructure and drove down prices.

Oysters are another key species in Maine's coastal economy that can be affected by marine heatwaves. Like coral, oysters are immobile and cannot escape warming waters. While oysters can survive brief periods of high temperatures, the physiological stress can stunt their growth and increase their vulnerability to disease, exacerbated by pathogens thriving in warmer waters.

Marine heatwaves can also trigger harmful algal blooms (HABs) — rapid algae growth that can be dangerous to marine life. These blooms can disrupt aquaculture operations, kill marine life, and threaten coastal economies. As warming waters create more favorable conditions for HABs, understanding and monitoring these events is critical to protecting ocean-dependent industries in the Gulf of Maine.

Navigating a Warming Future

Describing and predicting extreme events in a rapidly changing ocean is challenging, but that isn’t stopping scientists and fishery managers as they work to understand and respond to these warming events.

In some regions, early detection systems have allowed for short-term adaptation. In Australia, for example, forecasts obtained using an artificial intelligence model trained on ocean data has been used to predict marine heatwaves. These predictions are being used by both fisheries and conservationists alike to prepare for extreme warming events along Australia's western coast.

While this is a promising development in planning for marine heatwaves, it doesn’t translate perfectly to other oceans. In the Gulf of Maine’s chaotic waters for example, accurate prediction remains difficult due to the rapid changes in our ocean conditions year-to-year.

Marine heatwaves are no longer rare anomalies — they are recurring disruptions reshaping life in the Gulf of Maine and in our global oceans. From shifting species distributions to harmful algal blooms and economic upheaval in ocean-dependent industries, the ripple effects of these warming events are wide-reaching and complex.

But by studying marine heatwaves — their patterns, biological effects, and socioeconomic consequences — scientists, managers, and communities are better positioned to adapt. Continued research will be critical to building early warning systems, informing adaptive fisheries management, and supporting the resilience of coastal economies in a warming world.

Understanding marine heatwaves isn’t just a scientific endeavor — it’s a necessary step toward sustaining the people, species, and systems that rely on the Gulf of Maine.