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?

Heatwaves are periods of several days or more when temperatures are beyond historical averages for a given area. MHWs follow this logic, but as a special case occurring within the ocean. MHWs are prolonged periods during which ocean temperatures remain well 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”, a large patch of unusually warm water with sea surface temperatures as much as 3oC above average. The Blob first appeared off of California in 2013, caused by a stubborn high-pressure system that blocked the winds that usually mix and cool the ocean. Heat built up in the water which gradually spread northward from California to Alaska over a period of several years. The Blob was unusual not only for how warm it was, but also because the heat extended deep into the ocean, reaching depths of up to 150 meters. This intense, deep heating disrupted marine food webs, contributed to the occurrence of harmful algal blooms, prompted shifts in important fish species like Pacific cod and salmon, and led to catastrophic die-offs of seabirds and marine mammals. Effects of the Blob on marine communities and food webs persisted for years after the event.

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, as stalled high-pressure systems led to calm skies and prolonged solar heating over the Northeast Atlantic. Coupled with an El Niño event, the region 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. Prolonged hot ocean conditions led to mass coral bleaching and widespread mortality, and was also linked to fish kills, changes in biodiversity, production, and broader ocean health.

Florida's Coral Reefs (2023)

The summer heating of 2023 also brought unprecedented marine heat to the coastal waters of Florida, 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 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 warmed nearly three times faster than the rest of the world’s oceans during the period from 1982 – 2024. It is now understood that the rapid change in SST is a product of both a slow, steady warming from climate change and abrupt oceanographic changes that have had a wide-reaching and lasting impact. Around 2010, a shift in the positioning of nearby ocean currents had a cascade of downstream effects on regional circulation which itself influences temperatures, stratification, and nutrient supply to the region. The rapid shift to persistently higher temperatures along the Northeast US Continental shelf was one such effect. SSTs in the decade that followed were routinely higher than the long-term averages, and the Gulf of Maine experienced more frequent and longer-lasting marine heatwaves.

In our 2021 reporting, we concluded that the Gulf of Maine had experienced its warmest year on record. Daily SSTs that year were above marine heatwave conditions for 360 days compared to the 1982 – 2011 climatological baseline used at that time. The Gulf of Maine had warmed so quickly that the entire year could be considered a single MHW event using the standard MHW definition. This phenomenon, sometimes called "saturation," occurs when background ocean warming causes temperatures to frequently exceed thresholds that historically would have marked extreme events.

More recently, SSTs in the GOM have been much cooler following a quick transition into below average temperatures. As early as 2023 there were signs that the amount of cooler water that enters the Gulf of Maine from the Northeast Channel had increased. SSTs have since fallen to ranges closer to the long-term average, a trend projected to continue through 2025 (NOAA).

Researchers have developed a number ways to adapt the definition of a MHW which better separate MHWs from baseline warming due to climate change. As with anything, different approaches excel in different settings, and each comes with its own tradeoffs. The proliferation of competing definitions for MHWs has made it challenging to communicate trends in the number and durations of MHW events, metrics that are dependent on the approach used. Regardless of the approach taken, it is undeniable that the Gulf of Maine (and the Northeast US more broadly) has experienced major changes in the thermal landscape. The abrupt increase in GOM SSTs that occurred around 2010, and the quick drop seen more recently present a challenge for all approaches: how to determine what conditions to consider “normal" in the presence of rapid, un-characteristic, and potentially reversible changes. Which approach provides the clearest signal into how a given marine species responds to these changes in the environment is case-specific and is an area of active research.

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. More mobile species may move to deeper or cooler waters during short-term events. Prolonged exposure from longer-lasting MHWs, however, could lead to physiological stress or permanent relocation to new areas.

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 early in the year which marked a dramatic shift in regional temperature patterns, triggering noticeable effects on local fisheries — particularly the lobster fishery. Higher ocean temperatures over winter and in the spring 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. Following this incident the industry in Maine took a number of actions to shield itself from future price shocks. This included increasing its processing capacity and expanding the state’s capacity for cold storage. These efforts and other collaborative efforts between State, industry, and community members appear to have paid off in the short-term, preventing subsequent supply-chain shock events in spite of record high SSTs.

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), events where ocean conditions support rapid algae growth. Some algae species produce harmful toxins, which during HABs can reach concentrations in the environment dangerous to nearby marine life and people. HABs can disrupt aquaculture operations, kill marine life, make people sick, 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 for example, accurate projections of ocean conditions have only recently become available.

Marine heatwaves are a recurring disruption that reshapes 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 can gain information that enables them to be 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.