A record-shattering marine heatwave that pushed sea temperatures to unprecedented highs off the coast of northern Europe in 2023 was an extreme event that scientists, in the context of a rapidly warming climate, had anticipated. New research shows that while the heatwave was unparalleled in the observational record, the conditions that created it are becoming increasingly common, shifting such events from a distant risk to a present-day reality.
The study, led by researchers from the University of Exeter, the Met Office, and Cefas, serves as a stark confirmation that steady global warming is leading to an exponential increase in the frequency of extreme climate events. During the June 2023 heatwave, ocean temperatures in the shallow seas around the United Kingdom soared to nearly 3 degrees Celsius above normal, disrupting marine ecosystems and contributing to record heat and rainfall on land. By analyzing a large suite of climate model simulations, the scientists determined that the annual probability of a heatwave of this magnitude has risen dramatically over the past three decades, making what was once a rare phenomenon now a roughly 1-in-10 chance each year.
Anatomy of a Record-Breaking Event
The marine heatwave of June 2023 was notable for its intensity and duration, establishing the highest temperature anomalies in the North Sea and Celtic Sea since satellite observations began in the 1980s. For approximately 16 days, temperatures in these regions registered 2.9°C above the long-term average for that time of year. This sustained period of anomalous warmth placed significant stress on the regional marine environment. The event’s striking magnitude captured public attention and highlighted the vulnerability of temperate marine ecosystems to the kind of extreme heat more often associated with tropical waters. According to Jamie Atkins, the study’s lead author from Utrecht University, the speed and scale of the warming were remarkable at the time, yet entirely consistent with expectations under climate change.
From Rare Anomaly to Increased Probability
A key finding of the research is the dramatic shift in the likelihood of such extreme heat events. The study used extensive climate model simulations to quantify how the odds have changed over time. In the Celtic Sea, located off the southern coast of Ireland, the annual probability of a heatwave matching the intensity of the June 2023 event surged from 3.8% in 1993 to 13.8% in the present climate. A similar, even more pronounced trend was identified in the central North Sea, where the chance rose from just 0.7% in 1993 to 9.8% today. This represents a more than tenfold increase in risk. Co-author Professor Adam Scaife, of the University of Exeter and the Met Office, stated that this pattern exemplifies how consistent background warming dramatically amplifies the occurrence of extreme events.
Primary Drivers of the Atlantic Warming
While long-term climate change set the stage, specific atmospheric conditions were the direct cause of the 2023 North Atlantic heatwave. Research published in the journal Nature by a separate team from the University of New South Wales identified record-breaking weak winds combined with increased solar radiation as the primary culprits. Typically, strong winds help mix the ocean’s warmer surface water with cooler water from below, distributing heat and moderating surface temperatures. However, in June and July of 2023, the winds over the North Atlantic were the weakest ever recorded.
This lack of wind led to an unprecedented thinning of the ocean’s upper mixed layer, the stratum directly affected by the sun. In some areas, this layer was reduced to just 10 meters deep, compared to a more typical 20 to 40 meters. This shallow layer of surface water heated up much more rapidly than usual under the sun, similar to how a small amount of water in a pan boils faster than a full one. The intensity of this warming within a single summer was equivalent to two decades of average warming for the entire North Atlantic.
Impacts on Marine Life and Weather
The consequences of the heatwave extended far beyond the water. The sudden and intense warming significantly disrupted phytoplankton blooms, the foundational base of the marine food web. Although the full ecological damage is still being assessed, such events are known to cause severe stress to marine species and can promote the growth of harmful bacteria that pose a risk to humans. The vast expanse of warmed water also had a direct influence on weather patterns. The warmer sea surface provided a major source of heat that contributed to record-breaking land temperatures across the British Isles. Furthermore, because warmer air can hold more moisture, the heatwave fueled increased rainfall when that air cooled over land. The heat radiating from the ocean was also implicated in deadly heatwaves across Europe in cities in Germany, France, and Italy, as well as torrential rains in Spain and Eastern Europe.
A New and Unstable Normal
Scientists emphasize that the 2023 event is a clear sign that marine heatwaves are a pressing contemporary problem, not merely a future threat. The findings underscore that in today’s climate, such “unprecedented” events should no longer be considered unexpected. Globally, the trend is accelerating, with marine heatwaves in 2023 affecting 96% of the ocean’s surface and lasting, on average, four times longer than they did between 1980 and 2023. Researchers warn that the ocean may be approaching a tipping point where average sea temperatures are permanently reset to a higher baseline, preventing some ecosystems from ever recovering. Professor Matthew England of UNSW has called for urgent and decisive global action, stating that the only way to curtail the escalating frequency and intensity of marine heatwaves is to accelerate the transition away from fossil fuels and achieve net-zero carbon emissions.