The coastal waters around the UK and Ireland are experiencing an unusual heatwave, with temperatures soaring as much as 4°C above normal in some areas. This marine heatwave, affecting regions off the west coast of Ireland, as well as Cornwall and Devon, is sparking concerns among marine biologists and coastal communities alike. Scientists from the National Oceanography Centre and the Met Office report that sea temperatures in April and early May have reached levels not seen in the past 45 years. The unusual rise in the UK’s sea temperatures warrants a closer look at the causes, potential impacts, and what it signifies for the future health of our oceans.

The surge in the UK’s sea temperatures can be attributed to several factors. Unusually warm spring temperatures combined with weak winds have contributed to the warming of the ocean’s top layer. However, the underlying driver is undeniably climate change, which is causing oceans worldwide to warm, increasing the likelihood and intensity of marine heatwaves. According to Dr. Ségolène Berthou at the Met Office, “Before we started having this exceptional weather this spring, the waters were already in a hotter state,” underscoring the pre-existing conditions that exacerbated the recent temperature spike. Indeed, historical data confirms that April and the first half of May saw the highest sea temperatures recorded in 45 years of monitoring.

The most intense warming is concentrated off the west coast of Ireland and in certain areas off the coasts of Cornwall and Devon. Regional variations exist, with the west coast of the UK experiencing temperatures about 2.5°C above average, while Scottish waters are 2-3°C warmer than usual. “It started in the North Sea and the Celtic Sea. Now the North Sea has cooled down a bit but the west of Ireland is extremely hot,” notes Dr. Barthou, highlighting the dynamic nature of the marine heatwave’s spread.

The potential impacts on marine life are considerable. Warmer waters can disrupt the breeding patterns of various marine species, potentially leading to population declines or shifts in distribution. An influx of jellyfish, including large barrel jellyfish, could also occur. Furthermore, out-of-control algae growth, known as harmful algal blooms, can create toxic conditions, poisoning marine life and impacting fisheries. Warmer waters may also attract different fish species, such as bluefin tuna, into UK waters, altering the existing ecosystem dynamics. Dr. Jacobs emphasizes the unusual timing, stating, “The interesting thing is that this started in winter and spring, when most people assume marine heatwaves are only in summer.” Previous heatwaves have offered grim previews, leading to harmful algal blooms and mass mussel mortality in 2018, as well as increased jellyfish sightings in 2023.

For swimmers off the west coast of the UK and Ireland, the immediate effect is simply warmer temperatures. While temperatures have not yet exceeded the upper limits that marine life can tolerate, prolonged exposure to these conditions could have cascading effects throughout the food chain.

Marine heatwaves typically last about two weeks, but this one has persisted for over two months, raising serious concerns. “It is exceptional. We are about two and half months in, which is very long,” says Dr. Barthou. While a break in the warm, dry weather may lead to a temporary cooling of the ocean, longer-range forecasts suggest that the weather will warm up again, potentially prolonging the marine heatwave.

Higher sea temperatures can also influence land temperatures via sea breezes. In May 2024, a short marine heatwave contributed to above-average land temperatures, according to the Met Office, illustrating the interconnectedness of marine and terrestrial climates.

Marine heatwaves in the UK are a relatively new phenomenon, and they are expected to increase in frequency and intensity as global warming continues. A study by Dr. Jacobs identified the southern North Sea and the English Channel as UK hotspots for these events.

Globally, marine heatwaves can devastate coral reefs, fisheries, and seagrass meadows. While the UK is usually more protected due to cooler waters overall, the current event underscores the vulnerability of even temperate marine ecosystems to climate change. Oceans have absorbed 90% of the excess heat from burning fossil fuels, resulting in an overall warming of 1°C, a change that is now manifesting in events like the current UK marine heatwave.

The current surge in the UK’s sea temperatures serves as a stark reminder of the far-reaching consequences of climate change. While the immediate effects may seem subtle, the potential disruptions to marine life and coastal ecosystems are concerning. As scientists continue to study these marine heatwaves, it’s crucial to understand their long-term consequences and implement strategies to mitigate the effects of climate change on our precious marine environments. The UK’s marine environment is undeniably changing, and understanding the implications is paramount.