Record-Breaking Marine Heatwave Drives UK and Irish Coastal Temperatures 4°C Above Normal

UK and Ireland Hit by ‘Extreme’ Marine Heatwave

Waters around the UK and Ireland are experiencing an unprecedented marine heatwave, with temperatures up to 4°C (7.2°F) above average, reaching levels typical of mid-June. Experts attribute the spike to an exceptionally warm spring, calm winds, and prolonged high-pressure systems. Since March 2023, sea temperatures have been the highest recorded in 45 years, particularly off Ireland’s west coast and parts of Devon and Cornwall.

Swimmers enjoy unusually warm waters, with temperatures 4°C above seasonal norms.

Record-Breaking Heat
April and early May saw sustained warmth, pushing UK coastal temperatures 2–4.6°C above average. The North Sea near Yorkshire hit 4.6°C (8.28°F) higher than usual. While swimmers welcome the warmth, temperatures remain below summer peaks, and colder depths mean waters may still feel brisk.

A marine heatwave is declared when temperatures exceed seasonal thresholds for five consecutive days. The current event, ongoing since March, is among the longest recorded. Dr. Ségolène Berthou of the Met Office notes that calm winds and high-pressure systems created a thin surface layer of warm water, akin to "olive oil on water," accelerating heating by trapping solar radiation.

Areas in red and black indicate severe to extreme heatwave conditions as of March 19.

Ecological Impacts
While immediate mass mortality of marine life is unlikely, scientists warn of disruptions. Warmer waters may shift species distributions, boost jellyfish populations (which surged 32% after a 2023 heatwave), and affect fisheries. Prof. Tim Smyth of Plymouth Marine Laboratory reports phytoplankton patterns resembling summer, altering food chains. Octopus and tuna are appearing more frequently, impacting crab fisheries as they raid pots.

Climate Change Connection
The Met Office links the heatwave to long-term warming trends, with UK seas rising 0.3°C per decade. By 2050, summer sea temperatures could climb 2.5°C, making such events more frequent. Dr. Jonathan Tinker warns this offers a glimpse into future ocean conditions under climate change.

Weather patterns triggered February warming, a month earlier than usual.

Cooler Weather Ahead
Forecasts predict cooler temperatures and stronger winds may soon disrupt the surface warmth, easing the heatwave. However, the event underscores the growing climate threat. As emissions continue, rising sea levels and ocean heating could reshape coastlines and ecosystems permanently.

Key Fact: Marine heatwaves are projected to increase in frequency and intensity, challenging biodiversity and coastal communities worldwide.

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