An “unheard of” marine heatwave off the coasts of the UK and Ireland poses a serious threat to species, scientists have warned. Sea temperatures, particularly off the north-east coast of England and the west of Ireland, are several degrees above normal, smashing records for late spring and early summer. The North Sea and north Atlantic are experiencing higher temperatures, data shows. The Met Office said global sea surface temperatures in April and May reached an all-time high for those months, according to records dating to 1850, with June also on course to hit record heat levels. The US National Oceanic and Atmospheric Administration has categorised parts of the North Sea as being in a category four marine heatwave, which is considered “extreme”, with areas off the coast of England up to 5C above what is usual. The Met Office says temperatures are likely to remain high because of the emerging El Niño weather phenomenon. Daniela Schmidt, a professor of earth sciences at the University of Bristol, said: “The extreme and unprecedented temperatures show the power of the combination of human-induced warming and natural climate variability like El Niño. “While marine heatwaves are found in warmer seas like the Mediterranean, such anomalous temperatures in this part of the north Atlantic are unheard of. They have been linked to less dust from the Sahara but also the North Atlantic climate variability, which will need further understanding to unravel. “Heat, like on land, stresses marine organisms. In other parts of the world, we have seen several mass mortalities of marine plants and animals caused by ocean heatwave which have caused hundreds of millions of pounds of losses, in fisheries income, carbon storage, cultural values and habitat loss. As long as we are not dramatically cutting emissions, these heatwaves will continue to destroy our ecosystems. But as this is happening below the surface of the ocean, it will go unnoticed.” Dr Dan Smale from the Marine Biological Association has been working on marine heatwaves for more than a decade and was surprised by the temperatures. He said: “I always thought they would never be ecologically impactful in the cool waters around UK and Ireland but this is unprecedented and possibly devastating. Current temperatures are way too high but not yet lethal for majority of species, although stressful for many … If it carries on through summer we could see mass mortality of kelp, seagrass, fish and oysters.” Piers Forster, a professor of climate physics at the University of Leeds, said: “Both Met Office and NOAA analyses of sea-surface temperature show temperatures are at their highest ever level – and the average sea-surface temperature breached 21C for the first time in April. These high temperatures are mainly driven by unprecedented high rates of human-induced warming. “Cleaning up sulphur from marine shipping fuels is probably adding to the greenhouse gas driven warming. The shift towards El Niño conditions is also adding to the heat. There is also evidence that there is less Saharan dust over the ocean this year. This normally reflects heat away from the ocean. So in all, oceans are being hit by a quadruple whammy – it’s a sign of things to come.” Marine heatwaves are becoming more numerous, a 2019 study found, with the number of heatwave days having tripled in the past couple of years studied. The number of heatwave days rose by more than 50% in the 30 years to 2016, compared with 1925-54. Scientists said at the time the heat destroyed swathes of sealife “like wildfires take out huge areas of forest”. The damage caused in these hotspots is also harmful to humanity, which relies on the oceans for oxygen, food, storm protection and the removal of climate-heating carbon dioxide in the atmosphere. A report by the Met Office says warmer than usual marine temperatures are contributing to a reduction in sea ice. It found the amount of Arctic ice was well below average for the time of year but still above record low levels after near-average ice loss during May. However, Antarctic sea ice extent is exceptionally low, the lowest on record for this date by a wide margin. Dr Ed Blockley, the lead of the polar climate group at the Met Office, said: “Over recent decades we have seen a sustained loss in Arctic sea ice extent in every month of the year – especially in late summer to early autumn. Although the current Arctic sea ice extent is considerably higher than the record low for the time of year, it is still well below the long-term average. “Antarctic sea ice has been at very low levels since November 2016. This year we have seen Antarctic sea ice shrink to a record low point for the time of year, following a second successive annual record minimum sea ice extent in February.”