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Marine biodiversity is under threat from record high global sea surface temperatures and more frequent marine heatwaves in unexpected zones, the latest European earth observation agency oceans report has found.
Sea surface average temperatures reached 21.1C in April and large areas experienced “intense heatwaves” from June to August. The previous record temperature of 21C was in 2016, the report by Copernicus said.
Marine heatwaves had become more frequent everywhere except the Southern Ocean, with their occurrence increasing by one extra event every five to 10 years, the research showed. Conversely, marine cold spells were reduced by one event every five years.
“It’s not just the frequency,” said the University of Bristol’s Daniela Schmidt. “Marine heatwaves have happened before but they would have been isolated at the west coast of Australia, Alaska or the Mediterranean. It has now happened in areas we would not have expected, like around the British Isles.”
Marine heatwaves are defined as temporary events in specific locations where the water is unusually warm for a minimum of five consecutive days.
“What is clear is that every increase in global warming will affect climate extremes, including in the ocean,” said Karina Von Schuckmann, chair of the Copernicus ocean state report. “What is needed is mitigation by reducing emissions to bring Earth back to its energy balance.”
Marine heatwaves pose a threat to a large range of species including corals, seagrasses, fish and birds. They can also cause coral bleaching, changes in fish communities and deterioration of physical reef structures. Effects on marine natural habitats and food chains can have further consequences for the entire food chain.
In the north-east Atlantic, where Copernicus detected moderate to strong and even extreme heatwaves, there was a 50 to 60 per cent decrease in phytoplankton concentrations compared with 1998-2018.
“The impacts we’re seeing are massive, and they ricochet out across the food chain,” said the Marine Biological Association’s Katie Smith. Decreases in plankton, for example, could mean that ground fish and, in turn, the larger fish such as cod which feed on them, would not find enough to eat, endangering food supplies and fishing industries.
In the tropical north Atlantic, a moderate to severe heatwave covered the entire basin from July onwards, while there were moderate to extreme heatwaves across the Mediterranean in June, July and August.
Moderate to strong heatwaves were observed in the north-east Pacific during July, August and September, and sea surface temperatures increased over the same period in the tropical Pacific due to the El Niño conditions.
Sea ice is also rapidly shrinking. Antarctica sea ice reached its lowest levels in May and June since records began, losing an area seven times the size of Poland. Arctic sea ice has lost 3.5mn square kilometres since 1979, an area seven times the size of Spain.
Sea ice helps to cool the Earth, and provides habitat for krill and microorganisms, as well as places for larger animals such as whales to find food.
“The Earth’s albedo, its ability to reflect sunlight, is heavily influenced by the extent of ice cover on the planet,” said Clément Bricaud, oceanographer at Mercator Ocean International. “This dramatic reduction in polar sea ice in the Arctic and Antarctic exacerbates global warming and threatens to disrupt fragile ecosystems and impact weather patterns worldwide.”
New research published this week in Nature suggested that humanity had crossed a tipping point to avoid instability in the West Antarctic Ice Sheet, and that it would continue to melt in the coming decades and centuries under every scenario, speeding up sea level rise and disrupting ocean currents that regulate global climate.
Alberto Naveira Garabato, professor in physical oceanography at the University of Southampton, said it was “a sobering piece of research”.
“It illustrates how our past choices have likely committed us to substantial melting of the West Antarctic Ice Sheet and its consequent sea level rise — to which we will inevitably have to adapt as a society over coming decades and centuries.”
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