A marine heatwave off the UK poses a serious threat to ecosystems and the coastal communities that rely on them and raises questions about longer term climate impacts.
Sea temperatures, particularly off the north-east coast of England and the west of Ireland, are up to five degrees above normal, noted Dr Jules Kajtar of the National Oceanography Centre, Southampton.
The US National Oceanic and Atmospheric Administration has categorised parts of the North Sea as being in a category four marine heatwave, the most extreme. ‘This is the highest category we ever see,’ he said.
Though the sea temperatures will be far from tropical, ‘the reason we are worried is the ecosystem has not experienced these temperatures at this time of year before.’ Warming oceans can make waters more acidic and drive a decrease in oxygen levels in the water.
Moreover, Dr Kajtar said global sea surface temperatures in April and May reached an all-time high for those months, according to records dating back to 1850. Variations of around a tenth of a degree are common but he said it was double that in this case. ‘Temperatures are 0.2 degrees warmer than it has ever been at this time of year, which is a record. This is highly unusual.’
The reason for the marine heatwave is down to a combination of factors: human-induced climate heating, as seen in the warmest start to June in decades. One of Dr Kajtar’s colleagues at the National Oceanography Centre, Dr Marilena Oltmanns, has found that recent atmospheric warmth is one factor in driving this particular marine heatwave.
Another factor is natural climate variability like El Niño, a climate pattern currently developing in the Pacific Ocean, where waters are warming, that can affect weather worldwide.
Others have cited a lack of dust from the Sahara, which is known to have a cooling effect when blown over the ocean, as another factor.
Because the heat capacity of water far exceeds that of the atmosphere, marine heatwaves last longer than atmospheric heatwaves and can also impact local climate: though too early to say what these impacts might be, one concern is that they might lead to a permanent change in weather patterns if systems of currents pass a tipping point.
The excess temperatures will also stress marine organisms such as kelp, seagrass, fish and oysters and the impact depend on the temperature excess and how long it persists. In worst case scenarios, the seas can become less economically productive, also harming tourism. ‘The impacts are not always clear or direct,’ he said.
In western Australia, for example, he studied a marine heatwave in 2011 that adversely affected kelp forests, seagrass meadows and coral reefs, so called ‘benthic species’ that cannot move, unlike fish. Turf seaweed grew back but the marine ecosystem shift remained after water temperatures returned to normal, with fewer blue crabs, scallops, and lobster. ‘Some parts of the marine ecosystem have never recovered.’
Overall, marine heatwaves have increased in frequency by 50% in the past 10 years and are becoming more severe, as a result of global heating, he said. ‘Marine heatwaves are becoming increasingly worrying and increasingly impactful.’