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Record highs in the North Sea

2023 was the warmest year in the AWI’s time series – Even the German Bight is warmer than ever before.

Researchers around the globe are sounding the alarm: ocean temperatures are the warmest ever recorded. In 2023, the North Sea also experienced dramatic record highs, as readings taken by the Alfred Wegener Institute’s Biological Institute Helgoland indicate. As data from the time series “Helgoland Reede” also reveal: it’s not the first year in which the German Bight experienced marine heatwaves. The high temperatures and extreme weather events are a product of climate change and could have substantial impacts on the ecosystem.

Last year, the oceans were warmer than at any time since the beginning of record-keeping. Our own North Sea was no exception, as experts from the Biological Institute Helgoland (BAH), part of the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI), determined. And judging by the first months of 2024, this trend shows no signs of stopping: the mean values for January, February, March and April 2024 are among the “Top 10” warmest months since 1962. March 2024, with a mean water temperature of 6.9 degrees Celsius, was even the warmest March since 1962. Temperature data from the Helgoland Reede time series shows that the mean water temperature in 2023 was nearly 11.9 degrees Celsius. “Accordingly, 2023 was a record-breaking year since the beginning of our time series in 1962,” says Dr Inga Kirstein, a researcher at the BAH. This began in January, which, at ca. 7.2 degrees, was the second-warmest ever recorded. Though it wasn’t a record-breaking day in the time series, 12 September was the day with the highest recorded water temperature in 2023; the AWI experts measured a temperature of 19.5 degrees

Grafik: Inga Kirstein
© AWI – Die Temperaturdaten der Langzeitreihe Helgoland Reede zeigen, dass die mittlere Wassertemperatur im Jahr 2023 bei knapp 11,9 Grad Celsius lag. Damit war 2023 ein Rekordjahr seit Beginn der Langzeitdatenreihe in 1962. (Graphic: Inga Kirstein)

Though the year 2023 was shaped by marine heatwaves, it wasn’t the first year in which the North Sea experienced them. In a recent study, AWI researchers analysed sea-surface temperature data from 1962 to 2018, taken from the Helgoland Reede. Marine heatwaves aren’t limited to the summer; they can also occur in other seasons, even in winter, when water temperatures are significantly above the normal values. As they data showed, the frequency of intense heatwaves has risen since the 1990s, particularly in the months March to April and July to September. In this regard, the third quarter of the year shows the highest frequency of marine heatwaves. Since 1990, AWI researchers on Helgoland and Sylt have observed new temperature patterns: there are far more warmer days in summer and far fewer extremely cold days in winter.

“For example, from 1962 to 1990 we had a total of 24 months with a mean temperature below 3 degrees Celsius; since 1990, there have only been five months. At the same time, up to 1990 there had only been eight months with mean temperatures above 17 degrees, and from 1990 to the end of 2023, there’ve been no less than 53 months.” Further, unusually high temperatures are now occurring earlier in the year. “The German Bight in particular has seen a major temperature increase since the 1990s,” says Prof Karen Wiltshire, Director of the BAH. The data also shows a connection between the monthly temperatures in the German Bight and on the German mainland: marine heatwaves occurred more frequently in late summer, during or shortly after atmospheric heatwaves, when temperatures were at their highest, which points to a coupling between ocean temperatures and atmospheric temperatures. “The North Sea warms so quickly because it’s a shelf sea surrounded by landmasses, like a giant puddle. As such, the temperature trends for the mainland are absolutely consistent with those for the water temperature.”

The experts consider climate change and the resultant global warming as a main reason for the high sea-surface temperatures and increased frequency of extreme events like marine heatwaves.

Effects on ecosystems

© AWI – Das Jahr 2023 war geprägt von marinen Hitzewellen. Es ist nicht das erste Jahr, in dem die Nordsee marine Hitzewellen erlebt. Seit 1990 beobachten die AWI-Forschenden auf Helgoland und Sylt neue Temperaturmuster. Im Sommer gibt es demnach deutlich mehr w (Graphic: Luis Giménez)

What do these findings mean for the North Sea and its ecosystem? Rising water temperatures in the sea and extreme temperature events like marine heatwaves can produce biological responses. Due to the mixing of the water column at the coast, marine heatwaves will most likely affect not just the upper water layers, but also habitats on the seafloor. In the North Sea, decadal changes have already been observed, e.g. in the occurrence of species or the composition of biotic communities. In this regard, temperature is one of the most important drivers for biodiversity and species distribution.

“Marine organisms respond to climate change in a number of different ways. We can see these changes in our own research and are currently investigating how marine heatwaves are affecting the planktonic food web, for example in the composition or frequency (abundance) of plankton communities and individual species,” says Inga Kirstein. In a mesocosmos study, the AWI researcher has already demonstrated that the combined effects of warming, acidification and changed food availability are impacting plankton dynamics, in favour of smaller plankton species. This can in turn affect food webs, since plankton is a staple for many marine organisms. Rising temperatures and the increasing frequency of marine heatwaves in the past few decades, which are connected to the fundamental changes in the German Bight, are a cause of concern for the ecology and society alike.

The ecological time series “Helgoland Reede”

Since 1962, researchers at the Biological Institute Helgoland have recorded the temperature, salinity and nutrient load in the German Bight, and assessed the plankton composition, on a nearly daily basis. Today, the Helgoland Reede is one of the most important and detailed ecological time series available. It allows researchers at the AWI and from around the world to seamlessly document the impact of climate change in the North Sea in the past 60 years, and to determine whether specific changes constitute natural, cyclical fluctuations or anthropogenic trends. The data is archived and made available for generations to come in the Alfred Wegener Institute’s “World Data Bank PANGAEA”.

  • Luis Giménez, Maarten Boersma, and Karen H. Wiltshire. “A multiple baseline approach for marine heatwaves.” Limnology and Oceanography (2024). doi: doi.org/10.1002/lno.12521
  • Felipe de Luca Lopes de Amorim, Karen Helen Wiltshire, Peter Lemke, Kristine Carstens, Silvia Peters, Johannes Rick, Luis Gimenez, Mirco Scharfe (2023). Investigation of marine temperature changes across temporal and spatial Gradients: Providing a fundament for studies on the effects of warming on marine ecosystem function and biodiversity. Progress in Oceanography 216. doi: 103080 
  • Moreno, H.D., Köring, M., Di Pane, J., Tremblay, N., Wiltshire, K.H., Boersma, M., et al. (2022). An integrated multiple driver mesocosm experiment reveals the effect of global change on planktonic food web structure. Communications Biology 5(1), 179. doi: 10.1038/s42003-022-03105-5

Alfred-Wegener-Institut, Helmholtz-Zentrum für Polar- und Meeresforschung (AWI) 2024

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