How a single 2003 heat wave triggered lasting upheaval in the North Atlantic

By Nadine Kraft, Phys.Org

Major currents in the North Atlantic. Case study areas highlighted in green. Credit: Science Advances (2026). DOI: 10.1126/sciadv.adt7125

The ecology of the North Atlantic is constantly changing. Sometimes it changes abruptly. Extreme events are one driver of such sudden changes. A team of researchers has discovered that a single, large-scale heat wave has affected ecosystems and trophic interactions in the subpolar North Atlantic, and the effects are still felt today. The study has now been published in Science Advances.

“Such an event even impacts us humans because it changes the distribution of fish species we are adapted to fishing and eating for decades,” explains Dr. Karl-Michael Werner from the Thünen Institute of Sea Fisheries and first author of the study.

Marine heat waves can have sudden and unpredictable effects on life in the ocean. One of the questions arising from the study is under which conditions such heat waves can develop. “We found that more marine heat waves occurred after 2003, but they did not have a similar effect compared to the one in 2003,” says Werner.

A ‘perfect storm’

The researchers investigated how a large-scale physical extreme event affected the distribution of organism and species communities in the ocean. They examined about 100 biological time series for this purpose. Important data came, for example, from the LTER (Long-Term Ecological Research) observatory HAUSGARTEN of the Alfred-Wegener-Institute Helmholtz Center for Polar and Marine Research (AWI) in Bremerhaven. Here, in the Fram Strait between Svalbard and Greenland, long-term ecological data have been collected for 25 years.

A majority of the examined parameters showed abrupt changes in 2003 or shortly after. In 2003, a so-called perfect storm happened in and over the subpolar North Atlantic. A perfect storm describes a combination of events with dramatic or highly surprising implications. Unusually low amounts of arctic water flew southwards off the East coast of Greenland while unusually high quantities of warm, subtropical waters entered the North Atlantic between Iceland and Scotland. Atmospheric temperatures above the North Atlantic also reached record high values in 2003.

The effects grasped Europe with thousands dying from atmospheric heat waves. The combination of atmospheric and hydrographic events led to record high water temperatures between the waters of the coast off western Greenland and the Norwegian coast, thereby spanning the entire subpolar North Atlantic.

From unicellular algae to whales—all organisms were affected by the unexpected heat. Capelin, a cold-water fish species, started to suffer, while warmer-water-loving species, such as cod and haddock expanded their distribution northwards. Capelin is the most important subpolar forage fish species in the North Atlantic, and the fish moved their spawning areas northwards from south-western Iceland.

This had far-reaching consequences: Eggs and larvae started drifting to unfavorable habitats close to the coast off East Greenland, where they have a lower probability of survival because they are not adapted to the local conditions. Species, such as humpback whales, benefit from these changes: the whales follow capelin and are much more regularly seen in South-East Greenland than they historically used to be.

Although the Fram Strait, the passage between Svalbard and Greenland, is thousands of kilometers away from the starting point of the marine heat wave, the authors were able to link a warm-water anomaly starting in the Fram Strait approximately two years after the origin of the heat wave. The warm waters moved northwards, transported new organisms to the Fram Strait and supported local boreal species, where they abruptly changed the entire ecosystem from the sea surface to the deep ocean floor.

Warmer waters led to higher biomass in surface waters, which ultimately sinks to the seafloor, providing food for organisms living on and in the sediments, such as brittle stars and nematodes. However, more food does not necessarily mean better food. There are signs that the nutritional quality of the sinking organic matter declined at the same time.

“Our results show that unexpected extreme events can lead to unpredictable ecological cascades,” says Werner. How rising temperatures will affect marine biodiversity is unknown at present. “One can predict how rising temperatures affect organisms’ metabolism. But a species won’t benefit from such changes if it is eaten by predators after moving northwards or does not find suitable spawning grounds in the new environment.”

The only species most likely benefitting from such changes in the subpolar North Atlantic is Atlantic cod, a typical opportunistic predator. “It spreads and eats everything it can find, if the conditions allow,” says the researcher.