- It is incredibly worrying that the warming is going so fast and that there are such extreme heat waves in the oceans. There have been strong climate fluctuations in the past in the history of the earth, but then it took place over thousands of years. Now it goes on for a few decades. Many animals and plants have no chance to adapt so quickly, says Fredrik Jutfelt, professor of zoophysiology at the University of Gothenburg.
The world's oceans have so far absorbed around 90 percent of the global warming caused by human emissions of greenhouse gases. Since pre-industrial times, ocean temperatures have increased by an average of 0.9 degrees Celsius. However, the biggest change has occurred since the 1980s, and now the temperature in the water is rising by about 0.3 degrees per decade.
But the warming is unevenly distributed and is significantly stronger in Europe's sea areas.
This makes extreme periods even more extreme. This summer, large parts of Europe have been hit by stronger marine heat waves than previously measured. These are periods when the temperature for at least five consecutive days is abnormally high for that time of year compared to 90 percent of previous measurements (today the reference period is 1981–2011).
The strongest deviations were noted in the Mediterranean Sea, where the water temperature off Mallorca for a period amounted to 33 degrees Celsius, which is 6 degrees above normal.
The international research network World Weather Attribution (WWA) shows in a new report that this year's marine heat waves in Europe have a clear connection to climate change. The researchers have made calculations for four sea areas. In three of them – the Bay of Biscay and the eastern and western Mediterranean – the probability of a heat wave in a pre-industrial climate would have been largely non-existent under the same meteorological conditions. In the fourth area investigated, the Celtic Sea area in the Atlantic off Ireland, calculations show that climate change has made this year's heat wave 30 times more likely.
The researchers also show that the spread of heat waves would have been significantly less in a pre-industrial climate. For example, only 9 percent of the Eastern Mediterranean and 40 percent of the Bay of Biscay would have been affected by heat waves in a world without human influence. But during the summer, the heat waves actually covered 70 and 90 percent of the respective areas.
- These are not isolated warm events, but a pattern that lasted for large parts of the summer. And we see that marine heat waves are becoming more common, says Friederike Otto, a professor at Imperial College London in the UK and one of the researchers behind the WWA report.
It is not only marine life that is threatened when the oceans warm up. Marine heat waves charge the atmosphere with more moisture and energy, which increases the risk of extreme weather along the coasts in the form of storms and heavy downpours, she explains.
- Our research clearly shows that it is climate change that is driving this development. Every extra ton of carbon dioxide that we emit pushes Europe's coastal waters and ecosystems further and further into unknown territory, says another of the researchers behind the report, Professor Thomas Frölicher at the University of Bern in Switzerland.
