Extreme heat is the new normal for the world’s oceans, study finds | science and technology

In the 120 months that made up the decade from 2010 to 2019, the waters off the coast of the Maldives in the Indian Ocean recorded extreme temperatures in 111 of them. A century ago, this only happened a few days a year. The phenomenon is not limited to this area of ​​the planet; the warming of the seas and oceans is as global a problem as it is on dry land. Analysis of temperatures from 150 years ago to today shows that what was once a rare extreme thermal event is now the norm. The impact on plant and animal life in the oceans is already being felt, with more and more species migrating further north, or further into the colder depths.

The first time that thermal extremes became normal in a specific part of the seas was in 1998, in the South Atlantic. This point of no return was defined by the study authors as the year in which more than half of a given area reaches and maintains the average maximum temperature recorded during the first 50 years of the study (from 1870 to 1919). The Indian Ocean reached this threshold in 2007. In 2014, half of the entire marine surface of the planet suffered from these heat waves. Now the survey, titled The recent normalization of historical marine heat extremes and published in a specialized scientific journal Climate PLOS states that in 2019, 57% of Earth’s ocean surface reached and sustained what researchers describe as extreme marine heat events.

The study covers historical temperature records of every sea and ocean on the planet from 1870. Using this data, obtained from institutions such as the British Meteorological Office, the researchers compiled an index of temperatures average monthly extremes since 1919. Then they compared the data. of this index with those observed over the past few decades. What they discovered is that today more than two-thirds of the Earth’s marine surface is affected by extreme temperatures, compared to only 2% historically.

Kyle Van Houtan, director of the Loggerhead Marinelife Center in Florida, co-authored the study when he was chief scientist at the Monterey Bay Aquarium. According to him, “Climate change is not something that will happen in the future; the reality is that it has been affecting us for some time now. The temporal and spatial increase in maximum temperatures in some regions, such as the territorial waters of Indian Ocean countries such as the Maldives and Tanzania, means that “in these regions, extreme temperatures have increased up to 47 times” , explains Van Houtan.

The large maritime basins where the increase in extreme temperatures has been most pronounced are, as well as the Indian Ocean; the central Atlantic region, the part of the Pacific closest to Asia and the polar oceans, while the areas where the sea surface has been most successful in maintaining its historical averages are the American Pacific, in particular the southern part, and the North Atlantic. However, there appear to be no ocean areas where temperatures have dropped. “We haven’t analyzed the occurrence of extreme cold, but it would be quite easy to do so,” Van Houtan says via email. “Based on the results of this study, where we showed that extreme heat is increasing and has become commonplace, we believe that extremely cold temperatures in the oceans have decreased significantly.”

Climate change is not something that will happen in the future; the reality is that it has been affecting us for quite some time now

Kyle Van Houtan, director of the Loggerhead Marinelife Center in Florida

Although this is not the primary objective of the study, its authors also discuss the possible causes of ocean warming, which have been highlighted by recent reports by the Intergovernmental Panel on Climate Change in the United Nations. One of these causes is the flow of warm air from the Earth’s atmosphere, which is getting warmer and warmer due to the concentration of greenhouse gases. Another is an increase in the frequency, duration and intensity of heat waves. In a 2018 book, the authors estimated that the frequency of these multi-day periods of extreme heat had increased by 34% over the past century, their duration by 17% and, in total, the number of days when temperatures exceeded the extreme average. has increased by 54% since the beginning of the 20th century.

This “new normal” is creating a series of changes in the marine environment that the scientific world has been highlighting for several years. In the same way that global warming is causing many terrestrial species to migrate to colder latitudes, in the oceans, more fish are doing the same and heading further north. José Luis Sánchez, professor of marine sciences and applied biology at the University of Alicante, published in 2017 a study on the main tuna species in the Atlantic, Indian and Pacific oceans. Since 1965, the catches of subtropical species have been increasing more and more towards the north, showing a link between the increase in sea surface temperature and migration.

“They are spreading to higher latitudes and this is happening in all oceans,” says Sánchez, adding that the changes “are happening at all levels, not only with tuna and other fish species, but also with phytoplankton and zooplankton”. The other great migration is vertical, with many species seeking cooler temperatures in the depths of the ocean.

But these migrations have their limits. For example, species that depend on photosynthesis cannot descend into the water column. Moreover, cold-water species cannot extend far beyond the Arctic and Antarctic oceans. But the problem is more pronounced in tropical waters. As Sánchez notes: “The hottest areas are depleting of species.”

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