Extreme temperatures are the new normal in the oceans | Science


The map shows the grids that suffer the most thermal extremes, in green, and those that suffer the least, in blue.
The map shows the grids that suffer the most thermal extremes, in green, and those that suffer the least, in blue.Van Houtan et al./PLoS Climate

In the 120 months of the decade from 2010 to 2019, the waters that bathe the coasts of the Maldives (Indian Ocean) had extreme temperatures in 111 of them. A century ago, that only happened a few days a year. The phenomenon is not limited to this area of ​​the planet, the warming of seas and oceans is as global as that which occurs on land. Analysis of temperatures going back 150 years shows that what was once an extreme thermal event is now normal. The impact on marine fauna and flora is already being felt, with species migrating ever further north or to the bottom of the sea.

The first time temperature extremes became the norm anywhere in the sea was in 1998, in the South Atlantic. This point of no return is defined by the authors of the study as the year in which more than half of a given area reaches and remains at the maximum average temperature of the first 50 years (from 1870 to 1919). The Indian Ocean reached this threshold in 2007. It was in 2014 that half of the sea surface suffered from these heat waves. Now, this research published in the specialized scientific journal PLoS Climate maintains that in 2019 57% of the marine surface had and maintained these extremes.

The work used the historical records of temperatures of all the seas of the planet since 1870. With these data, obtained from institutions such as the British Meteorological Agency, the authors elaborated an index of average monthly extreme temperatures until 1919. Then they compared the values ​​of this index of the past with those observed in the most recent decades. They have verified that what is produced today in almost two thirds of the planet’s marine surface, previously only happened in 2% of salt water.

“Climate change is not something of the future. The reality is that it has been affecting us for a long time”

Kyle Van Houtan, President of the Loggerhead Center for Marine Life, United States

Kyle Van Houtan is the president of the Loggerhead Center for Marine Life (USA) and conducted the study while he was chief scientist at the Monterey Bay Aquarium. For him, “climate change is not something of the future; the reality is that it has been affecting us for a long time”. The temporal and spatial increase in maximum temperatures in some areas, such as the territorial waters of Indian Ocean countries such as the Maldives or Tanzania, means that “in these areas, extreme heat has increased by up to 47 times,” says Van Houtan.

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The large marine basins where temperature extremes have increased the most are, in addition to the Indian Ocean, the central region of the Atlantic, the portion of the Pacific closest to Asia and the polar oceans. The areas where the sea surface has best maintained its past averages are the American Pacific, especially in the south, and the North Atlantic, but it seems that there are no marine regions where the thermometer has dropped. “We have not analyzed the occurrence of extreme cold, but it would be easy to do so,” Van Houtan says in an email. “Based on the results of this work, where we show that extreme heat has increased and become common, we believe that extreme cold ocean temperatures have decreased significantly.”

Although it is not the object of this investigation, the authors of the study mention the possible causes, already highlighted by the latest reports of the Intergovernmental Panel on Climate Change, IPCC. One is the flow of warm air from an atmosphere increasingly warmed by the concentration of greenhouse gases. Another is the increase in the frequency, duration and intensity of heat waves. In a study published in 2018, its authors estimated that the frequency of these episodes of several days of extreme heat had increased by 34% in the last century, their duration by 17% and, in total, the number of days with temperatures beyond the extreme average they have grown by 54% since the beginning of the last century.

“The warmer areas are emptying of species”

José Luis Sánchez, from the Department of Marine Sciences of the University of Alicante

This new normality is causing a series of changes in the marine environment that science has been highlighting for a few years. Just as, on land, global warming is driving many species to migrate to colder latitudes, in the sea, an increasing number of fish are heading north. José Luis Sánchez is a professor at the Department of Marine Sciences and Applied Biology at the University of Alicante. In 2017 he published a study on the main tuna species of the Atlantic, the Indian and the Pacific. Since 1965, catches of subtropical species have multiplied further north, observing a relationship between increased sea surface temperature and migration.

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“They are expanding to higher latitudes and it is happening in all the oceans,” says Sánchez. And the changes, he adds, “are taking place at all levels, not only with tuna or other fish, but also with phytoplankton and zooplankton.” The other big direction that many marine species are taking is going lower.

But these displacements have their limits. For example, species that depend on photosynthesis cannot descend in the water column. In addition, those of colder waters cannot go beyond the Arctic and Antarctic oceans. But the worst is for tropical waters. As Sánchez says, “the warmest areas are emptying of species.”

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George Holan

George Holan is chief editor at Plainsmen Post and has articles published in many notable publications in the last decade.

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