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Research that was published in this journal PLOS ClimateResearchers used historical sea surface temperature data from 1870 to 1919. This is thanks to thousands of ship’s captains and harbour masters across the globe, who made these meticulous measurements daily.
It shows the rapid warming of oceans due to what researchers believe to be climate change.
“For me, it is how fast this change is happening. Marine systems, fish and other living things do not respond so well to change that happens so abruptly,” says Dr Kisei Tanaka, a co-author in the study.
The marine biologist who works at the National Oceanic and Atmospheric Administration and is based in Hawaii, said that he saw this happen in 2014 after a heatwave in the Pacific Ocean caused warm water to disrupt fisheries and affect other ecosystems.
“It changed the timing of fish migrating from one place to another,” recalls Tanaka. Tanaka also studied white sharks and found that the heatwave had an affect on them in a separate study. They moved further up the coast of the US due to ocean warming.
“In central California, juvenile white sharks were an extremely rare sighting; suddenly they started to show up more frequently, the warming oceans pushed their thermal habitat toward the North.”
California is not the only state that has seen unusual white shark movements.
White sharks have been disappearing in South Africa over the past two years.
It is not clear what caused these apex predators leave their seal feeding grounds. One theory suggests that they may have moved further north along the Eastern Cape coastline.
Tanaka and Dr Kyle Van Houtan were the chief scientists at the New Monterey Bay Aquarium in America. They used 150 years’ worth of sea surface temperature data to create an average that allowed them establish a historical benchmark to measure extremes in marine heat.
They then looked at the frequency with which this benchmark was broken. In 2014, the first year when more than half of the oceans were subject to heat extremes, was it.
It was a trend that was going to continue. In 2019, that number had risen to 57%, the last year the study was done.
Comparatively, the benchmark figure indicates that only 2% experienced extremely high temperatures at the end 19th century.
“These dramatic changes we’ve recorded in the ocean are yet another piece of evidence that should be a wake-up call to act on climate change,” says Van Houtan.
Researchers discovered that the tropics are the ecosystems most at risk from rising sea temperature.
“These ecosystems are already closer to their upper thermal tolerances which means that any push of it getting warmer puts them at risk,” Tanaka explains, adding that coral reefs are the most threatened.
Researchers warn that the collapse of marine ecosystems could have a devastating knock-on effect for humans.
“Altering ecosystem structure and function threatens their capacity to provide life-sustaining services to human communities like supporting healthy and sustainable fisheries, buffering low-lying coastal regions from extreme weather events, and serving as a carbon sink to store the excess carbon put in the atmosphere from human-generated greenhouse emissions,” Van Houtan says. DM/OBP
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