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Climate change could be about to alter the biggest environment on the planet: the ocean.
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In the last seven years, more than half of all ocean surfaces have been hotter than at any time in human history, and since 2014 these heat extremes have happened on a regular basis, according to a new study.
And a second and separate research team has found that as the seas warm, so too they could literally start to run out of breath: mid-ocean depths in many places worldwide are losing oxygen at unnatural rates, precisely because of temperature rises.
Meanwhile, a third study has confirmed not only that the planet’s coral reefs—the richest ecosystems in the ocean—are endangered by global heating, but that, even if the world’s nations stick to their promise made in Paris seven years ago and contain global temperatures to a rise of no more than 1.5°C, nine-tenths of all the reefs could still suffer intolerable thermal stress.
The message is that human-induced warming driven by ever-rising greenhouse gas emissions from fossil fuel combustion is altering the biggest single living room on the planet. Blue ocean defines the colour of the planet from distant space. It is Earth’s ultimate reservoir of water. It has made all the limestone and sandstone of the continents. It is the biggest single terrestrial player in the climate machine. It is the birthplace and nursery for life on Earth.
And thanks to the actions of one species, it is changing, and not to human advantage.
The big player is heat. Oceanographers have 150 years of ocean temperatures, both directly measured, and deduced from proxy evidence. U.S. scientists report in a new journal from the Public Library of Science, PLOS Climate, that they used a century and a half of data to settle on average temperatures for the years 1870 to 1919, and then a definition of heat extremes, and then to fix a benchmark for dramatic warming. This is not a new worry, but it has become increasingly urgent.
During the first 50 years of their study, they found that ocean temperatures reached extremes as a once-in-50-years event, over 2% of the sea surfaces. Then they looked at the following century. By 2014, more than half the ocean had surpassed the level defined as extreme heat, and by 2019, that proportion has reach 57%.
In effect, the Atlantic, Pacific, and Indian oceans now have new average temperatures, and have passed a point of no return.
“Climate change is not a future event. The reality is that it’s been affecting us for a while. Our research shows that for the last seven years, more than half the ocean has experienced extreme heat,” said Kyle Van Houtan, chief scientist at the Monterey Bay Aquarium, one of the researchers.
“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. We are experiencing it now, and it is speeding up.”
The Monterey study grew from research into changes in the kelp forests off the California coast. Kelp forests and sea grass meadows, like coral reefs, are vital for the health not just of oceans, but also as nurseries for the growth of commercial fish species. These ecosystems can, and do, collapse as the thermometer rises.
Even more alarmingly, levels of dissolved gas in waters fall as the mercury goes up. And Chinese scientists warn in the journal Geophysical Research Letters that oxygen levels at mid-ocean depths are already falling at unnatural rates, and passed some kind of threshold in 2021.
The researchers worked from computer simulations of temperature changes and gas levels in the mesopelagic zones: the water layers between 200 and 1,000 metres deep, home to many of the world’s commercially-fished species. These could be the first to lose significant levels of oxygen as global temperatures rise. By 2080, deoxygenation could begin to affect all zones of the ocean. Tropical seas are naturally marked by lower oxygen levels, and these oxygen minimum zones could be spreading. More unexpectedly, oceans closer to the poles could be particularly vulnerable.
“Oxygen minimum zones actually are spreading into high latitude areas, both to the north and south,” said Yuntao Zhou, of Shanghai’s Jiao Tong University, who led the study. “That’s something we need to pay more attention to.”
The ocean ecosystems most obviously vulnerable to heat extremes are the coral reefs. Researchers have worried for decades about the tendency of coral to bleach—to interrupt a vital symbiotic relationship with tiny algae and to cease to function—as the thermometer soars. Given time, a reef can recover. More than 84% of the world’s tropical coral reefs have had time to recover from heat waves since 1980.
But time may be running out. Climate scientists report in PLOS Climate that on the basis of past data and future climate projections, if even if global heating stops at the promised average worldwide of 1.5°C above the historic normal, only 0.2% of the reefs will have time to recover between increasingly frequent bouts of extreme ocean heat. And 90.6% of them will suffer intolerable thermal stress, which is bad news for the myriad species that flourished in the coral.
“This reinforces the stark reality that there is no safe limit of global warming, and we need to act urgently to save what we can,” said Piers Forster of the University of Leeds in the UK, one of the authors.