Marlowe HOOD, AFP Yahoo News 5 Mar 19;
Paris (AFP) - Invisible to people but deadly to marine life, ocean heatwaves have damaged ecosystems across the globe and are poised to become even more destructive, according to the first study to measure worldwide impacts with a single yardstick.
The number of marine heatwave days has increased by more than 50 percent since the mid-20th century, researchers reported in the journal Nature Climate Change.
"Globally, marine heatwaves are becoming more frequent and prolonged, and record-breaking events have been observed in most ocean basins in the past decade," said lead author Dan Smale, a researcher at the Marine Biological Association in Plymouth, Britain.
Above the ocean watermark, on Earth's surface, 18 of the last 19 years have been the warmest on record, leading to more severe storms, droughts, heatwaves and flooding.
"Just as atmospheric heatwaves can destroy crops, forests and animal populations, marine heatwaves can devastate ocean ecosystems," Smale told AFP.
Compared to hot spells over land, which have claimed tens of thousands of lives since the start of the century, ocean heatwaves have received scant scientific attention.
But sustained spikes in sea-surface temperatures can also have devastating consequences.
A 10-week marine heatwave near western Australia in 2011, for example, shattered an entire ecosystem and permanently pushed commercial fish species into colder waters.
Corals have been the marquee victims of shallow-water heatwaves, and face a bleak future. Even if humanity manages to cap global warming at 1.5 degrees Celsius -- mission impossible, according to some scientists -- up to 90 percent of corals are likely to die, the UN's top climate science body said in October.
But other bedrock species have suffered too: the 2011 surge of heat killed off large swathes of seagrass meadows and kelp forests, along with the finfish and abalone that depend on them.
- Heat sponge -
Another ocean hot spell off the coast of California warmed waters by 6 C (10.8 F) and lasted for more than a year.
Known at "The Blob", it generated toxic algae blooms, caused the closure of crab fisheries, and led to the death of sea lions, whales and sea birds.
More frequent and intense ocean heatwaves also have a direct impact on people by reducing fisheries harvests and adding to global warming, the researchers noted.
"Species of fish and crustaceans targeted for human consumption may be locally wiped out," Smale said.
"And carbon stored by sea grasses and mangroves may be released if they are hit by extreme temperatures."
To determine the full extent of marine heatwave impacts across different oceans, Smale and an international team from 19 research centres crunched data from more than 1,000 field studies that reported on how organisms and ecosystem responded.
By definition, marine heatwaves last at least five days. Sea water temperatures for a given location are "extremely high" -- the top 5-to-10 percent on record for that time and place.
"Marine heatwaves can penetrate to hundreds of metres, though for our analysis we used data which only captures warming at the surface," Smale said.
As manmade global warming heats the planet, oceans have absorbed some 90 percent of the extra heat generated.
Without that heat sponge, air temperatures would be intolerably higher.
Even if humanity does manage to cap global warming at "well below" 2C (3.6 F), as called for in the Paris climate treaty, marine heatwaves will sharply increase in frequency, intensity and duration, earlier research has shown.
Heatwaves sweeping oceans ‘like wildfires’, scientists reveal
Extreme temperatures destroy kelp, seagrass and corals – with alarming impacts for humanity
Damian Carrington The Guardian 4 Mar 19;
The number of heatwaves affecting the planet’s oceans has increased sharply, scientists have revealed, killing swathes of sea-life like “wildfires that take out huge areas of forest”.
The damage caused in these hotspots is also harmful for humanity, which relies on the oceans for oxygen, food, storm protection and the removal of climate-warming carbon dioxide the atmosphere, they say.
Global warming is gradually increasing the average temperature of the oceans, but the new research is the first systematic global analysis of ocean heatwaves, when temperatures reach extremes for five days or more.
The research found heatwaves are becoming more frequent, prolonged and severe, with the number of heatwave days tripling in the last couple of years studied. In the longer term, the number of heatwave days jumped by more than 50% in the 30 years to 2016, compared with the period of 1925 to 1954.
As heatwaves have increased, kelp forests, seagrass meadows and coral reefs have been lost. These foundation species are critical to life in the ocean. They provide shelter and food to many others, but have been hit on coasts from California to Australia to Spain.
“You have heatwave-induced wildfires that take out huge areas of forest, but this is happening underwater as well,” said Dan Smale at the Marine Biological Association in Plymouth, UK, who led the research published in Nature Climate Change. “You see the kelp and seagrasses dying in front of you. Within weeks or months they are just gone, along hundreds of kilometres of coastline.”
As well as quantifying the increase in heatwaves, the team analysed 116 research papers on eight well-studied marine heatwaves, such as the record-breaking “Ningaloo Niño” that hit Australia in 2011 and the hot “blob” that persisted in the north-east Pacific from 2013 to 2016. “They have adverse impacts on a wide range of organisms, from plankton to invertebrates, to fish, mammals and seabirds,” Smale said.
The scientists compared the areas where heatwaves have increased most with those areas harbouring rich biodiversity or species already near their temperature limit and those where additional stresses, such as pollution or overfishing, already occur. This revealed hotspots of harm from the north-east Atlantic to the Caribbean to the western Pacific. “A lot of ocean systems are being battered by multiple stresses,” Smale said.
The natural ocean cycle of El Niño is a key factor in pushing up temperatures in some parts of the ocean and the effect of global warming on the phenomenon remains uncertain, but the gradual overall heating of the oceans means heatwaves are worse when they strike.
“The starting temperature is much higher, so the absolute temperatures [in a heatwave] are that much higher and more stressful,” said Smale. Some marine wildlife is mobile and could in theory swim to cooler waters, but ocean heatwaves often strike large areas more rapidly than fish move, he said.
The researchers said ocean heatwaves can have “major socioeconomic and political ramifications”, such as in the north-west Atlantic in 2012, when lobster stocks were dramatically affected, creating tensions across the US-Canada border.
“This [research] makes clear that heatwaves are hitting the ocean all over the world … The ocean, in effect, is spiking a fever,” said Prof Malin Pinsky, at Rutgers University, US, and not part of the team. “These events are likely to become more extreme and more common in the future unless we can reduce greenhouse gas emissions.”
Dr Éva Plagányi at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Australia also likened ocean heatwaves to wildfires. “Frequent big hits can have long-lasting effects,” she said. “This study shows that record-breaking events are becoming the new normal.”
The damage global warming is causing to the oceans has also been shown in a series of other scientific papers published in the last week. Ocean warming has cut sustainable fish catches by 15% to 35% in five regions, including the North Sea and the East China Sea, and 4% globally, according to work published by Pinsky and colleagues.
“We were stunned to find that fisheries around the world have already responded to ocean warming,” he said. Another study showed that achieving the 2C climate change target set out in the Paris agreement would protect almost 10m tonnes of fish catches each year, worth tens of billions of dollars.
Separate work by Plagányi’s team showed that climate change will reverse the recovery of whales in the Southern Ocean by damaging the krill on which they feed. “Models predict concerning declines, and even local extinctions by 2100, for Pacific populations of blue and fin whales, and Atlantic and Indian Ocean fin and humpback whales,” they said.
“In the space of one week, scientific publications have underscored that unless we take evasive action, our future oceans will have fewer fish, fewer whales and frequent dramatic shifts in ecological structure will occur, with concerning implications for humans who depend on the ocean,” said Plagányi.
