Fish Friday: Marine heatwave killing critters as scientists record underwater temperatures

It’s been a record-breaking summer for many parts of the world and the oceans are also feeling the heat.

Off the coast of San Diego, scientist recorded the highest seawater temperatures since daily measuares began 102 years ago in 1916.

Researchers at Scripps Institution of Oceanography at UC San Diego have predicted faster rates of warming than previously predicted for the North Atlantic Ocean in a recent paper published in the Journal of Climate.

This warming could disrupt major oceanic cycles and have worldwide impacts on climate systems.

The researchers modeled scenarios based on possible future greenhouse gas and aerosol emission rates. One likely scenario focuses on future decline in aerosols and continued increase of greenhouse gases in the atmosphere. Aerosols are minute particles suspended in the atmosphere. Some scatter sunlight, thereby actually acting as cooling agents.

The aerosol cooling effect is about 50 percent of the warming effect of anthropogenic carbon dioxide at present. Aerosols released from human activities are pollutants, however, and their health concerns have triggered worldwide efforts to curb emissions. An aerosol decline could spark an interesting catch-22: Because of their cooling effect, this decline would accelerate ocean warming that is already being caused by increasing carbon dioxide emissions–most notably initiating major warming in the North Atlantic.

Historically, the Southern Ocean has been the predominant heat absorber, accounting for roughly 72 percent of uptake of anthropogenic greenhouse heat in the oceans, due in part to the area’s low levels of cooling aerosols. The opposite is true of the North Atlantic: under strong aerosol cooling, the North Atlantic has not taken up much heat, meaning that most of the warming in the Northern Hemisphere is happening in the atmosphere and not in the ocean.

“The ocean heat uptake moderates atmospheric warming by storing much of the greenhouse heat below the surface,” said Shang-Ping Xie, a climate researcher at Scripps and co-author of the study. “We now show that the ocean uptake is not only uneven, but its distribution also evolves with time.”

The model presented by the researchers predicts a massive switch. With a decrease in cooling aerosols, which are concentrated in the Northern Hemisphere, over time the ocean would need to absorb more heat. The researchers predict that the North Atlantic’s share of the uptake could increase from 6 percent to about 27 percent.

“It’s important to note that the Southern Ocean has been heating over the past few decades, and this will continue,” said Lynne Talley, a professor at Scripps and co-author of the study. “What’s changing is that the North Atlantic is starting to join in as a second major heat sink, which will affect circulation and climate around the North Atlantic.”

Increased rates of warming in the North Atlantic could impact a key oceanic cycle, the Atlantic Meridional Overturning Circulation, or AMOC. An important part of the Earth’s climate system, the AMOC is a large system of ocean currents – such as the Gulf Stream – that pulls warmer water from the tropics toward the North Atlantic where it cools, sinks into the deep ocean, and flows southward again. Eventually the water is pulled back to the surface, where it warms and completes the circulation. Because it distributes heat and energy, the AMOC plays a major role in sustaining worldwide climate patterns, and is especially important in regulating climate in northern Europe and coastal North America.

A warmer North Atlantic could significantly weaken the AMOC. The system is driven by changes in salinity and temperature; as the warm water moves north, it evaporates to become saltier and gets cooler in the northern latitudes, making it denser so it sinks. Changes in water temperature in the North Atlantic could throw a wrench into this whole process. In fact, recent studies have shown that the AMOC may have started weakening over the past decade. Greater weakening could have widespread impacts on climate and weather patterns. While all of the exact consequences are still unknown, for instance, warmer water on the surface could fuel stronger hurricanes.

“The changing pattern of ocean heat uptake will cause the atmospheric circulation to reorganize, bringing changes in rainfall and storms,” Xie said.

This study was funded by the National Science Foundation.

-Chase Martin

“Just like we have heat waves on land, we also have heat waves in the ocean,” said Art Miller of the Scripps Institution of Oceanography.

Between 1982 and 2016, the number of “marine heat waves” roughly doubled, and likely will become more common and intense as the planet warms, a study released Wednesday found.

Prolonged periods of extreme heat in the oceans can damage kelp forests and coral reefs, and harm fish and other marine life.

Some free-swimming sea animals like bat rays or lobsters may shift their routines. But stationary organisms like coral reefs and kelp forests “are in real peril,” said Michael Burrows, an ecologist at the Scottish Marine Institute, who was not part of the research.

In 2016 and 2017, persistent high ocean temperatures off eastern Australia killed off as much as half of the shallow water corals of the Great Barrier Reef — with significant consequences for other creatures dependent upon the reef.

“One in every four fish in the ocean lives in or around coral reefs,” said Ove Hoegh-Guldberg, a marine biologist at the University of Queensland. “So much of the ocean’s biodiversity depends upon a fairly small amount of the ocean floor.”

The latest study in Nature relied on satellite data and other records of sea-surface temperatures including from ships and buoys.

Marine biologists nicknamed a patch of persistent high temperatures in the Pacific Ocean between 2013 and 2016 “the Blob.” During that period, decreased phytoplankton production led to a cascading lack of food for many species, causing thousands of California sea lion pups to starve, said Miller, who had no role in the Nature study.

“We’ve repeatedly set new heat records. It’s not surprising, but it is shocking,” he said.


h/t: AP, Scripps

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