Oceans

Sunlight may have helped remove as much as 17 percent of the oil slicking the surface of the Gulf of Mexico following the 2010 Deepwater Horizon spill. That means that sunlight plays a bigger role in cleaning up such spills than previously thought, researchers suggest February 16 in Science Advances.

When sunlight shines on spilled oil in the sea, it can kick off a chain of chemical reactions, transforming the oil into new compounds (SN: 6/12/18). Some of these reactions can increase how easily the oil dissolves in water, called photodissolution. But there has been little data on how much of the oil becomes water-soluble.

To assess this, environmental chemists Danielle Haas Freeman and Collin Ward, both of Woods Hole Oceanographic Institution in Massachusetts, placed samples of the Macondo oil from the Deepwater Horizon spill on glass disks and irradiated them with light using LEDs that emit wavelengths found in sunlight. The duo then chemically analyzed the irradiated oil to see how much was transformed into dissolved organic carbon.

The most important factors in photodissolution, the researchers found, were the thickness of the slick and the wavelengths of light. Longer wavelengths (toward the red end of the spectrum) dissolved less oil, possibly because they are more easily scattered by water, than shorter wavelengths. How long the oil was exposed to light was not as important.

Though the team didn’t specifically test for seasonal or latitude differences, computer simulations based on the lab data suggested that those factors, as well as the oil’s chemical makeup, also matter.

The researchers estimate irradiation helped dissolve from 3 to 17 percent of surface oil from the Deepwater Horizon spill, comparable to processes such as evaporation and stranding on coastlines. What impact the sunlight-produced compounds might have on marine ecosystems, however, isn’t yet known.  More

Yesterday’s scorching ocean extremes are today’s new normal. A new analysis of surface ocean temperatures over the past 150 years reveals that in 2019, 57 percent of the ocean’s surface experienced temperatures rarely seen a century ago, researchers report February 1 in PLOS Climate.

To provide context for the frequency and duration of modern extreme heat events, marine ecologists Kisei Tanaka, now at the National Oceanographic and Atmospheric Administration in Honolulu, and Kyle Van Houtan, now at the Loggerhead Marinelife Center in Juno Beach, Fla., analyzed monthly sea-surface temperatures from 1870 through 2019, mapping where and when extreme heat events occurred decade to decade.

Looking at monthly extremes rather than annual averages revealed new benchmarks in how the ocean is changing. More and more patches of water hit extreme temperatures over time, the team found. Then, in 2014, the entire ocean hit the “point of no return,” Van Houtan says. Beginning that year, at least half of the ocean’s surface waters saw temperatures hotter than the most extreme events from 1870 to 1919.

Marine heat waves are defined as at least five days of unusually high temperatures for a patch of ocean. Heat waves wreak havoc on ocean ecosystems, leading to seabird starvation, coral bleaching, dying kelp forests, and migration of fish, whales and turtles in search of cooler waters (SN: 1/15/20; SN: 8/10/20).

In May 2020, NOAA announced that it was updating its “climate normals” — what the agency uses to put daily weather events in historical context — from the average 1981–2010 values to the higher 1991–2020 averages (SN: 5/26/21). 

This study emphasizes that ocean heat extremes are also now the norm, Van Houtan says. “Much of the public discussion now on climate change is about future events, and whether or not they might happen,” he says. “Extreme heat became common in our ocean in 2014. It’s a documented historical fact, not a future possibility.”

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The Southern Ocean is still busily absorbing large amounts of the carbon dioxide emitted by humans’ fossil fuel burning, a study based on airborne observations of the gas suggests. The new results counter a 2018 report that had found that the ocean surrounding Antarctica might not be taking up as much of the emissions as previously thought, and in some regions may actually be adding CO₂ back to the atmosphere.    

It’s not exactly a relief to say that the oceans, which are already becoming more acidic and storing record-breaking amounts of heat due to global warming, might be able to bear a little more of the climate change burden (SN: 4/28/17; SN: 1/13/21). But “in many ways, [the conclusion] was reassuring,” says Matthew Long, an oceanographer at the National Center for Atmospheric Research in Boulder, Colo.  

That’s because the Southern Ocean alone has been thought to be responsible for nearly half of the global ocean uptake of humans’ CO₂ emissions each year. That means it plays an outsize role in modulating some of the immediate impacts of those emissions. However, the float-based estimates had suggested that, over the course of a year, the Southern Ocean was actually a net source of carbon dioxide rather than a sink, ultimately emitting about 0.3 billion metric tons of the gas back to the atmosphere each year.

In contrast, the new findings, published in the Dec. 3 Science, suggest that from 2009 through 2018, the Southern Ocean was still a net sink, taking up a total of about 0.55 billion metric tons of carbon dioxide each year.

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The 2018 study had used newly deployed deep-diving ocean floats, now numbering almost 200, that are part of a project called Southern Ocean Carbon and Climate Observations and Modeling, or SOCCOM. Calculations based on data collected from 2014 through 2017 by 35 of the floats suggested that parts of the ocean were actually releasing a great deal of carbon dioxide back into the atmosphere during winter (SN: 6/2/19). That sparked concerns that the Southern Ocean’s role in buffering the impacts of climate change on Earth might not be so robust as once thought.

Long says he and other researchers were somewhat skeptical about that takeaway, however. The floats measure temperature, salinity and pH in the water down to about 2,000 meters, and scientists use those data to calculate the carbon dioxide concentration in the water. But those calculations rest on several assumptions about the ocean water properties, as actual data are still very scarce. That may be skewing the data a bit, leading to calculations of higher carbon dioxide emitted from the water than is actually occurring, Long suggests.

Another way to measure how much carbon dioxide is moving between air and sea is by taking airborne measurements. In the new study, the team amassed previously collected carbon dioxide data over large swaths of the Southern Ocean during three separate series of aircraft flights — one series lasting from 2009 to 2011, one in the winter of 2016 and a third in several periods from 2016 to 2018 (SN: 9/8/11). Then, the researchers used those data to create simulations of how much carbon dioxide could possibly be moving between ocean and atmosphere each year.

The float-based and aircraft-based studies estimate different overall amounts of carbon dioxide moving out of the ocean, but both identified a seasonal pattern of less carbon dioxide absorbed by the ocean during winter. That indicates that both types of data are picking up a real trend, says Ken Johnson, an ocean chemist at the Monterey Bay Aquarium Research Institute in Moss Landing, Calif., who was not involved in the research. “We all go up and down together.”

It’s not yet clear whether the SOCCOM data were off. But to better understand what sorts of biases might affect the pH calculations, researchers must compare direct measurements of carbon dioxide in the water taken from ships with pH-based estimates at the same location. Such studies are under way right now off the coast of California, Johnson says.

The big takeaway, Johnson says, is that both datasets — as well as direct shipboard measurements in the Southern Ocean, which are few and far between — are going to be essential for understanding what role these waters play in the planet’s carbon cycle. While the airborne studies can help constrain the big picture of carbon dioxide emissions data from the Southern Ocean, the floats are much more widely distributed, and so are able to identify local and regional variability in carbon dioxide, which the atmospheric data can’t do.

“The Southern Ocean is the flywheel of the climate system,” the part of an engine’s machinery that keeps things chugging smoothly along, Johnson says. “If we don’t get our understanding of the Southern Ocean right, we don’t have much hope for understanding the rest of the world.” More

Stony corals that build reefs have been hiding their diversity in plain sight. A genetic analysis of the most widespread reef coral in the Indo-Pacific revealed that rather than being a single species (Pachyseris speciosa), it was actually four distinct species of coral, researchers report April 2 in Current Biology.

Coral reefs are the condominiums of ocean biodiversity, supporting more species per square meter than any other marine habitat. Understanding which coral species foster that biodiversity and how those corals behave is vital to taking care of them, especially as a warming climate threatens overall ocean biodiversity (SN: 5/6/20). “Just knowing what’s there is critical to tracking what we are losing,” says Rebecca Vega-Thurber, a marine microbiologist at Oregon State University in Corvallis, who was not involved in the new study. The results suggest other corals thought to be a single species may actually be much more diverse than researchers realized.

Using a combination of scuba gear and remotely operated vehicles, marine biologist Pim Bongaerts of the California Academy of Sciences in San Francisco and colleagues sampled more than 1,400 P. speciosa corals from the ocean surface down to 80 meters. In the lab, the sampleslooked identical, and their internal structures were indistinguishable in scanning electron microscope images. Yet, their genomes — their full genetic instruction books — revealed the corals had diverged millions of years ago. That made sense for one of the species in the Red Sea’s Gulf of Aqaba, which was geographically separated from the others. But the other three newly identified species lived together on the same reefs in the waters off South Asia. If the corals were living together, why didn’t one overtake the other two, the team wondered.

Examining habitat data from their dives, the researchers found the three distinct coral species favored different water depths, with one abundant in the top 10 meters and the other two flourishing deeper down. The three coral species also had different concentrations of photosynthetic algae and pigments, suggesting they had distinct strategies for hosting their algae partners that provide food. And spawning times of these three species were slightly spread out too. One released most of its gametes five days after the full moon, another seven days after, and the third at nine days and counting. The separation of spawning could help the eggs and sperm of each species hook up with its correct species match.

Marine biologists Pim Bongaerts and Norbert Englebert collect coral samples during a dive at Holmes Reef in the Coral Sea north of Australia.David Whillas

This study is the first to show how a set of cryptic reef corals are splitting up their shared ecological space — by depth, physiology and spawning time, Bongaerts says. “There are all these cryptic lineages occurring, but they’ve largely been ignored from an ecological point of view.”

The results open the door to the possibility that many other doppelgänger corals may be multiple species that coexist thanks to ecological differences, says reef genomicist Christian Voolstra at the University of Konstanz in Germany. “There is a minimal chance that they picked the unicorn, but I highly doubt it. This paper shows that in all likelihood there is a huge diversity of reef corals with distinct ecologies.” More

Things are heating up at the seafloor.
Thermometers moored at the bottom of the Atlantic Ocean recorded an average temperature increase of about 0.02 degrees Celsius over the last decade, researchers report in the Sept. 28 Geophysical Research Letters. That warming may be a consequence of human-driven climate change, which has boosted ocean temperatures near the surface (SN: 9/25/19), but it’s unclear since so little is known about the deepest, darkest parts of the ocean.
“The deep ocean, below about 2,000 meters, is not very well observed,” says Chris Meinen, an oceanographer at the U.S. National Oceanic and Atmospheric Administration in Miami. The deep sea is so hard to reach that the temperature at any given research site is typically taken only once per decade. But Meinen’s team measured temperatures hourly from 2009 to 2019 using seafloor sensors at four spots in the Argentine Basin, off the coast of Uruguay.
Temperature records for the two deepest spots revealed a clear trend of warming over that decade. Waters 4,540 meters below the surface warmed from an average 0.209° C to 0.234° C, while waters 4,757 meters down went from about 0.232°C to 0.248°C. This warming is much weaker than in the upper ocean, Meinen says, but he also notes that since warm water rises, it would take a lot of heat to generate even this little bit of warming so deep.
It’s too soon to judge whether human activity or natural variation is the cause, Meinen says. Continuing to monitor these sites and comparing the records with data from devices in other ocean basins may help to clarify matters. More

The retreat of North America’s ice sheets in the latter years of the last ice age may have begun with “catastrophic” losses of ice into the North Pacific Ocean along the coast of modern-day British Columbia and Alaska, scientists say. 
In a new study published October 1 in Science, researchers find that these pulses of rapid ice loss from what’s known as the western Cordilleran ice sheet contributed to, and perhaps triggered, the massive calving of the Laurentide ice sheet into the North Atlantic Ocean thousands of years ago. That collapse of the Laurentide ice sheet, which at one point covered large swaths of Canada and parts of the United States, ultimately led to major disturbances in the global climate (SN: 11/5/12).
The new findings cast doubt on the long-held assumption that hemispheric-scale changes in Earth’s climate originate in the North Atlantic (SN: 1/31/19). The study suggests that the melting of Alaska’s remaining glaciers into the North Pacific, though less extreme than purges of the past, could have far-ranging effects on global ocean circulation and the climate in coming centuries.
“People typically think that the Atlantic is where all the action is, and everything else follows,” says Alan Mix, a paleoclimatologist at Oregon State University in Corvallis. “We’re saying it’s the other way around.” The Cordilleran ice sheet fails earlier in the chain of reaction, “and then that signal is transmitted [from the Pacific] around the world like falling dominoes.”

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In 2013, Mix and colleagues pulled sediment cores from the seafloor of the Gulf of Alaska in the hope of figuring out how exactly the Cordilleran ice sheet had changed prior to the end of the last ice age. These cores contained distinct layers of sand and silt deposited by the ice sheet’s calved icebergs during four separate occasions over the last 42,000 years. The team then used radiocarbon dating to determine the chronology of events, finding that the Cordilleran’s ice purges “surprisingly” preceded the Laurentide’s periods of abrupt ice loss, known as “Heinrich events,” by 1,000 to 1,500 years every single time.
“We’ve long known that these Heinrich events are a big deal,” says coauthor Maureen Walczak, a paleoceanographer also at Oregon State University. “They have global climate consequences associated with increases in atmospheric CO2, warming in Antarctica … and the weakening of the Asian monsoon in the Pacific. But we’ve not known why they happened.”  
Though scientists can now point the finger at the North Pacific, the exact mechanism remains unclear. Mix proposes several theories for how Cordilleran ice loss ultimately translated to mass calving of ice along North America’s east coast. It’s possible, he says, that the freshwater deposited in the North Pacific traveled northward through the Bering Strait, across the Arctic and down into the North Atlantic. There, the buoyant freshwater served as a “cap” on the ocean’s denser saltwater, preventing it from overturning. This process could have led to the water getting warmer, destabilizing the adjacent ice sheet.
Another theory posits that the lower elevation of the diminished Cordilleran ice sheet altered how surface winds entered North America. Normally, the ice sheet would act like a fence, diverting winds and their water vapor southward as they entered North America. Without this barrier, the transport of heat and freshwater between the Pacific and Atlantic Ocean basins is disrupted, changing the salinity of the Atlantic waters and ultimately delivering more heat to the ice there.
Today, Alaska’s glaciers serve as the last remnants of the Cordilleran ice sheet. Many are in a state of rapid retreat due to climate change. This melting ice, too, drains into the Pacific and Arctic oceans, raising sea levels and interfering with normal ocean mixing processes. “Knowing the failure of ice in the North Pacific seemed to presage really rapid ice loss in the North Atlantic, that’s kind of concerning,” Walczak says.
If the ice melt into the North Pacific follows similar patterns to the past, it could yield significant global climate events, the researchers suggest. But Mix cautions that the amount of freshwater runoff needed to trigger changes elsewhere in the global ocean, and climate, is unknown. “We know enough to say that such things happened in the past, ergo, they are real and could happen again.”
It’s not clear, though, what the timing of such global changes would be. If the ice losses in the Atlantic occurred in the past due to a change in deep ocean dynamics triggered by Pacific melting, that signal would likely take hundreds of years to reach the other remaining ice sheets. If, however, those losses were triggered by a change in sea levels or winds, other ice sheets could be affected a bit faster, though still not this century.
The Laurentide ice sheet is, of course, long gone. But two others remain, in Greenland and Antarctica (SN: 9/30/20, 9/23/20). Both have numerous glaciers that terminate in the ocean and drain the interior of the ice sheets. This makes the ice sheets susceptible to both warmer ocean water and sea level rise.
Alaska’s melting glaciers have already fueled about 30 percent of global sea level rise. “One of the hypotheses we have is that sea level rise is going to destabilize the ice shelves at the mouths of those glaciers, which will break off like champagne corks,” Walczak explains. When that happens, the idea goes, the ice sheets will start collapsing faster and faster.
Records of climate change in the Pacific, like the one Walczak and colleagues have compiled, have been hard to come by, says Richard Alley, a glaciologist at Pennsylvania State University who wasn’t involved with the study. “These new data may raise more questions than they answer,” he says. “But by linking North Pacific Ocean circulation … to the global template of climate oscillations, the new paper gives us a real advance in understanding all of this.” More

Sound waves traveling thousands of kilometers through the ocean may help scientists monitor climate change.
As greenhouse gas emissions warm the planet, the ocean is absorbing vast amounts of that heat. To monitor the change, a global fleet of about 4,000 devices called Argo floats is collecting temperature data from the ocean’s upper 2,000 meters. But that data collection is scanty in some regions, including deeper reaches of the ocean and areas under sea ice.
So Wenbo Wu, a seismologist at Caltech, and colleagues are resurfacing a decades-old idea: using the speed of sound in seawater to estimate ocean temperatures. In a new study, Wu’s team developed and tested a way to use earthquake-generated sound waves traveling across the East Indian Ocean to estimate temperature changes in those waters from 2005 to 2016.
Comparing that data with similar information from Argo floats and computer models showed that the new results matched well. That finding suggests that the technique, dubbed seismic ocean thermometry, holds promise for tracking the impact of climate change on less well-studied ocean regions, the researchers report in the Sept. 18 Science.

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Sound waves are carried through water by the vibration of water molecules, and at higher temperatures, those molecules vibrate more easily. As a result, the waves travel a bit faster when the water is warmer. But those changes are so small that, to be measurable, researchers need to track the waves over very long distances.
Fortunately, sound waves can travel great distances through the ocean, thanks to a curious phenomenon known as the SOFAR Channel, short for Sound Fixing and Ranging. Formed by different salinity and temperature layers within the water, the SOFAR channel is a horizontal layer that acts as a wave guide, guiding sound waves in much the same way that optical fibers guide light waves, Wu says. The waves bounce back and forth against the upper and lower boundaries of the channel, but can continue on their way, virtually unaltered, for tens of thousands of kilometers (SN: 7/16/60).
In 1979, physical oceanographers Walter Munk, then at the Scripps Institution of Oceanography in La Jolla, Calif., and Carl Wunsch, now an emeritus professor at both MIT and Harvard University, came up with a plan to use these ocean properties to measure water temperatures from surface to seafloor, a technique they called “ocean acoustic tomography.” They would transmit sound signals through the SOFAR Channel and measure the time that it took for the waves to arrive at receivers located 10,000 kilometers away. In this way, the researchers hoped to compile a global database of ocean temperatures (SN: 1/26/1991).
But environmental groups lobbied against and ultimately halted the experiment, stating that the human-made signals might have adverse effects on marine mammals, as Wunsch notes in a commentary in the same issue of Science.
Forty years later, scientists have determined that the ocean is in fact a very noisy place, and that the proposed human-made signals would have been faint compared with the rumbles of quakes, the belches of undersea volcanoes and the groans of colliding icebergs, says seismologist Emile Okal of Northwestern University in Evanston, Ill., who was not involved in the new study.
Still, Wu and colleagues have devised a work-around that sidesteps any environmental concerns: Rather than use human-made signals, they employ earthquakes. When an undersea earthquake rumbles, it releases energy as seismic waves known as P waves and S waves that vibrate through the seafloor. Some of that energy enters the water, and when it does, the seismic waves slow down, becoming T waves.
Those T waves can also travel along the SOFAR Channel. So, to track changes in ocean temperature, Wu and colleagues identified “repeaters” — earthquakes that the team determined to originate from the same location, but occurring at different times. The East Indian Ocean, Wu says, was chosen for this proof-of-concept study largely because it’s very seismically active, offering an abundance of such earthquakes. After identifying over 2,000 repeaters from 2005 to 2016, the team then measured differences in the sound waves’ travel time across the East Indian Ocean, a span of some 3,000 kilometers. 
The data revealed a slight warming trend in the waters, of about 0.044 degrees Celsius per decade. That trend is similar to, though a bit faster than, the one indicated by real-time temperatures collected by Argo floats. Wu says the team next plans to test the technique with receivers that are farther away, including off of Australia’s west coast.
That extra distance will be important to prove that the new method works, Okal says. “It’s a fascinating study,” he says, but the distances involved are very short as far as T waves go, and the temperature changes being estimated are very small. That means that any uncertainty in matching the precise origins of two repeater quakes could translate to uncertainty in the travel times, and thus the temperature changes. But future studies over greater distances could help mitigate this concern, he says.
The new study is “really breaking new ground,” says Frederik Simons, a geophysicist at Princeton University, who was not involved in the research. “They’ve really worked out a good way to tease out very subtle, slow temporal changes. It’s technically really savvy.”
And, Simons adds, in many locations seismic records are decades older than the temperature records collected by Argo floats. That means that scientists may be able to use seismic ocean thermometry to come up with new estimates of past ocean temperatures. “The hunt will be on for high-quality archival records.” More

When an intense heat wave strikes a patch of ocean, overheated marine animals may have to swim thousands of kilometers to find cooler waters, researchers report August 5 in Nature.
Such displacement, whether among fish, whales or turtles, can hinder both conservation efforts and fishery operations. “To properly manage those species, we need to understand where they are,” says Michael Jacox, a physical oceanographer with the National Oceanographic and Atmospheric Administration based in Monterey, Calif.
Marine heat waves —  defined as at least five consecutive days of unusually hot water for a given patch of ocean — have become increasingly common over the past century (SN: 4/10/18). Climate change has amped up the intensity of some of the most famous marine heat waves of recent years, such as the Pacific Ocean Blob from 2015 to 2016 and scorching waters in the Tasman Sea in 2017 (SN: 12/14/17; SN: 12/11/18).
“We know that these marine heat waves are having lots of effects on the ecosystem,” Jacox says. For example, researchers have documented how the sweltering waters can bleach corals and wreak havoc on kelp forests. But the impacts on mobile species such as fish are only beginning to be studied (SN: 1/15/20).

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“We have seen species appearing far north of where we expect them,” Jacox says. For example, in 2015, the Blob drove hammerhead sharks — which normally stay close to the tropics, near Baja California in Mexico — to shift their range at least hundreds of kilometers north, where they were observed off the coast of Southern California.
To see how far a mobile ocean dweller would need to flee to escape the heat, Jacox and colleagues compared ocean temperatures around the globe. First, they examined surface ocean temperatures from 1982 to 2019 compiled by NOAA from satellites, buoys and shipboard measurements. Then, for the same period, they identified marine heat waves occurring around the world, where water temperatures for a region lingered in the highest 10 percent ever recorded for that place and that time of year. Finally, they calculated how far a swimmer in an area with a heat wave has had to go to reach cooler waters, a distance the team dubs “thermal displacement.”

In higher-latitude regions, such as the Tasman Sea, relief tended to be much closer, within a few tens of kilometers of the overheated patch, the researchers found. So while ocean heat waves in that region might spell doom for firmly rooted corals and kelp, mobile species might fare better. “We were surprised that the displacements were so small,” Jacox says.
But in the tropics, where ocean temperatures are more uniform, species may have had to travel thousands of kilometers to escape the heat.  
Projecting how species might move around in the future due to marine heat waves gets increasingly complicated, the researchers found. That’s because over the next few decades, climate change is anticipated to cause not just an increase in frequency and intensity of marine heat waves, but also warming of all of Earth’s ocean waters (SN: 9/25/19). Furthermore, that rate of warming will vary from place to place. As a result, future thermal displacement could increase in some parts of the ocean relative to today, and decrease in others, writes marine ecologist Mark Payne of the Technical University of Denmark in Copenhagen, in a commentary in the same issue of Nature.
That complexity highlights the task ahead for researchers trying to anticipate changes across ocean ecosystems as the waters warm, says Lewis Barnett, a Seattle-based NOAA fish biologist, who was not involved in the study. The new work provides important context for data being collected on fish stocks. For example, surveys of the Gulf of Alaska in 2017 noted a large decline in the abundance of valuable Pacific cod, now known to be linked to the Blob heatwave that had ended the year before.
But there’s a lot more work to be done, Barnett says.
The study focuses on surface ocean temperatures, but ocean conditions and dynamics are different in the deep ocean, he notes. Some species, too, move more easily between water depths than others. And heat tolerance also varies from species to species. Biologists are racing to understand these differences, and how hot waters can affect the life cycles and distributions of many different animals.
The effects of marine heat waves might be ephemeral compared with the impacts of long-term climate change. But these extreme events offer a peek into the future, says Malin Pinsky, a marine ecologist at Rutgers University in New Brunswick, N.J., who was not involved in the study. “We can use these heat waves as lessons for how we’ll need to adapt.” More