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  • richardmitnick 11:51 am on February 6, 2020 Permalink | Reply
    Tags: "Climate change may be speeding up ocean circulation", AMOC-Atlantic meridional overturning circulation, , , , , The Great Ocean Conveyor Belt   

    From Science News: “Climate change may be speeding up ocean circulation” 

    From Science News

    2.5.20
    Carolyn Gramling

    Since the 1990s, wind speeds have picked up, making surface waters swirl faster.

    1
    Argo floats, such as this one being deployed in the Southern Ocean, measure water temperature, salinity and current speeds. Data from such floats suggest that ocean circulation has sped up. SOCCOM Project/Cara Nissen/Flickr (CC BY 2.0)

    Winds are picking up worldwide, and that is making the surface waters of the oceans swirl a bit faster, researchers report. A new analysis of the ocean’s kinetic energy, measured by thousands of floats around the world, suggests that surface ocean circulation has been accelerating since the early 1990s.

    Some of that sped-up circulation may be due to naturally recurring ocean-atmosphere patterns, such as the Pacific Decadal Oscillation, researchers report February 5 in Science Advances. But the acceleration is greater than can be attributed to natural variability alone — suggesting that global warming may also be playing a role, says a team led by oceanographer Shijian Hu of the Chinese Academy of Sciences in Qingdao.

    3

    The connected system of massive currents that swirl between the world’s oceans, sometimes called the Great Ocean Conveyor Belt, redistributes heat and nutrients around the globe and has a powerful effect on climate. Winds dominate mixing in the surface ocean: Prevailing winds in the tropics, for example, can push water masses aside, allowing deeper, nutrient-rich waters to surge upward.

    In the deeper ocean, differences in water density due to salt and heat content keep the currents flowing (SN: 1/4/17). For example, in the North Atlantic Ocean, surface currents carry heat north from the tropics, helping to keep northwestern Europe warm. As the waters arrive at the Labrador Sea, they cool, sink and then flow southward, keeping the conveyor belt humming along.

    How climate change might affect this Atlantic Meridional Overturning Circulation, or AMOC, has garnered headlines, as some simulations have predicted that global warming would lead to a slowdown in which could eventually bring a deep chill to Europe. In 2018, paleoceanographer David Thornalley of University College London and colleagues reported evidence that the AMOC has weakened over the last 150 years, although the question remains uncertain (SN: 1/31/19).

    But the new study focuses on “the amount of swirling around of upper ocean waters due to wind,” rather than the speed of that overturning circulation, says Thornalley, who was not involved in the work.

    Global warming has long been predicted to slow global wind speeds, called “global stilling.” That’s because the poles are warming faster than the equatorial region, and a smaller temperature gradient between the two zones might be expected to result in weaker winds (SN: 3/16/18). But recent studies, such as a report published November 2019 in Nature Climate Change, suggest that wind speeds around the world have actually been speeding up, at least since about 2010.

    The new study suggests that winds have actually been picking up over the oceans for several decades, leading to the faster-swirling surface waters especially in the tropics. The study used data collected by over 3,000 Argo floats, which measure temperature, salinity and speeds of currents down to about 2,000 meters, in oceans around the world. Then, the team combined these data with a variety of climate simulations to calculate the change in kinetic energy —energy from the wind motion that gets transferred to the water — in that upper part of the ocean.

    Each of the analyses that the team performed showed the same trend: On average around the world, there was a distinct uptick in kinetic energy beginning around 1990.

    The new analyses of wind speeds come from satellite, shipboard and other data previously collected and analyzed by other scientists. The team considered one possible culprit for those changing winds: the late-1990s onset of a “cold” phase of an El Niño–like ocean-atmosphere pattern called the Pacific Decadal Oscillation, which can bring stronger winds to the tropics. But, the researchers say, the observed acceleration is much larger than would be expected from natural variability alone, suggesting that it is part of a longer-term trend.

    Simulations of increasing greenhouse gas emissions over the last two decades, the team found, produce a similar uptick in winds, suggesting that climate change may be speeding up the winds too.

    See the full article here .


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  • richardmitnick 8:32 am on May 8, 2019 Permalink | Reply
    Tags: "North Atlantic Ocean productivity has dropped 10 percent during Industrial era", AMOC-Atlantic meridional overturning circulation, DMS-dimethylsulfide, , , MSA-methanesulfonic acid, , Phytoplankton, The decline coincides with steadily rising surface temperatures over the same period of time.   

    From MIT News: “North Atlantic Ocean productivity has dropped 10 percent during Industrial era” 

    MIT News
    MIT Widget

    From MIT News

    May 6, 2019
    Jennifer Chu

    Phytoplankton decline coincides with warming temperatures over the last 150 years.

    1
    Matt Osman, a graduate student in MIT’s Department of Earth, Atmospheric, and Planetary Sciences, overlooking a frozen Baffin Bay to the west, Nuussuaq Peninsula Ice Cap, west Greenland. Image: Luke Trusel (Rowan University).

    2
    Ice core field camp on a clear spring evening, Disko Island Ice Cap, west Greenland. Image: Luke Trusel (Rowan University).

    3
    Iceberg in Disko Bay, west Greenland. Image: Luke Trusel (Rowan University)

    4
    Retrieving an ice core section from the drill barrel during a west Greenland snowstorm, west Greenland Ice Sheet. Image: Sarah Das (WHOI).

    Virtually all marine life depends on the productivity of phytoplankton — microscopic organisms that work tirelessly at the ocean’s surface to absorb the carbon dioxide that gets dissolved into the upper ocean from the atmosphere.

    Through photosynthesis, these microbes break down carbon dioxide into oxygen, some of which ultimately gets released back to the atmosphere, and organic carbon, which they store until they themselves are consumed. This plankton-derived carbon fuels the rest of the marine food web, from the tiniest shrimp to giant sea turtles and humpback whales.

    Now, scientists at MIT, Woods Hole Oceanographic Institution (WHOI), and elsewhere have found evidence that phytoplankton’s productivity is declining steadily in the North Atlantic, one of the world’s most productive marine basins.

    In a paper appearing today in Nature, the researchers report that phytoplankton’s productivity in this important region has gone down around 10 percent since the mid-19th century and the start of the Industrial era. This decline coincides with steadily rising surface temperatures over the same period of time.

    Matthew Osman, the paper’s lead author and a graduate student in MIT’s Department of Earth, Atmospheric, and Planetary Sciences and the MIT/WHOI Joint Program in Oceanography, says there are indications that phytoplankton’s productivity may decline further as temperatures continue to rise as a result of human-induced climate change.

    “It’s a significant enough decine that we should be concerned,” Osman says. “The amount of productivity in the oceans roughly scales with how much phytoplankton you have. So this translates to 10 percent of the marine food base in this region that’s been lost over the industrial era. If we have a growing population but a decreasing food base, at some point we’re likely going to feel the effects of that decline.”

    Drilling through “pancakes” of ice

    Osman and his colleagues looked for trends in phytoplankton’s productivity using the molecular compound methanesulfonic acid, or MSA. When phytoplankton expand into large blooms, certain microbes emit dimethylsulfide, or DMS, an aerosol that is lofted into the atmosphere and eventually breaks down as either sulfate aerosol, or MSA, which is then deposited on sea or land surfaces by winds.

    “Unlike sulfate, which can have many sources in the atmosphere, it was recognized about 30 years ago that MSA had a very unique aspect to it, which is that it’s only derived from DMS, which in turn is only derived from these phytoplankton blooms,” Osman says. “So any MSA you measure, you can be confident has only one unique source — phytoplankton.”

    In the North Atlantic, phytoplankton likely produced MSA that was deposited to the north, including across Greenland. The researchers measured MSA in Greenland ice cores — in this case using 100- to 200-meter-long columns of snow and ice that represent layers of past snowfall events preserved over hundreds of years.

    “They’re basically sedimentary layers of ice that have been stacked on top of each other over centuries, like pancakes,” Osman says.

    The team analyzed 12 ice cores in all, each collected from a different location on the Greenland ice sheet by various groups from the 1980s to the present. Osman and his advisor Sarah Das, an associate scientist at WHOI and co-author on the paper, collected one of the cores during an expedition in April 2015.

    “The conditions can be really harsh,” Osman says. “It’s minus 30 degrees Celsius, windy, and there are often whiteout conditions in a snowstorm, where it’s difficult to differentiate the sky from the ice sheet itself.”

    The team was nevertheless able to extract, meter by meter, a 100-meter-long core, using a giant drill that was delivered to the team’s location via a small ski-equipped airplane. They immediately archived each ice core segment in a heavily insulated cold storage box, then flew the boxes on “cold deck flights” — aircraft with ambient conditions of around minus 20 degrees Celsius. Once the planes touched down, freezer trucks transported the ice cores to the scientists’ ice core laboratories.

    “The whole process of how one safely transports a 100-meter section of ice from Greenland, kept at minus-20-degree conditions, back to the United States is a massive undertaking,” Osman says.

    Cascading effects

    The team incorporated the expertise of researchers at various labs around the world in analyzing each of the 12 ice cores for MSA. Across all 12 records, they observed a conspicuous decline in MSA concentrations, beginning in the mid-19th century, around the start of the Industrial era when the widescale production of greenhouse gases began. This decline in MSA is directly related to a decline in phytoplankton productivity in the North Atlantic.

    “This is the first time we’ve collectively used these ice core MSA records from all across Greenland, and they show this coherent signal. We see a long-term decline that originates around the same time as when we started perturbing the climate system with industrial-scale greenhouse-gas emissions,” Osman says. “The North Atlantic is such a productive area, and there’s a huge multinational fisheries economy related to this productivity. Any changes at the base of this food chain will have cascading effects that we’ll ultimately feel at our dinner tables.”

    The multicentury decline in phytoplankton productivity appears to coincide not only with concurrent long-term warming temperatures; it also shows synchronous variations on decadal time-scales with the large-scale ocean circulation pattern known as the Atlantic Meridional Overturning Circulation, or AMOC. This circulation pattern typically acts to mix layers of the deep ocean with the surface, allowing the exchange of much-needed nutrients on which phytoplankton feed.

    In recent years, scientists have found evidence that AMOC is weakening, a process that is still not well-understood but may be due in part to warming temperatures increasing the melting of Greenland’s ice. This ice melt has added an influx of less-dense freshwater to the North Atlantic, which acts to stratify, or separate its layers, much like oil and water, preventing nutrients in the deep from upwelling to the surface. This warming-induced weakening of the ocean circulation could be what is driving phytoplankton’s decline. As the atmosphere warms the upper ocean in general, this could also further the ocean’s stratification, worsening phytoplankton’s productivity.

    “It’s a one-two punch,” Osman says. “It’s not good news, but the upshot to this is that we can no longer claim ignorance. We have evidence that this is happening, and that’s the first step you inherently have to take toward fixing the problem, however we do that.”

    This research was supported in part by the National Science Foundation (NSF), the National Aeronautics and Space Administration (NASA), as well as graduate fellowship support from the US Department of Defense Office of Naval Research.

    See the full article here .


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  • richardmitnick 12:34 pm on April 8, 2019 Permalink | Reply
    Tags: "Carbon Lurking in Deep Ocean Threw Ancient Climate Switch, A key Atlantic Ocean current system that today once again is slowing, AMOC-Atlantic meridional overturning circulation, , , , , Lamont-Doherty Earth Observatory, Say Researchers", Slowdown of Atlantic Circulation Sent Planet Into Deep Freeze, State of the Planet, The researchers reached their findings by analyzing cores of deep-sea sediments taken in the south and north Atlantic   

    From Columbia University: “Carbon Lurking in Deep Ocean Threw Ancient Climate Switch, Say Researchers” 

    Columbia U bloc

    From Columbia University

    April 8, 2019
    Kevin Krajick

    Slowdown of Atlantic Circulation Sent Planet Into Deep Freeze.

    A million years ago, a longtime pattern of alternating glaciations and warm periods dramatically changed, when ice ages suddenly became longer and more intense. Scientists have long suspected that this was connected to the slowdown of a key Atlantic Ocean current system that today once again is slowing. A new study of sediments from the Atlantic bottom directly links this slowdown with a massive buildup of carbon dragged from the air into the abyss. With the system running at full speed, this carbon would have percolated back into the air fairly quickly, but during this period it just stagnated in the depths. This suggests that the carbon draw down cooled the planet—the opposite of the greenhouse effect we are seeing now, as humans pump carbon into the atmosphere. But if the current keeps slowing now, we should not expect it to help us out by storing our emissions; possibly to the contrary. The study, led by researchers at Columbia University’s Lamont-Doherty Earth Observatory, appears this week in the journal Nature Geoscience.

    The scientists targeted a system of currents called the Atlantic meridional overturning circulation, or AMOC. Flowing northward near the surface, it transports warm, salty water from near the equator into the latitudes near Greenland and northern Europe. Here, it hits colder water from the Arctic, becomes denser and sinks into the abyss, taking with it large amounts of carbon absorbed from the atmosphere. The deep water then circles back south, where much of it re-merges in the Southern Ocean, to release carbon back to the air. The journey takes place over decades to centuries.

    A 2014 study [Science] by Lamont-Doherty scientists Steven Goldstein and Leopoldo Pena–both of whom also are coauthors of the new study–showed that this current system abruptly slowed around 950,000 years ago. The new study shows that this slowdown correlated directly with a huge buildup of carbon in the deep Atlantic, and corresponding decline of carbon in the air. This event was the apparent trigger for a series of ice ages that came every 100,000 years, versus previous ones that occurred about every 40,000 years, and which built up less ice than those that came later. Scientists call this turning point the Mid-Pleistocene Transition, and the new pattern has persisted right through the last ice age, which ended about 15,000 years ago. Exactly why the pattern has continued no one knows, but the study clearly demonstrates that the carbon missing from the air ended up in the ocean, and had a powerful effect on climate.

    2
    The Atlantic meridional overturning circulation, seen here in simplified form, brings warm water northward (red arrows) until it reaches the region around Greenland and northern Europe. Here, it sinks and travels southward (yellow arrows). Much of the water re-emerges in the Southern Ocean. (Courtesy Francesco Muschitiello/Lamont-Doherty Earth Observatory)

    “It’s a one-to-one relationship. It was like flipping a switch,” said lead author Jesse Farmer, who did the work while a PhD. student at Lamont-Doherty. “It shows us that there’s an intimate relationship between the amount of carbon stored in the ocean, and what the climate is doing.”

    The researchers reached their findings by analyzing cores of deep-sea sediments taken in the south and north Atlantic, where ancient deep waters passed by and left chemical clues about their contents in the shells of microscopic creatures. Their analysis confirmed the 2014 study showing that the AMOC weakened to an extent not seen before, around 950,000 years ago, and for an unusually long time. Because of this, the deep water collected about 50 billion tons more carbon than it had during previous glaciations—equivalent to about one third of the human emissions that all the world’s oceans have so far absorbed today. (For context, the oceans today absorb roughly a quarter of what we emit; land and vegetation take up a third. The rest stays in the air.)

    In the warm period leading up to this event, the atmosphere had held about 280 parts per million carbon; with the slowdown, airborne carbon dioxide went down to 180 ppm, as measured by ice cores. Atmospheric carbon had sunk during previous glaciations as well, but from 280 ppm down only to about 210 ppm. (Because of human emissions during the past two centuries, this normal repeating 280 ppm warm-era figure has become obsolete; atmospheric carbon is now up to about 410 ppm.)

    3
    Left: Before about 950,000 years ago, waters reached the deep Atlantic Ocean from the north (black arrows) and south (purple arrows). Right: Using data from two sediment cores (yellow stars), scientists showed that a weakening of the northern-sourced circulation (thinner black arrows) after that led to more carbon storage in the Atlantic. Under a weaker circulation, more of the deep Atlantic water was sourced from the south. (Jesse Farmer)

    At some point, the current woke up again, and things warmed for a while before dropping back into another similarly extreme ice age, after 100,000 years. “There are lots of ideas about what caused these changes to happen, but it’s hard to say what the trigger was,” said Bärbel Hönisch, Farmer’s advisor and coauthor of the study. “There are several different screws you could imagine turning, and lots of loose screws.”

    One idea, espoused by Goldstein’s group [Science] among others: In the north, repeated build-ups of glaciers ultimately scrape everything on land down to bedrock. Subsequent glaciers are then able to stick fast to the bedrock and bulk up even more, before discharging icebergs into the ocean. This introduces more freshwater to mix with the AMOC, making it less dense and eventually unable to sink. On the other end, ice would also grow in Antarctica and discharge more icebergs, which would make the ocean waters colder and less salty, thus encouraging the growth of more sea ice. This, theoretically, would cap the surface and keep deep water from rising and releasing its carbon. But if this is indeed the way it works, it is not clear what starts or ends any of the processes; it is a chicken-and-egg kind of question.

    The strength of the AMOC is believed to fluctuate naturally, but it appears to have weakened by an unusual 15 percent since the mid-20th century. No one is sure what is behind that, or what effects it might produce if the slowdown continues. Another Lamont-Doherty study [Nature] last month showed that a slowdown around 13,000 years ago, at the tail end of the last ice age, was followed 400 years later by an intense cold snap that lasted centuries.

    “We have to be careful about drawing parallels with that,” said Farmer, now a postdoctoral researcher at Princeton University. “We see a similar weakening today, and one might say, ‘Great! Ocean circulation is going to save us from warming climate!’ But that’s not correct, because of the way different parts of the climate system talk to each other.” Farmer said that if the AMOC continues weakening now, it is probable that less carbon-laden water will sink in the north, at the same time, in the Southern Ocean, any carbon already arriving in the deep water will likely keep bubbling up without any problem. The result: carbon will continue to build in the air, not the ocean.

    The researchers point out that the AMOC is only part of a much larger system of global circulation that connects all the oceans—the so-called Great Ocean Conveyor, a term coined by the late Lamont-Doherty scientist Wallace Broecker, who laid the groundwork for much of the current research. Much less is known about the carbon dynamics of the Indian and Pacific, which together dwarf the Atlantic, so there are many missing pieces to the puzzle. Ongoing research at Lamont-Doherty is aimed at building carbon chronologies of those other waters in the next few years.

    The study was also coauthored by Laura Haynes, Heather Ford, Maureen Raymo, Maria Jaume-Seguí, Steven Goldstein, Maayan Yehudai and Joohee Kim, all of Lamont-Doherty; Dirk Kroon, Simon Jung and Dave Bell of the University of Edinburgh; and Leopoldo Pena of the University of Barcelona.

    See the full article here .

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  • richardmitnick 10:02 am on August 23, 2018 Permalink | Reply
    Tags: AMOC-Atlantic meridional overturning circulation, , , Seafloor cores suggest sticky thick glaciers caused mysterious shift in ice age rhythms   

    From Science Magazine: “Seafloor cores suggest sticky, thick glaciers caused mysterious shift in ice age rhythms” 

    AAAS
    From Science Magazine

    Aug. 22, 2018
    Paul Voosen

    1
    The Russell Glacier in Greenland. After ancient glaciers scoured away soil and reached bedrock, they may have grown thicker—triggering a global cooling that shifted the ice age cycles. Jason Edwards/National Geographic/Getty Images.

    About 1 million years ago, one of Earth’s most important metronomes mysteriously shifted: Ice ages went from occurring every 40,000 years to every 100,000 years. At the same time, the “conveyor belt” of warming currents in the North Atlantic Ocean slowed sharply. Last week, scientists here at the Goldschmidt Conference presented a clue to these twin mysteries: evidence that glaciers in the Northern Hemisphere suddenly began to stick to their beds. Growing thicker, they might have triggered a cooling that disrupted the conveyor belt and allowed the 100,000-year cycle that we see today to take root.

    “The system basically crashed,” says Steve Goldstein, an ocean geochemist at Columbia University who led the study. Other scientists welcome the new clues to the transition. “This is really exciting new evidence,” says Henrieka Detlef, a paleoclimatologist at Cardiff University in the United Kingdom. But she and others aren’t sold yet on the long causal chain that Goldstein’s team posits.

    Scientists have long known that tiny changes in Earth’s orbit around the sun, called Milankovitch cycles, drive the planet in and out of ice ages. But nothing changed in those orbital patterns 1 million years ago. Recently, Goldstein and his colleagues found signs of a possible contributor to the ice age transition: a near-collapse of the Atlantic meridional overturning circulation (AMOC). The AMOC shepherds shallow warm water to the North Atlantic, where it cools and sinks before returning south along the sea floor to the Southern Ocean to meet Pacific Ocean waters.

    Goldstein’s group deduces the overall strength of the AMOC from geochemical markers in ocean sediment cores. The researchers take advantage of a ratio between two isotopes of neodymium that varies with the age of their source rocks: ancient crust runs negative, whereas younger rocks are more positive. As it happens, the North Atlantic is surrounded by ancient crust, whereas the Pacific, thanks to its volcanic Ring of Fire, tilts younger. The neodymium-carrying grit ends up incorporated into the shells of single-celled foraminifera or fish teeth, both of which accumulate over time on the sea floor. Changes in the isotope ratio record the wax and wane of intruding North Atlantic or Pacific waters.

    Earlier this decade, the Columbia group tested its approach on two archived sediment cores from the South Atlantic. About 950,000 years ago, they saw the isotopic signals shoot up, reflecting an incursion of Pacific waters, with little evidence of returning North Atlantic waters—suggesting a stark “AMOC crisis.” The slowdown could have sharply cooled the North Atlantic region—and might have lengthened the ice age rhythm.

    Now, the team has analyzed five other ocean cores that also show signs of a weak AMOC. Two of the cores, from the North Atlantic, suggest a possible trigger for the AMOC crisis. In the millennia leading up to it, the neodymium signal sharply trended negative before abating—a sign that an influx of older and older grit from the North Atlantic region had suddenly stopped.

    The only plausible explanation, they say, is a long-standing hypothesis advanced by Peter Clark, a glaciologist at Oregon State University in Corvallis, and several others: that the northern ice sheets had finally ground their way to bedrock. Before Earth’s current ice age cycles began 3 million years ago, a long warm period had allowed a thick soil layer to build up on northern landmasses. At first, the soil acted as a grease that caused early ice sheets to collapse before they could thicken much. But repeated glaciations gradually scoured this grit away, and meltwater swept it into the ocean. As the glaciers dug deeper into older rock, the neodymium signal in ocean sediment became more negative. Eventually, the glaciers reached bedrock and began to stick to their base, allowing them to grow thicker—leading to a more profound and persistent cooling that somehow caused the AMOC to crash and the glacial cycle to lengthen. “We think we’re seeing the trigger,” says Maayan Yehudai, the Columbia graduate student who presented the work. (Scientists believe pronounced global warming—like the warming underway now—could also disrupt the AMOC.)

    The neodymium evidence supports this geological story, Clark says. “It’s a pretty clear signal that you should see.” Detlef notes, however, that there is no conclusive evidence that northern ice sheets were increasing in thickness prior to the AMOC slowdown. But she accepts that something important happened in the North Atlantic leading up to the AMOC crisis.

    One hypothesis that does seem ruled out, however, is the notion that the growth of Antarctic ice sheets 900,000 years ago played a pivotal role in the tempo change. “Everything that’s happening in the North Atlantic is happening before [that],” Yehudai says.

    The AMOC and the glaciers may not have been the only factors in the transition, however. Some scientists have suggested that a small drawdown of carbon dioxide (CO2), perhaps driven by a dust-fertilized plankton bloom in the Southern Ocean, would have been enough to shift the ice age rhythm. Yair Rosenthal, a paleo-oceanographer at Rutgers University in New Brunswick, New Jersey, thinks a CO2 drop, thickening ice sheets, and a weak AMOC could have all played a role. “I’m not a fan of single triggers of anything.”

    See the full article here .


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