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5/1/18

The underestimated danger of a breakdown of the Gulf Stream System

Filed under:   — stefan @ 4 January 2017
A new model simulation of the Gulf Stream System shows a breakdown of the gigantic overturning circulating in the Atlantic after a CO2 doubling.
new study in Science Advances by Wei Liu and colleagues at the Scripps Institution of Oceanography in San Diego and the University of Wisconsin-Madison has important implications for the future stability of the overturning circulation in the Atlantic Ocean. They applied a correction to the freshwater fluxes in the Atlantic, in order to better reproduce the salt concentration of ocean waters there. This correction changes the overall salt budget for the Atlantic, also changing the stability of the model’s ocean circulation in future climate change. The Atlantic ocean circulation is relatively stable in the uncorrected model, only declining by about 20% in response to a CO2 doubling, but in the corrected model version it breaks down completely in the centuries following a CO2 doubling, with dramatic consequences for the climate of the Northern Hemisphere.
The potential instability of the Atlantic Meridional Overturning Circulation or AMOC – commonly known as the Gulf Stream System – has been a subject of research since the 1980s, when Wallace Broecker warned in an essay in Nature of Unpleasant Surprises in the Greenhouse. The reason for this was growing evidence of abrupt climate changes in the history of the Earth due to instability of Atlantic currents.


Fig. 1 Schematic of the Atlantic ocean circulation (simplified). In red the relatively warm surface flow is seen, in blue the cold deep water flow. The northward surface flow and southward deep flow together make up the Atlantic Meridional Overturning Circulation (AMOC), popularly dubbed Gulf Stream System. Image by S. Rahmstorf (Nature 1997), Creative Commons BY-SA 4.0. 
Why the AMOC has a tipping point
The basic physical mechanism of this instability was already described by Henry Stommel in 1961. The freshwater balance (precipitation minus evaporation), which determines the salt content, is central to this. Freshwater continually flows into the northern Atlantic through precipitation, rivers and ice-melting. But supply of salty waters from the south, through the Gulf Stream System, balances this. If however the current slows, there is less salt supply, and the surface ocean gets less salty. This fresher water is lighter than saltier water and therefore cannot sink into the depths so easily. Since this sinking – the so-called deep water formation – drives the Gulf Stream System, the current continues to weaken. There is a critical point when this becomes an unstoppable vicious circle. This is one of the classic tipping points in the climate system.
However, it’s still unclear where exactly this tipping point is. Most models show a significant slowdown in the Gulf Stream System by 20% to 50% in typical global warming scenarios up to the year 2100, but do not exceed the tipping point that would lead to its collapse. However, there is a large spread between different models – which is not surprising since the stability of the Atlantic flow depends on a subtle balance in the salinity and thus also in the freshwater budget, which is only inaccurately known. In addition, there have long been serious indications that the models are not only inaccurate, but perhaps all systematically biased towards an exceedingly stable AMOC. We discussed these papers in a review article in PNAS in 2009.
What makes the new study different?
According to lead author Wei Liu, the starting point of the new study was my paper from 1996 on the relationship between the freshwater balance and stability of the flow. Back then I showed how to determine the stability of the AMOC from an analysis of the freshwater transport in the Atlantic at 30° south. The decisive factor is whether the AMOC brings freshwater into the Atlantic basin or whether it exports it (in the latter case, working to increase salinity in the Atlantic). My article ended with the suggestion to clarify this from observational data. That was later done by colleagues from Holland (Weijer et al. 1999). Several studies followed which performed this diagnosis for different climate models (e.g., Pardaens et al. 2003de Vries and Weber 2005Dijkstra 2007Drijfhout et al. 2010Hawkins et al. 2011). According to the observational data, the AMOC is exporting freshwater, which is why freshwater will accumulate in the Atlantic when the AMOC breaks down. That is precisely the instability described by Stommel 1961 and Broecker 1987. In the models, on the other hand, the AMOC in most cases imports freshwater, so the flow is fundamentally stable there. The differences in AMOC stability between different models cannot be understood without the fundamental criterion of whether the AMOC imports or exports freshwater, and by what amount. Liu et al. 2014 have identified a known common bias in all coupled climate GCMs without flux adjustments, the “tropical bias”, which makes them import freshwater in contrast to what observations show for the real ocean. A model bias towards stability is also consistent with the fact that most models underestimate the cooling trend observed in the subpolar Atlantic, which is indicative of an ongoing significant AMOC weakening, as we have argued (Rahmstorf et al. 2015).
The new study attempts to correct this model deficit by modifying the freshwater exchange at the sea surface in a model by using a so-called flux correction (which also involves the heat exchange, but this should be secondary). As a result, the salinity distribution in the ocean of the model for today’s climate is in better agreement with that of the real ocean. This is an important criterion: while precipitation and evaporation over the oceans are difficult to measure and therefore only very imprecisely known, we have detailed and precise information about the salinity distribution from ship measurements. Apart from the improved salinity distribution, this correction has no significant influence on the model climate for the present.
And now the result …
With both model variants – with and without the subtle correction of the salinity distribution – an experiment was performed in which the amount of CO2 in the air was doubled. The reaction of the Atlantic circulation is shown in the following graph. Without correction, the AMOC once again proves to be very stable against the massive disturbance. With the correction, in contrast, the flow breaks down in the course of about 300 years. It has lost a third of its strength after 100 years.




Fig. 2 Time series of the Atlantic flow (AMOC) in the two model variants: without correction (blue) and with correction (orange). In model year 201, the CO2 concentration in the model is doubled and then left at this level. Source: Liu et al., Science Advances 2016.
As expected, the breakdown of the heat-bringing Gulf Stream System leads to a cooling in the northern Atlantic, as shown in Figure 3. Land areas are also affected: besides Greenland and Iceland mainly Great Britain and Scandinavia.


Fig. 3 Temperature change in the winter months (DJF), 300 years after CO2 doubling in the experiment. Due to the almost completely extinct Atlantic flow, the northern Atlantic region has cooled significantly. Source: Wei Liu, with permission.
This new study is certainly not the last word on this important question. Compared to the measured data the correction appears to be somewhat too strong – the adjusted model version might therefore be too unstable. As computing time is scarce and expensive, the CO2concentration in the experiments was abruptly doubled, rather than gradually ramped up in a more realistic emission scenario. The experiment was carried out with only one climate model; for robust conclusions, one usually waits until a series of models shows consistent results. (However, consistent with the new results two earlier climate GCMs and a number of simpler models have shown an AMOC that exports freshwater and is bistable, i.e. could potentially pass a tipping point and break down, as discussed by Liu et al. 2014.)
Also, no meltwater influence from the dwindling continental ice on Greenland was taken into account, which could additionally weaken the flow. On this topic, only three weeks ago a new study was published (Bakker et al. 2016) comparing future warming scenarios, once with and once without consideration of the influx of Greenland meltwater. (An emulator was used for this study; that is a highly simplified computer model that reproduces the results of complex circulation models in a time-saving way, so that many experiments can be performed with it.) With unmitigated emissions (RCP8.5 scenario), the Gulf Stream System weakens on average by 37% by the year 2300 without Greenland melt. With Greenland meltwater this doubles to 74%. And a few months ago, a study with a high-resolution ocean model appeared, suggesting that the meltwater from Greenland is likely to weaken the AMOC considerably within a few decades (Böning et al. 2016 – as we reported).
There are, therefore, two reasons why thus far we could have underestimated the risk of a breakdown of the Gulf Stream System. First, climate models probably have a systematic bias towards stable flow. Secondly, most of them do not take into account the melting ice of Greenland. As the new studies show, each of these factors alone can lead to a much stronger weakening of the Gulf Stream system. We now need to study how these two factors work together. I hope these worrying new results will encourage as many other research groups as possible to pursue this question with their own models!
Weblinks

19/6/17

With enough evidence, even skepticism will thaw

THE WASHINGTON POST

CHAPTER 3

Opinion evolves with evidence

As nervous as he was about their equipment, Muenchow was much more in his element here than he had been in 2010, testifying before Congress. Then, Muenchow’s scientific caution and compunction for rigor didn’t translate very well for a political audience.
Petermann Ice Shelf, GREENLAND
Half a decade before he took this trip to the farthest reaches of the north, Andreas Muenchow had his doubts about whether warming temperatures were causing one of the world’s great platforms of ice to melt and fall apart.
He even stood before Congress in 2010 and balked on whether climate change might have caused a mammoth chunk of ice, four times the size of Manhattan, to break off from this floating, 300-square-mile shelf. The University of Delaware oceanographer said he wasn’t sure. He needed more evidence.

Petermann Glacier
North
Pole
Arctic
Circle
Qaanaaq
Nuuk
Pacific
Ocean
Ottawa
Atlantic
Ocean
D.C.
Arctic Ocean
Ellesmere Island
(CANADA)
GREENLAND
(DENMARK)
Petermann
Glacier
Inland
ice sheet
40 MILES
Source: NASA MODIS
But then the Petermann Ice Shelf lost another two Manhattans of ice in 2012, and Muenchow decided to see for himself, launching a project to study the ice shelf intensively.
He was back again in late August, no longer a skeptic. It was hard not to be a believer here at 81 degrees north latitude, where Greenland and Canada very nearly touch. The surface of the bumpy and misshapen ice was covered with pools and puddles, in some cases frozen over but with piercing blue water beneath. Streams carved through the vast shelf, swelling into larger ponds or even small lakes.
The meltwater was a sign the ice shelf was growing more fragile, moving closer to the day when it might give up more city-size chunks of ice.
The Petermann Ice Shelf serves as a plug of sorts to one of Greenland’s largest glaciers, lodged in a fjord that, from the height of its mountain walls down to the lowest point of the seafloor, is deeper than the Grand Canyon. There’s enough ice piled up behind Petermann to raise oceans globally by nearly a foot someday.
The question for Muenchow is no longer whether Petermann is changing — it’s how fast it could give up still more ice to the seas. That’s why he and British Antarctic Survey colleague Keith Nicholls ventured here by helicopter to take the measure of the Petermann shelf, which had been shifting and surging in a way that damaged the scientific instruments they had left behind a year earlier — behaving as though it didn’t want to be known.

CHAPTER 1

Hard data, hard to reach

Greenland is the largest island on Earth and home to its second-largest ice sheet after East Antarctica. It’s pouring 281 billion tons of that ice into the ocean each year, a major contribution to rising seas. Much of the loss comes from some 200 outlet glaciers, which extend out to the sea like fingers of the larger ice sheet.
The great fear is that Greenland’s ice loss is accelerating, and that’s why much attention has been directed at Petermann. One expert has called it one of the island’s three major “floodgates,” and the only one that has not yet opened. In part, the Petermann Ice Shelf has been slower to disintegrate simply because it is in a much colder place.
But that is beginning to change, and Muenchow and Nicholls are trying to understand the mechanics of how it might break apart.
They are old-school scientists, focused on gathering hard data in the world’s most remote places. Each has a “great record in terms of publications,” says Marco Tedesco, a Greenland researcher at Columbia University’s Lamont-Doherty Earth Observatory.
Muenchow, who was born in Germany, traveled to the United States to pursue oceanography and got his PhD studying the Delaware River. But before long he became infatuated with the idea of probing places that few have reached before, despite the hardships of leaving family and the comforts of home. The search took him five times to the Nares Strait, a tiny ocean passageway between northwest Greenland and Canada near Petermann glacier.

How the ice shelf edge has
changed since 2008
10 MILES
Sea ice
2008
shelf
edge
2016
shelf edge
2014
Rift in ice
Data
stations
along
meltstream
Grounding
line
Ice moves
roughly one
kilometer a year
Summer
meltwater
Greenland ice sheet
Composite satellite image from ESA Sentinel
LARIS KARKLIS/THE WASHINGTON POST
“To me, this hardship is pleasure as it always shatters prior expectations,” he explained later. “The only constant, it feels, is change and new insights. This drives me. Perhaps I am addicted to it. . . . The field work gives me this chance or opportunity to ‘reset’ and take a new look at what I thought I knew or I knew I did not understand.”
Nicholls, meanwhile, is an expert in his own sort of extreme pursuit — using hot water to drill holes hundreds of feet through the enormous ice shelves of Antarctica and now Greenland, and then analyzing data from the ocean beneath them.
They first worked together in 2015 as part of a major National Science Foundation-sponsored ship voyage to Petermann, where Muenchow was taking ocean measurements and Nicholls was busy drilling through the ice. Now, they had returned as part of a much smaller mission to recover data and determine why their instruments had gone dead. Two Washington Post journalists accompanied them with the support of the foundation which, in keeping with its policies, provided transportation and accommodations.
Their expedition began a day earlier from the United States’ Thule Air Base and the small village of Qaanaaq, Greenland’s most northern permanent settlement. This time they sought to reach Petermann by helicopter. The 300-mile journey was so long they had to break it in stages, picking up fuel from caches they strategically placed a year earlier when they had visited aboard a foundation-supported icebreaker.
A deep gulley with rushing water feeds into a river on Petermann Glacier. The shelf has reached a record low size after losing pieces larger than Manhattan in recent years.. (Whitney Shefte / The Washington Post)

CHAPTER 2

The data stopped coming

As they crossed the desolate landscape, their Air Greenland chopper finally emerged from a series of inland canyons into the air above the ice shelf, which was streaked with thick veins of blue ice amid a sea of white, the landscape covered with meltwater pools. It was as though a fly had suddenly buzzed in through the window of a cathedral. The ice shelf was its sprawling floor, and it was rimmed on either side by enormous, symmetrical mountain walls sculpted into shapes resembling flying buttresses.
Along the shelf’s central aisle ran one of Petermann’s most distinctive features — a 30-mile-long meltwater river. A year ago, Muenchow and Nicholls had established three scientific data-collection stations on its banks — drilling through the football-field-thick ice and extending ocean sensors, attached to a long cable, into the dark and half-mile-deep waters beneath the shelf. These were to detect whether warming ocean water was causing a double-whammy of damage to Petermann by melting it from below, even as the warm air temperatures melt it from above.
But the main station had stopped feeding back any data in February. Now, Muenchow and Nicholls were here to see what had happened.
Muenchow sat in the second row of seats in the helicopter with earphones on to muffle the noise.
His chief fear, he had explained before the trip, was that this might just be a mop-up mission: That the flowing ice might have damaged the stations beyond repair, snapping the cables extending into the ocean below, and that there would be no data to retrieve.
Muenchow said he had prepared himself to be devastated if the data was lost. But he said he would give himself about “15 minutes” to mourn before adjusting to see what could be salvaged.

Petermann glacier had just lost a chunk of its ice shelf, and NASA satellite images of the enormous ice “island” were circulating widely. At the hearing, Jay Inslee (D), then a congressman and now the governor of Washington, pressed Muenchow to be more outspoken about what was happening to the planet. The scientist demurred.
The evidence “does not conclusively prove that this specific event is global warming,” Muenchow testified. The logic was simple — breaking off large pieces might just be something Petermann glacier does occasionally, if you go back far enough in time.
But two years later, another vast island of ice cleaved from Petermann. That’s when Muenchow began to change his mind. The shelf had by then lost 23 miles of its prior length, reaching a record low in size.
“It’s two extreme events in six years, so something is happening,” Muenchow said.
In science — unlike in politics — being hesitant when you don’t know something, and being willing to change your mind in the face of new evidence, are virtues. He has since joined a growing wave of researchers working to learn more.
Climate change doubters have continued to suggest — from a distance — that Petermann’s huge ice losses are just normal glacier behavior. Muenchow himself, no dogmatist about the matter, can still entertain the case for skepticism, in part because the glacier has never been as well observed as it now, by scientists and satellites. Conceivably, it lost as much ice during previous periods as it has lost in the present. Muenchow doubts that – the idea that the glacier has shifted to a new state, he says, is supported by the “preponderance of the evidence.”
Petermann is looking suspicious again: At its front edge near the ocean, it features several additional cracks, including one that penetrates further toward the center than the others, arcing inward toward the central river and the shelf’s thinnest region.
“I already see the beginning of a third break-up,” Muenchow said.

When ice shelves break away, the ice that had once fed the shelf instead flows directly into the ocean, helping to raise sea levels throughout the world. In the case of Petermann, that plug runs about 30 miles in length, floating over the fjord, to a “grounding line.” This is where the shelf ends and the ice touches the sea floor in 2,000-foot-deep waters. Farther back, the ice gets thicker and deeper.
Scientists worry about possible “marine ice sheet instability” in the region, which would allow warm ocean water to melt the base of the glacier and chase it backward — hastening its losses along the way.
It’s not clear where the retreat would end. Oregon State University geologist Alan Mix said researchers have recently discovered that behind Petermann glacier lies an enormous, ancient canyon that is nearly 500 miles long and cuts all the way to the center of the Greenland ice sheet. It was probably carved by a river long ago.
So if the ice shelf collapses and Petermann glacier starts breaking off large icebergs and retreating backward, the ocean could someday gain access to this canyon.
“You can think about this as a huge drain of Greenland,” Mix said of the Petermann fjord. “This is where the water gets out.”
A pool of frozen water rests on the bumpy, often wet surface of Petermann Glacier. (Whitney Shefte / The Washington Post)

CHAPTER 4

When warm and cold collide

As the helicopter circled the researchers’ central station, Nicholls spotted the non-responsive device first. Its weather beacon listed at a 30-degree angle, felled by the moving ice. That would explain why the station had not transmitted data in six months.
Nicholls turned to his partner in the helicopter and drew a finger across his neck in a sign recognized universally: dead.
Half an hour later, Muenchow was on the ice, busily using a hacksaw to cut through the weather station’s steel pole to try to free it. “I’m making progress,” he huffed, the temperatures at around 39 degrees in the early afternoon of an August day that, this far north, won’t ever lose its light.
Muenchow retrieved a memory card and plugged it into his computer. It was the moment of truth.
And also, it turned out, one of pure scientific joy — the data was there, and the sensors were still recording more.
After realizing that the last recording was just a few hours old, Muenchow was speechless. He covered his mouth with his hands.
“That’s good news,” Nicholls deadpanned.
Muenchow started clapping softly.
It would take weeks, after the scientists got back home, to analyze the data. But they already knew it would give them an unprecedented image of the behavior of ocean waters in the deep cavity beneath the ice.
There is still some mystery about how warm waters might be reaching and interacting with the Petermann Ice Shelf.
The Atlantic’s warm, salty waters reach this fjord through a convoluted route that takes them north off Greenland’s eastern coast, along a full circuit of the Arctic Ocean, and ultimately south through the Nares Strait. Here, warmer Atlantic-originating waters are found at the greatest depths because their saltiness gives them more density, while fresher and colder Arctic waters lie at the surface.
Sources: Andreas Muenchow and ESA Sentinel
The warm waters then penetrate beneath the ice shelf and to the base of the glacier, and are somehow managing to melt and thin it at a rate of 30 to 40 feet per year. And the Atlantic waters in the area are getting still warmer over time.
But the fundamental question is what’s pulling the Atlantic waters in and causing them to touch the shelf?
One key idea, Nicholls suspected, turns on all the wetness atop Petermann — a sign of ever-rising Arctic air temperatures. “Our major hypothesis,” he said, is that some of this water is running off somehow, entering the ocean, and in the process, helping to draw in the warm water that causes the most extensive melting.
It isn’t clear where the fresh water is spilling out, but it could be further up the fjord from here, at the grounding line. That would mean cold, fresh and buoyant water is suddenly pouring into warm, salty Atlantic-originating water at extreme, dark ocean depths. This interaction is probably very turbulent and dramatic, and it could be the key to growing melting.
“Because the base of the ice shelf is sloping upwards, this water flows quickly up the bottom of the ice shelf, and as it does that, it mixes and stirs in the warm water from beneath,” said Nicholls.
This may help explain the most dramatic feature atop Petermann — its central river. It has a frozen surface in some areas, but flowing water underneath. It’s noisy — constantly the source of cracking, crashing and sliding sounds. It’s fed by a seemingly endless network of ice-banked tributaries that, amid above-freezing temperatures on the second day of the team’s trip, were roaring with water.
But why is it here in the first place? The reason seems to be below the surface, where ocean waters have carved an undersea channel into the bottom of the shelf. That changes the surface of the shelf, too, because thinner ice won’t float as high above the surface of the water. The result is a depression or chasm at the surface, which meltwater, flowing downhill, naturally fills.
This river, and the channel beneath it, seems implicated in the ice shelf’s undoing. According to Muenchow, the previous major ice loss events seemed to occur whenever a crack in the shelf, coming in from the side, finally extends as far as the river.
From above Petermann Glacier, cracks are visible on its surface. (Whitney Shefte / The Washington Post)

CHAPTER 5

Time to get out


After an intense 24 hours of work in near-freezing temperatures — tearing apart, rebuilding and reprogramming scientific stations, and consigning four out of nine ocean sensors to a watery oblivion — the researchers had one remaining quest to complete.
With their time on the ice dwindling, they wanted to install a radar that measures the ice’s thickness roughly a mile from the main station. This would let them compare the shelf’s thinning in different places.
The helicopter had dropped off equipment at the spot, but to save fuel and flying time, they decided to hike the distance, guided by a GPS device.
The hike was at a slight incline, out of Petermann’s riverine depression and into higher terrain. At one point, the trek required crossing a small, flowing tributary. Nicholls used the long bamboo poles and drill bits he was carrying to test the opposite ice bank, making sure it provided a good foothold, before they did so.
After three hours of work, trouble arose. A cold rain had arrived, turning the ice treacherously slick. “The one thing you do not want is rain on an ice shelf,” Nicholls said.
The water complicated the trip back to the helicopter. The team made it to within about a football field of the aircraft but could go no further, blocked by a rain-swollen series of streams flowing so fast that their roars were audible.
So with time running short before the pilots would start out to look for them, the researchers had to backtrack half a mile, where they found a crossing at higher ground.
Returning to the helicopter, they considered the expedition a success — while the scientists had to jettison two scientific stations, they repaired one and established another. Most of all, they retrieved key ocean data. But they acknowledge that in the vastness of the Petermann Ice Shelf, those are still just two small points taking measurements.
A month and a half later, as he was about to embark from Alaska on a research vessel bound for a different part of the Arctic, Muenchow told The Post what the data from beneath Petermann revealed.
A sensor in 3,000-foot-deep waters had found that in the warm, salty Atlantic layer, temperatures were even warmer than just a year earlier, in 2015. Those waters are likely flowing toward Petermann glacier’s grounding line and helping to melt the shelf from below.
“The temperatures at the bottom end of the array continue to increase,” said Muenchow. “It’s getting warmer.”
In a recent paper he and Nicholls pointed out that several other glaciers in Greenland have already lost their ice shelves. Their work suggests that Petermann is now following this path.


4/6/17

Crippled Atlantic currents triggered ice age climate change


Melting icebergs may have triggered or exacerbated ice age slowdowns to Atlantic Ocean currents.

SCIENCE

By Eric Hand
The last ice age wasn’t one long big chill. Dozens of times temperatures abruptly rose or fell, causing all manner of ecological change. Mysteriously, ice cores from Greenland and Antarctica show that these sudden shifts—which occurred every 1500 years or so—were out of sync in the two hemispheres: When it got cold in the north, it grew warm in the south, and vice versa. Now, scientists have implicated the culprit behind those seesaws—changes to a conveyor belt of ocean currents known as the Atlantic Meridional Overturning Circulation (AMOC).
These currents, which today drive the Gulf Stream, bring warm surface waters north and send cold, deeper waters south. But they weakened suddenly and drastically, nearly to the point of stopping, just before several periods of abrupt climate change, researchers report today in Science. In a matter of decades, temperatures plummeted in the north, as the currents brought less warmth in that direction. Meanwhile, the backlog of warm, southern waters allowed the Southern Hemisphere to heat up.
AMOC slowdowns have long been suspected as the cause of the climate swings during the last ice age, which lasted from 110,000 to 15,000 years ago, but never definitively shown. The new study “is the best demonstration that this indeed happened,” says Jerry McManus, a paleo-oceanographer at Columbia University’s Lamont-Doherty Earth Observatory, and a study author. “It is very convincing evidence,” adds Andreas Schmittner, a climate scientist at Oregon State University, Corvallis. “We did not know that the circulation changed during these shorter intervals.”
To assess the strength of ancient ocean currents over the course of 35,000 years in the middle of the ice age, McManus and his colleagues examined a 10-meter section of a 38-meter sediment core drilled from the bottom of the Atlantic Ocean. The core came from an elevated patch of sea floor known as the Bermuda Rise, where sediments accumulate abnormally fast. The thick layer allows for a more detailed reading of chemical changes within the sediments when they were buried.

Circulation slowdown

Ice Age climate swings may have come from the weakening of powerful Atlantic currents, in which shallow, warm waters move north (red), and deep, cold waters move south (blue).

The researchers measured the ratio of two products of radioactive decay: protactinium-231 and thorium-230. These daughter isotopes come from trace amounts of uranium that are dissolved everywhere in seawater. If the ocean were as still as a bathtub, the two daughter isotopes would bind to sediment particles, settle downward, and become buried at a constant ratio. But thorium gloms on to particles more readily than protactinium. It is therefore buried more readily, whereas protactinium tends to be carried away by ocean currents for burial elsewhere. At places like the Bermuda Rise, where the Atlantic conveyor belt is typically strong, little protactinium ends up in the sediments—except for four instances when the ratio to thorium rose sharply in a matter of decades to centuries, indicating the AMOC’s sudden weakening.
A 2014 study of the AMOC, based on a core from a nearby spot on the Bermuda Rise, found that the currents maintained much of their strength throughout the last glacial period. But because that team took fewer samples from the core and missed the sharp swings in strength. “Now these [new] guys have increased incredibly the data resolution,” says Jörg Lippold, a paleo-oceanographer at the University of Heidelberg in Germany and the leader of the 2014 study. “They found the peaks we missed.”
Still unclear is what triggered the AMOC’s sudden slowdowns. Many of the drops correspond to so-called Heinrich events: rapid releases of icebergs from Canada’s ice sheet. These iceberg armadas often chugged through the Hudson Strait of Canada and may have discharged more ice into the Atlantic Ocean than contained in the entire ice cap of Greenland, raising ancient sea levels by 10 meters. The meltwater brought incredible amounts of freshwater to the North Atlantic, precisely where ocean currents cool off and sink. Because freshwater is less dense than saltwater, it can plug up the AMOC, preventing the overturning and deep water formation the fuels the circulation’s engine. The slowdowns persisted for 1500 years or so, then the AMOC would suddenly regain strength as freshwater melt dissipated and the currents reached a certain threshold, McManus says.
But weak AMOCs are not always accompanied by a Heinrich event, and the timing is fuzzy—some Heinrich events seem to occur after the AMOC already began to weaken. “The Heinrich events may be a response to the change in the overturning circulation rather than a cause,” Schmittner says.
Another question is whether the AMOC—currently known to be in decline—could drop off suddenly today, as depicted in the 2004 movie The Day After Tomorrow, causing temperatures to plummet across northwestern Europe. Schmittner says the past provides an eye-opener. “It’s evidence that this really did happen in the past, on short time scales.” But McManus says that studies looking deeper into the ice ages have found that the 1500-year climate oscillations tend not to be nearly as strong during interglacial periods. “It would suggest that this kind of thing isn’t so likely to happen today,” he says. On the other hand, he adds, “In most interglacials, Greenland didn’t melt … and Greenland is currently melting.”
Posted in: 
DOI: 10.1126/science.aaf5828



Gulf Stream Slowing Down Faster Than Ever, Scientists Say

Home  Climate Science 
Posted on April 12, 2016Kevin Hester 

The Gulf Stream that helps to keep Britain from freezing over in winter is slowing down faster now than at any time in the past millennium according to a study suggesting that major changes are taking place to the ocean currents of the North Atlantic.
Scientists believe that the huge volumes of freshwater flowing into the North Atlantic from the rapidly melting ice cap of Greenland have slowed down the ocean “engine” that drives the Gulf Stream from the Caribbean towards north-west Europe, bringing heat equivalent to the output of a million power stations.’
———- ———-

And similarly: ‘While you were distracted, this climate change warning arrived‘:  ‘With dire warnings of catastrophic sea level rise and superstorms capable of pitching 1,000 tonne mega-boulders onto shorelines, scientist James Hansen sounded an alarm over continued global warming.  
In a video, coinciding with a release of a revised version of paper Hansen wrote  with 18 other authors, Hansen warned of “feedbacks” between the oceans and ice sheets.’
[dk:  like him or not, he does have gravitas here.  He seems more urgent than a year ago…]