There was a new paper out in Science last week about past changes in ocean circulation. It's from a team led by L. Gene Henry, a graduate student at Columbia University’s Lamont-Doherty Earth Observatory. What they were exploring was the past relationship between climate and ocean circulation in the North Atlantic, several thousand years ago. The paper focused on changes in the Atlantic meridional overturning circulation (AMOC).
The scientists compared climate changes with ocean circulation, during marine isotope stage three (MIS3). MIS3 spans the glacial time period from around twenty-five to sixty thousand years ago. Marine isotope stages are alternating warm and cool periods in the past. The isotope refers to oxygen isotope data (δ18O, which is the ratio of 18O to 16O). This is what's used to work out changes in temperature (from deep sea core samples, which is where the "marine" comes from). MIS1 is now. Even numbers have high δ18O and are cold. Odd numbers have low δ18O and are warm - or warmer. So MIS3 was "warm", but not really, as you can see from the chart below. This is from Lisiecki and Raymo (2015), where I've circled MIS3 to the left of the chart:
|Figure 1 | The LR04 benthic δ18O stack constructed by the graphic correlation of 57 globally distributed benthic δ18O records. The stack is plotted using the LR04 age model described in section 5 and with new MIS labels for the early Pliocene (section 6.2). Source: Lisiecki and Raymo (2015)|
In this new paper, Henry and his co-authors explain their work, and the climate back in MIS3, which was colder than now but not as cold as the last glacial maximum:
Here we investigate the relationship between Northern Hemispheric climate as recorded in Greenland ice cores and marine sediments, along with isotopic deep-sea paleoproxies sensitive to changes in North Atlantic Deep Water (NADW) production and AMOC transport during marine isotope stage three (MIS3). Throughout that time, when climate was neither as warm as today nor as cold as the last glacial maximum (LGM), ice sheets of intermediate size blanketed much of the northern hemisphere, and large millennial stadial - interstadial climate swings (6, 8) provide a wide dynamic range that allows examination of the ocean’s role in abrupt change..The Atlantic meridional overturning circulation (AMOC) brings warm, salty surface water up from the tropics. Salty water is more dense than fresher water, and the water sinks in the north Atlantic. In the deep ocean, the water flows south. You can see it in the top left of the diagram below. The red flow is in the upper ocean and the blue flow is in the deeper ocean.
|Figure 2 | Illustration depicting the overturning circulation of the global ocean. Throughout the Atlantic Ocean, the circulation carries warm waters (red arrows) northward near the surface and cold deep waters (blue arrows) southward. Image credit: NASA/JPL|
Back to the Henry paper. The scientists did a high resolution analysis of a 35 metre deep ocean core (at 4541m depth), which was recovered from the Bermuda Rise. The location of the core is shown below (click to enlarge as always):
|Figure 3 | Location sediment core CDH19 shown as star (33° 41.443’ N; 57° 34.559’ W, 4541m water depth) with Pa/Th ratios (black dots) in core top sediments used with ODV DIVA gridding to produce the color contours. White areas contain no data. Source: Henry16|
As you can guess from Figure 2 above, this core would have information about what happens in the deep ocean when water comes south from the North Atlantic. The authors explained:
Because this region of the deep North Atlantic is characterized by steep lateral gradients in tracers of NADW [North Atlantic Deep Water] and Antarctic Bottom Water (AABW), the Bermuda Rise has been intensively used to explore the connection between changes in ocean circulation and climate.
The bi-polar seesaw
The researchers used chemical tracers in the sediment to work out the speed of AMOC during each abrupt climate change during the last ice age. From the press release:
The chemical tracers show that the speed of the ocean overturning circulation changed first, and that sea surface temperature changed a while later. That suggests that cooling may start with changes in the ocean circulation, influencing the northern sea surface and atmosphere, said co-author Jerry McManus, a professor at Lamont-Doherty Earth Observatory. Evidence from ice cores and deep-sea sediment has shown that the northern climate also cooled before the southern climate during these abrupt changes, creating a “bipolar seesaw,” with the north cool while the south was warm, and the south cooling as the north warmed.
The chicken or the egg?
However the authors were cautious, as scientists are, and the press release continues (my emphasis):
The scientists stress that more work is needed to determine whether changes in ocean circulation initiated the abrupt climate changes or were an intermediary effect initially triggered by something else. “Our study supports the view that changes in ocean circulation were at least in part responsible for causing abrupt climate changes. However, what in turn caused those changes in circulation remains a mystery,” Henry said.They seem to be suggesting that it hasn't happened in the past 10,000 years because it hasn't been cold enough, so there hasn't been enough ice.
Also unclear is why these abrupt climate shifts, also seen in previous ice ages, haven’t happened in the past 10,000 years. The instability appears to occur only in certain temperature ranges, and when there is a large amount of land ice that could contribute freshwater.
Positive feedback from icebergs
The paper itself has more detail. The way I read it, the main points that emerge from the study relate to timing of the events. Possible answers to the chicken/egg conundrum. The authors were looking at the timing of the changes in ocean circulation, the timing of the warming and cooling in the northern hemisphere climate (particularly Greenland), and the timing of the advance and retreat of ice on land. They wrote about the possible scenarios:
Both SST and Greenland temperature proxies lag the ocean circulation in a consistent fashion, and in turn these northern changes have been demonstrated to lead Antarctic temperatures....
...The evidence here nevertheless indicates that the greatest AMOC reduction and the coldest stadial intervals accompanied the largest iceberg discharges. This suggests that the iceberg discharges may have provided a positive feedback mechanism to accelerate the initial cooling within each multi millennial climate cycle. In addition, the extended Heinrich-stadial reductions in AMOC observed in this study coincide with intervals of rising atmospheric CO2 (33), while CO2 declined when AMOC increased during the subsequent sharp transitions to northern interstadials, supporting a potential influence on the atmosphere by the deep circulation on millennial timescales.One reason the authors refer to a positive feedback from iceberg discharge is a study they referenced, by Stephen Barker et al from last year (in Nature). They looked over a much longer time span (400,000 years). What they found was that there was "a systematic delay between pronounced surface cooling and the arrival of ice-rafted debris at a site southwest of Iceland over the past four glacial cycles, implying that in general icebergs arrived too late to have triggered cooling." They posited that rather than trigger cooling, "freshwater derived from melting icebergs may provide a positive feedback for enhancing and or prolonging stadial conditions".
Henry et al (in Science) concluded with reference to changes in meridional heat transport by the ocean being a likely trigger for abrupt warming in the northern hemisphere back in MIS3:
The robust relationship of reductions in export of northern deep waters evident in reduced 231Paxs export and decreased δ13CBF before and during stadial periods [colder], and the dramatic increases in both during interstadials [warmer] provides direct evidence for the role of AMOC in abrupt glacial climate change. The sequence of marked circulation changes and northern hemisphere climate detailed here, combined with the demonstrated lag of Antarctic temperature variations (5), strongly implicates changes in meridional heat transport by the ocean as a trigger for abrupt northern hemisphere warming and the tipping of the “bipolar seesaw (26).”
AMOC is an active area of research
The topic of ocean circulation, ice and temperature seems to be an active area of research. One example (of several): there was another paper on a similar topic (from a different perspective) in February this year, this time in Nature's open access journal, Scientific Reports. The authors were looking at a similar period, 20,000 to 70,000 years ago. They describe a push and pull relationship. The press release states in part:
During the coldest periods of the last ice age the Nordic seas were covered with a permanent layer of sea ice. The pump stopped transporting the heat northward. The heat accumulated in the southern oceans. However, the warming was not restricted to the south.
"Our results show that it continued all the way to Iceland. The warming was slow and gradual, and happened simultaneously in both hemispheres. Little by little the warm Atlantic water penetrated into the Nordic sea underneath the ice cover. It melted the ice from below. Once the ice was gone, the pump started up again, bringing additional warm water into the Nordic seas. And we got a warmer period for 50 years. " says Rasmussen.
Large ice sheets continued however, to cover the continents around the Nordic seas. In contact with the warm ocean water they started calving. This delivered icebergs and fresh water into the sea and caused a cooling down of the surface water. The seas were again frozen. And the pump slowed down...
..."There are no symmetrical processes in the north and the south -- the climate changes were principally governed by simultaneous warming and the constant closing and re-opening of the sink pump in the Nordic seas" says Tine Rasmussen.
The AMOC now, and how to detect a major slowdown
The subject is also much studied to see what is happening now. This is in part because changes in the North Atlantic affect large populations, particularly in western Europe. Last year there was the paper by Stefan Rahmstorf and colleagues, which sent deniers at WUWT into a panic. It suggested that the melting of the Greenland ice sheet had slowed AMOC over the twentieth century. Before that, in 2014, there was a paper by Chris Boulton and co, about signals that could be used as an early warning of a collapse of AMOC. Neither of these papers suggested that AMOC was about to shut down. The Boulton paper was about signals that could detect changes 250 years before any collapse.
Back in May this year, Eric Hand wrote another article in Science about current monitoring of AMOC, using buoys, in which he wrote about the difficulties of directly measuring the strength of the AMOC, and recent observations:
Overall, the subtropical array has measured a decline in the AMOC from an average strength of 20 sverdrups in 2004 to about 15 sverdrups a decade later. (A sverdrup is a unit of flux of 1 million cubic meters of water per second—roughly the total flow of all the world's rivers into the oceans.) That decline is an order of magnitude more than models suggest could be due to climate change. Scientists suspect some natural cycle is to blame, such as the 60- to 70-year cycle of varying sea temperatures called the Atlantic Multidecadal Oscillation, says Meric Srokosz, an oceanographer at the University of Southampton in the United Kingdom, and the science coordinator for RAPID, the U.K.-funded portion of the 26.5°N array. Initial analysis of the latest unpublished data, from the 18 months through October 2015, shows the AMOC's average strength leveling out at about 15.5 sverdrups, Srokosz says. “It could be a hiatus, or it could start coming back up,” he says. It will take another decade of measurements to separate the climate change effect from natural variability, he says.
What happened 25,000 to 60,000 years ago sparks another conspiracy theory at WUWT
Over at WUWT, Eric Worrall decides that the Henry paper is part of a conspiracy to explain some mythical forthcoming global cooling (archived here). His headline was:
Are Scientists Preparing for a FlipFlop Back to Global Cooling Predictions?What he failed to understand was that any changes to AMOC would, at best, only affect part of the northern hemisphere. It would be not at all likely to result in a drop in global temperatures, given the amount of CO2 in the air.
Eric wrote about a provocative closing paragraph to an article by Eric Hand in Science, about the paper:
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.”You'll recall from above, the part in the press release:
Also unclear is why these abrupt climate shifts, also seen in previous ice ages, haven’t happened in the past 10,000 years. The instability appears to occur only in certain temperature ranges, and when there is a large amount of land ice that could contribute freshwater.At the end of his article, Eric expanded on his conspiratorial thinking, writing:
Naturally most of the climate scientists who make such predictions expect the cooling to occur over a relatively short timescale, before the ice melt forcing which causes the predicted cooling is overwhelmed by our continued sinful emissions of CO2. But a fallback prediction of imminent abrupt cooling does conveniently make it rather difficult to falsify anthropogenic climate theories based on temperature alone, should global temperatures suddenly drop.
From the WUWT comments
Would you be surprised if there were conspiracy theories at WUWT? If you've ever been there little would surprise you.
Jimmy Haigh seems to think that researchers get huge dollar. I don't think post-grads earn much money in academia. (Jimmy also probably thinks that climate science is a hoax.)
July 2, 2016 at 8:14 pm
Who gives a toss, eh? As long as the filthy lucre keeps rolling in…
prjindigo is patiently waiting for the temperature to drop. He's been waiting for his entire life and it hasn't happened yet. (The last time there was a "coldest" year on record was 1909.) I don't think the penny will drop ever, despite the temperature not.
July 2, 2016 at 8:15 pmJim Watson mutters something completely irrelevant to the subject. He might be thinking of S. Fred Singer's predictions.
Now I’m wanting to have stored up a whole bunch of popcorn like the preppers were six years ago… just so I have a munchy while I watch this freak weather show.
The reason the Earth STAYS in a specific temperature range is because any time it leaves it violently reacts to go back. Homeostasis isn’t just a definition on a science test!
July 2, 2016 at 8:20 pm
Predict all possible outcomes and when one of them happens–voila!–you’ll look like a genius!
TG is another typical denier, who sees everything through his own greed lens. He cannot conceive of a world where people choose to do work because they love it, or because they are curious, or that status might come from academic achievement not money:
July 2, 2016 at 8:24 pm
Warmist will blow hot or cold in the mildest breeze any which way for a free paid for climate conference (As long as it’s not in Detroit) and another further study grant.
Tom Halla has difficulty grasping the finer points of warmer Earth → Greenland ice melts → more fresh water in the North Atlantic → slowing the AMOC → less warm water from the tropics → colder weather in western Europe.
July 2, 2016 at 8:44 pmAs Eric Hand wrote in one of his Science articles:
So cooling will actually be due to global warming? Nice thing if you can get away with it.
...in 2009 to 2010, the average strength of the AMOC plunged by about 30%. Heat remained in the tropics rather than being delivered to northern latitudes. The consequences included an unusually harsh European winter, a strong Atlantic Basin hurricane season, and—because a strong AMOC keeps water away from land—an extreme sea level rise of nearly 13 centimeters along the North American coast north of New York City.As you'd expect, there were a lot of ignorant empty protests at science from the scientific illiterati, like this one from markl:
July 2, 2016 at 8:51 pm
When are people going to catch on to this ongoing charade? Or have they already?
Denier Don Easterbrook doesn't back up his claim, which is par for the course for deniers. Because he can't. Anyone who's looked at the subject knows there have been bi-polar temperature swings:
July 2, 2016 at 8:56 pm
“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).”
This statement is not true–hundreds of radiocarbon and beryllium isotope dates of glacial deposits in both hemispheres now show that glacial conditions were synchronous in both hemisphere during the last Ice Age. This undercuts the basic premise of this study.
Walter Sobchak opts for a Dunning-Kruger comment, thinking that a little knowledge means all knowledge. Someone once told him about the Coriolis effect but he missed the class on thermohaline circulation. Or maybe he can only hold one thought in his head at one time.
July 2, 2016 at 9:14 pm
Rubbish, the currents are driven by the Corriolis. They won’t change much unless Velikovsky was right.
Salvatore Del Prete says it's nonsense to think that cooler surface water in the North Atlantic could have any effect on temperature in the northern hemisphere.
July 2, 2016 at 9:36 pm
This is BS. The cooling will be due to weak solar conditions and the associated secondary effects.
higley7 calls scientists "idiots", which is a bit rich when he's writing on the "world's most widely read" idiot blog under an article from one of the idiot blog's most prolific idiots:
July 2, 2016 at 9:50 pm
Think interns of water viscosity. When the Atlantic cools, the Gulf Stream slows and logically Europe and New England cools. When the ocean warms, the system speeds up and delivers more heat to the north. The idiot warmists want to claim that a bit of fresh water from melting ice will stop this flow, such that warming slows the flow and cooling accelerates the flow—the opposite of whet is observed. As the above record indicates, melting ice had nothing to do with the thermostat effect of the AMOC.
Mark - Helsinki is another person who seems to think that cooling must be just around the corner. Surely it must cool soon, after 107 years of warming:
July 2, 2016 at 11:23 pm
Cooling is the death of AGW, no matter how they try wing it
Javier comments about Eric Worrall's reference to James Hansen's work. At least he understood it, which is more than Eric Worrall and most WUWT-ers can manage.
July 3, 2016 at 1:18 am
The cooling has to come accompanied of a slowdown of AMOC, and be of regional nature, otherwise it is of not use to Hansen’s hypothesis. Very difficult conditions to meet indeed. A global cooling wouldn’t cut it.
References and further reading
L. G. Henry, J. F. McManus, W. B. Curry, N. L. Roberts, A. M. Piotrowski, L. D. Keigwin. "North Atlantic ocean circulation and abrupt climate change during the last glaciation." Science 30 Jun 2016. DOI: 10.1126/science.aaf5529
- Ocean Circulation Implicated in Past Abrupt Climate Changes - press release at the Earth Institute Columbia University
- Crippled Atlantic currents triggered ice age climate change - article by Eric Hand, in Science News
- 'Ice age blob' of warm ocean water discovered south of Greenland - press release at ScienceDaily.com
Robson, Jon, Pablo Ortega, and Rowan Sutton. "A reversal of climatic trends in the North Atlantic since 2005." Nature Geoscience (2016). DOI: 10.1038/NGEO2727
Barker, Stephen, James Chen, Xun Gong, Lukas Jonkers, Gregor Knorr, and David Thornalley. "Icebergs not the trigger for North Atlantic cold events." Nature 520, no. 7547 (2015): 333-336. doi:10.1038/nature14330
- Iceberg armadas not the cause of North Atlantic cooling - press release at ScienceDaily.com
Park, Jong-yeon, Jürgen Bader, and Daniela Matei. "Anthropogenic Mediterranean warming essential driver for present and future Sahel rainfall." Nature Climate Change (2016). doi:10.1038/nclimate3065
- Sahel rainfall recovery linked to warming Mediterranean, study says - article by Robert McSweeney at the Carbon Brief
- Slowing ocean circulation prompts more Mann bashing from deniers at WUWT - March 2015
- An AMOC early warning signal prompts panic at WUWT - December 2014
- Taking a 68,000 year break from conspiracies, at the poles - May 2015
- Fred Singer's Predictions: Something for everyone - April 2015
- Denier Don is Angry - April 2013