Guest essay by Eric Worrall
Yesterday’s “The Day After Tomorrow” climate explainer’s excuse for cold winters is back – research suggests that the North Atlantic current is weaker than anytime for the last 1000 years
Climate change dials down Atlantic Ocean heating system
By Victoria Gill
Science correspondent, BBC News
11 April 2018
A significant shift in the system of ocean currents that helps keep parts of Europe warm could send temperatures in the UK lower, scientists have found.
They say the Atlantic Ocean circulation system is weaker now than it has been for more than 1,000 years – and has changed significantly in the past 150.
The study, in the journal Nature, says it may be a response to increased melting ice and is likely to continue.
Researchers say that could have an impact on Atlantic ecosystems.
Scientists involved in the Atlas project – the largest study of deep Atlantic ecosystems ever undertaken – say the impact will not be of the order played out in the 2004 Hollywood blockbuster The Day After Tomorrow.
But they say changes to the conveyor-belt-like system – also known as the Atlantic Meridional Overturning Circulation (Amoc) – could cool the North Atlantic and north-west Europe and transform some deep-ocean ecosystems.
That could also affect temperature-sensitive species like coral, and even Atlantic cod.
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Read more: https://www.bbc.com/news/amp/science-environment-43713719
The abstract of the paper;
Observed fingerprint of a weakening Atlantic Ocean overturning circulation
L. Caesar, S. Rahmstorf, A. Robinson, G. Feulner & V. Saba
The Atlantic meridional overturning circulation (AMOC)—a system of ocean currents in the North Atlantic—has a major impact on climate, yet its evolution during the industrial era is poorly known owing to a lack of direct current measurements. Here we provide evidence for a weakening of the AMOC by about 3 ± 1 sverdrups (around 15 per cent) since the mid-twentieth century. This weakening is revealed by a characteristic spatial and seasonal sea-surface temperature ‘fingerprint’—consisting of a pattern of cooling in the subpolar Atlantic Ocean and warming in the Gulf Stream region—and is calibrated through an ensemble of model simulations from the CMIP5 project. We find this fingerprint both in a high-resolution climate model in response to increasing atmospheric carbon dioxide concentrations, and in the temperature trends observed since the late nineteenth century. The pattern can be explained by a slowdown in the AMOC and reduced northward heat transport, as well as an associated northward shift of the Gulf Stream. Comparisons with recent direct measurements from the RAPID project and several other studies provide a consistent depiction of record-low AMOC values in recent years.
Read more (paywalled): https://www.nature.com/articles/s41586-018-0006-5
A NASA study in 2010 using direct satellite measurement rather than models suggested there is no evidence the North Atlantic Current is slowing.
NASA Study Finds Atlantic ‘Conveyor Belt’ Not Slowing
03.25.10
PASADENA, Calif. – New NASA measurements of the Atlantic Meridional Overturning Circulation, part of the global ocean conveyor belt that helps regulate climate around the North Atlantic, show no significant slowing over the past 15 years. The data suggest the circulation may have even sped up slightly in the recent past.
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Until recently, the only direct measurements of the circulation’s strength have been from ship-based surveys and a set of moorings anchored to the ocean floor in the mid-latitudes. Willis’ new technique is based on data from NASA satellite altimeters, which measure changes in the height of the sea surface, as well as data from Argo profiling floats. The international Argo array, supported in part by the National Oceanic and Atmospheric Administration, includes approximately 3,000 robotic floats that measure temperature, salinity and velocity across the world’s ocean.
With this new technique, Willis was able to calculate changes in the northward-flowing part of the circulation at about 41 degrees latitude, roughly between New York and northern Portugal. Combining satellite and float measurements, he found no change in the strength of the circulation overturning from 2002 to 2009. Looking further back with satellite altimeter data alone before the float data were available, [Josh] Willis found evidence that the circulation had sped up about 20 percent from 1993 to 2009. This is the longest direct record of variability in the Atlantic overturning to date and the only one at high latitudes.
The latest climate models predict the overturning circulation will slow down as greenhouse gases warm the planet and melting ice adds freshwater to the ocean. “Warm, freshwater is lighter and sinks less readily than cold, salty water,” Willis explained.
For now, however, there are no signs of a slowdown in the circulation. “The changes we’re seeing in overturning strength are probably part of a natural cycle,” said Willis. “The slight increase in overturning since 1993 coincides with a decades-long natural pattern of Atlantic heating and cooling.”
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Read more: https://www.nasa.gov/topics/earth/features/atlantic20100325.html
One model based study suggests a 15% slowdown, a direct measurement suggests a 20% acceleration. Settled science anyone?
Umm… if BBC is really all that worried about carbon emissions, they could contribute to a drop in it by putting clothes pins on their newscasters’ noses and sewing their mouths shut. That would solve that problem. In fact, if all these science people turned off their computers and did some real-world, outdoors observations, they could drop the CO2 load drastically in one single week. But will they do that? No. NO, they will not.
Sorry, but I’m in a foul mood because my lawn is barely one inch high instead of the two inches it should have reached by now and the trees are NOT leafing out. NOTHING. NADA. It is mid-April and in the upper Midwest it might as well be mid-March. No bugs emerging for birds to eat. Only a few robins have returned. And for the first time since Oct. 4, 2017, the temperature reached 65F in my area, then immediately began to drop again.
I do not enjoy the prospect of running my furnace into the mid-June.
It was still snowing here only last week, and we still have heavy night frosts all this week.
For a world where my children are not supposed to know what snow looks like, it looks like doing a pretty damn good job of being a warmer world!
From what I can see, we have the 2nd year in succession of an extremely cold delayed spring.
(Baltic and Finland area)
The positive feedback loop of salinity-downwelling-Greenland ice melt gives the AMOC its chaotic bistability. The AMOC and Gulf Stream are driven by the salinity-downwelling positive feedback. The Gulf Stream brings high salinity water to the North Atlantic. When it cools its higher salinity makes it exceptionally dense so that it downwells all the way down to the ocean floor. This is the deep water formation in the far North Atlantic and the Norwegian Sea. This deep cold dense water flows south, completing the loop of the AMOC. By doing so it in turn propels the northward Gulf Stream up on the surface, reinforcing the whole circuit with positive feedback. This feedback-reinforced AMOC considerably warms North West Europe and transports warmer water right up to the Arctic.
In the paradigm of current climate science this positive feedback at the heart of the AMOC would be assumed to be runaway and would soon be expected (projected) to turn the Atlantic Ocean into a whirling maelstrom washing machine. However in the real world of complex systems positive feedback does not do this, instead it causes self-limited excursions, oscillation and intermittency. Each “run” of the positive feedback causes eventually a negative feedback, which cuts it off. In the North Atlantic the negative feedback that cuts off the Gulf and cold downwelling feedback is Greenland ice melt and a resulting freshwater pulse, which chokes off the cold water formation and downwelling.
What results from these intermittent pulses or chaotic oscillations of the AMOC is what we call the AMO. This gives a chaotic instability to the whole NH climate.
It is not a regular 60 year oscillation. Instead it is an internal nonlinear oscillation of varying and irregular timing, with possible astrophysical (e.g. solar) external weak periodic forcing.
By contrast in the SH there is no such instability, there is no meridionally bounded ocean south of Africa and South America. In the Southern Ocean you have the unimpeded circumpolar circulation. Thus no salinity feedback driving meridional current loops. Thus in the SH both oceanography and climate are much more stable and changes occur more smoothly over much longer timescales.
I find it exceedingly hard to believe that a process as powerful as the AMOC can be driven by the cool part of the cycle (refer to the work of Nicolas Léonard Sadi Carnot). Your car may run better with a fully-functional cooling system, and a faulty cooling system could cause it to breakdown. But it will not run AT ALL without the burning of fuel in the hot part of the cycle.
This leads me to think that the Gulf Stream and AMOC must actually be driven by tropical heat. The cold down-welling part of the cycle must only determine the route and efficiency of these currents.
J
You’re right ultimately cold doesn’t drive a heat engine. The whole climate is a heat engine responding to the large asymmetry of solar heat delivery to earth, high intensity at the equator, low at the poles.
However in a feedback you can’t say what drives what. It’s the wrong question. The system drives itself.
AMOC is an oscillatory system with positive and negative feedbacks. There are several players – the Gulf Stream, atmospheric pressure gradients and their variations, meridional salinity transport, downwelling and deep water formation, Greenland melt, etc. Even inter hemispheric heat piracy south of the Caribbean inputs into it. Which of all these is the driver? None of them. The whole system drives itself.
The same is true of another intermittent oscillatory system, ENSO, and its players – Peruvian upwelling, trade winds, the SOI, etc.
Feedback? The AMOC is popularly portrayed as a conveyor – not as an oscillatory system. And the name “Atlantic meridional overturning circulation” corroborates this. I can quite believe that the AMO is an oscillation (the name Atlantic Multidecadal Oscillation rather gives it away) but not the AMOC.
Fake science relies on headlines for currency.
There are actually TWO studies on this issue and both are in the SAME ISSUE of Nature – April 12th !!
The study referred to by Eric Worrall, which is by Caesar et al., says: the “modelled response is entirely CO2-driven”. So with the evidence that such models are increasingly out of line with the facts this study can I think be safely discounted.
The second study is by Thornalley et al. and does not support or link to CO2 alarmism. It refers to a “transition to a weakened AMOC at the start of the industrial era” some 150 years ago and its explanation is of: “… either hysteresis of the AMOC in response to an early climate forcing – natural (solar, volcanic)or anthropogenic (greenhouse gases, aerosols, land-use change – or that continued climate forcing, such as the melting of the Greenland Ice Sheet, has been sufficient to keep the AMOC weak or cause further weakening )”. So the gradual solar forcing in the last 150 years could be the cause.
PLEASE would Eric Worrall do an amending post.
Tony Carey
Ganymede Climate Services