Has the puzzle of rapid climate change in the last ice age been solved?
Bremerhaven, Germany, August 19th, 2014. Over the past one hundred thousand years cold temperatures largely prevailed over the planet in what is known as the last ice age. However, the cold period was repeatedly interrupted by much warmer climate conditions. Scientists have long attempted to find out why these drastic temperature jumps of up to ten degrees took place in the far northern latitudes within just a few decades. Now, for the first time, a group of researchers at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI), have been able to reconstruct these climate changes during the last ice age using a series of model simulations.
The surprising finding is that minor variations in the ice sheet size can be sufficient to trigger abrupt climate changes. The new study was published online in the scientific journal Nature last week and will be appearing in the 21 August print issue.
During the last ice age a large part of North America was covered with a massive ice sheet up to 3km thick. The water stored in this ice sheet is part of the reason why the sea level was then about 120 meters lower than today. Young Chinese scientist Xu Zhang, lead author of the study who undertook his PhD at the Alfred Wegener Institute, explains. “The rapid climate changes known in the scientific world as Dansgaard-Oeschger events were limited to a period of time from 110,000 to 23,000 years before present. The abrupt climate changes did not take place at the extreme low sea levels, corresponding to the time of maximum glaciation 20,000 years ago, nor at high sea levels such as those prevailing today – they occurred during periods of intermediate ice volume and intermediate sea levels.” The results presented by the AWI researchers can explain the history of climate changes during glacial periods, comparing simulated model data with that retrieved from ice cores and marine sediments.
How rapid temperature changes might have occurred during times when the Northern Hemisphere ice sheets were at intermediate sizes
During the cold stadial periods of the last ice age, massive ice sheets covered northern parts of North America and Europe. Strong westerly winds drove the Arctic sea ice southward, even as far as the French coast. Since the extended ice cover over the North Atlantic prevented the exchange of heat between the atmosphere and the ocean, the strong driving forces for the ocean currents that prevail today were lacking. Ocean circulation, which is a powerful “conveyor belt” in the world’s oceans, was thus much weaker than at present, and consequently transported less heat to northern regions.
During the extended cold phases the ice sheets continued to thicken. When higher ice sheets prevailed over North America, typical in periods of intermediate sea levels, the prevailing westerly winds split into two branches. The major wind field ran to the north of the so-called Laurentide Ice Sheet and ensured that the sea ice boundary off the European coast shifted to the north. Ice-free seas permit heat exchange to take place between the atmosphere and the ocean. At the same time, the southern branch of the northwesterly winds drove warmer water into the ice-free areas of the northeast Atlantic and thus amplified the transportation of heat to the north. The modified conditions stimulated enhanced circulation in the ocean. Consequently, a thicker Laurentide Ice Sheet over North America resulted in increased ocean circulation and therefore greater transportation of heat to the north. The climate in the Northern Hemisphere became dramatically warmer within a few decades until, due to the retreat of the glaciers over North America and the renewed change in wind conditions, it began to cool off again.
“Using the simulations performed with our climate model, we were able to demonstrate that the climate system can respond to small changes with abrupt climate swings,” explains Professor Gerrit Lohmann, leader of the Paleoclimate Dynamics group at the Alfred Wegener Institute, Germany. In doing so he illustrates the new study’s significance with regards to contemporary climate change. “At medium sea levels, powerful forces, such as the dramatic acceleration of polar ice cap melting, are not necessary to result in abrupt climate shifts and associated drastic temperature changes.”
At present, the extent of Arctic sea ice is far less than during the last glacial period. The Laurentide Ice Sheet, the major driving force for ocean circulation during the glacials, has also disappeared. Climate changes following the pattern of the last ice age are therefore not to be anticipated under today’s conditions.
“There are apparently some situations in which the climate system is more resistant to change while in others the system tends toward strong fluctuations,” summarises Gerrit Lohmann. “In terms of the Earth’s history, we are currently in one of the climate system’s more stable phases. The preconditions which gave rise to rapid temperature changes during the last ice age do not exist today. But this does not mean that sudden climate changes can be excluded in the future.”
The paper:
Xu Zhang, Gerrit Lohmann, Gregor Knorr, Conor Purcell:
Abrupt glacial climate shifts controlled by ice sheet changes.
Nature, DOI: 10.1038/nature13592
During glacial periods of the Late Pleistocene, an abundance of proxy data demonstrates the existence of large and repeated millennial-scale warming episodes, known as Dansgaard–Oeschger (DO) events1. This ubiquitous feature of rapid glacial climate change can be extended back as far as 800,000 years before present (bp) in the ice core record2, and has drawn broad attention within the science and policy-making communities alike3. Many studies have been dedicated to investigating the underlying causes of these changes, but no coherent mechanism has yet been identified3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15. Here we show, by using a comprehensive fully coupled model16, that gradual changes in the height of the Northern Hemisphere ice sheets (NHISs) can alter the coupled atmosphere–ocean system and cause rapid glacial climate shifts closely resembling DO events. The simulated global climate responses—including abrupt warming in the North Atlantic, a northward shift of the tropical rainbelts, and Southern Hemisphere cooling related to the bipolar seesaw—are generally consistent with empirical evidence1, 3, 17. As a result of the coexistence of two glacial ocean circulation states at intermediate heights of the ice sheets, minor changes in the height of the NHISs and the amount of atmospheric CO2 can trigger the rapid climate transitions via a local positive atmosphere–ocean–sea-ice feedback in the North Atlantic. Our results, although based on a single model, thus provide a coherent concept for understanding the recorded millennial-scale variability and abrupt climate changes in the coupled atmosphere–ocean system, as well as their linkages to the volume of the intermediate ice sheets during glacials.
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“The surprising finding is that minor variations in the ice sheet size can be sufficient to trigger abrupt climate changes.”
The Northern hemisphere ice sheet melts by two thirds every summer. (The temperature fluctuation is also around 30C from mid winter to mid summer.) I wonder what they mean by “minor variations” because we see major variations every year.
Models again?
Seems to me like they programmed the response from the climate model that they wanted. Now, if they have physical evidence for their claims, then it might be worth considering. But really, what difference will it make to the life of any human in the next thousand years if they are right?
Alfred Wegener?….what did he ever know?!!!
Knows nothing about ocean currents too.
Is this more of that ‘settled science’ again ?
Seriously it seems that every real piece of research that comes out underlines just how little claimate scientists really know about the subject. Clearly they cannot ‘know’ that alterations in the height of ice sheets triggered a warm period, At best its a hypothesis (guess) based on computer models and a lot of assumptions.
I propose the CGAW crowd switch from computer to female models. It’d be a lot less tedious and have the same predictive powers.
They’re looking in the right direction. The ocean circulation system. Their model gains credibility by acting like a nonlinear chaotic system that in certain regimes can switch between alternative attractors. Such (Hopf) bifurcations are the classic hallmark of nonlinear dynamics approaching the border of chaos.
Note also no real role for CO2 which only gets a perfunctory nod.
A way to visualise such system is as a landscape of stability in which a high peak represents an unstable hard to achieve state while a low elevation is a valley of stability toward which the system like a ball rolled over the landscape naturally settles. In a bi-stable system you have two neighboring valleys of stability. High mountains between the two valleys would prevent the system moving from one valley to the next. However of you have a mountain pass or a “saddle” of lower elevation linking the two valleys, the system only needs a smaller “push” to roll it from one valley to the next through the pass. This is the thermodynamic landscape of bistability.
I occasionally comment about the importance of the N. Atlantic-Arctic link to climate change
Denmark straits, the major feature of the area, is the main bottleneck on the superhighway in the oceans global circulation. On both sides to the north and the south of the straits the Great Ocean Conveyer belt
http://www.grida.no/climate/ipcc_tar/slides/large/04.18.jpg
releases massive amounts of heat (several hundred watts/ m2).
Secular magnetic field delta (usually an indication of the earth’s crust-mantle events ) for the area (from both paleo and instrumental records) also closely matches the Loehle’s (non tree rings) GT reconstruction.
Tectonic activity (according to data 1860 to present) in the N. Atlantic – Arctic environs follows the closely integrated sunspot number for the period.
Solar magnetic field cannot penetrate to the depths of the Earth’s core, and yet the solar magnetic field (22 year cycle) and a much larger magnetic ripple superimposed on the Earth’s field (orders of magnitude greater than the heliospheric field at the earth’s orbit) have same frequency and are in phase.
Energy in the equal measure across the centuries, comes from the above, but the ‘control knob’ most likely comes from the below.
I don’t buy it. Firstly, the D/O events are VERY rapid in the rising temperature phase – some ice profiles from Greenland showing a major shift from one year to the next. Whilst it is the case that only a consistent shift in wind direction can do this, it is also the case that wind directions around the Greenland ice-cap and the Iceland/Norwegian Seas are controlled by the track of cyclones – which are themselves directed by the jetstream (and not the other way around, as some would imagine). A shift from a tight westerly jet to a loopy meridional jet is what best would account for major shifts in wind patterns in this region. And the jets are controlled by….? Well, some time back NASA’s Drew Shindell thought these changes were controlled by the UV status of the Sun. But he went on to work on other more important things like ‘global warming’ under James Hansen’s direction. Low UV leads to loose and loopy jets with less containment of Arctic cold air masses. Low UV is associated with very low sunspot numbers and low solar magnetic field status.
Further, between 50,000 and 30,000 BP there are two sets of warming cycles, of roughly 10k years each, but with a pronounced Fibonacci series periodicity – where each peak decays slowly, as would an harmonic series, and the breadth of the peak has an 8:5:3:2:1 ratio. This pattern does not entirely disappear during the Holocene – but is certainly obscured – for example, the 5-part of the cycle is suppressed, but the 8 (Holocene Climate Optimum), 3 (Minoan warm period), 2 (Roman warm period) and 1 (Medieval warm period) are clear. The current warm period may be the tail of the series – the final 1, or the beginning of a new series – implying a long period of warmth. Thus far, these patterns are ignored because nobody has a clue where such a series could originate!
Some researchers have held that the exit from an ice-age is actually a more extreme version of the D/O event. I would not be surprised if there was a 100,000 year magnetic periodicity – and this would then compete with the orbital theories – which are regarded by many as insufficient in any case to explain the sudden rises.
These Greenland changes correspond to temperature, wind and ocean currents shifts throughout the North Atlantic, with knock-on effects in other ocean basins, some of which are in any case subject to shifts in jetstream and vortex tracking, and hence these reflect global shifts – such as the Little Ice Age has proven to represent.
“But this does not mean that sudden climate changes can be excluded in the future.”
So tampering with the atmosphere could cause us to freeze or burn up.
10 degrees in 87000 years Hmm
I think that, by using the words “dramatic temperature changes,” they snuck this paper by the Alarmist gatekeepers. In actual fact they are describing a cycle, in some senses like an ice-age version of the AMO. This is dangerous turf to tread, for once you start to admit the existence of such cycles then, rather than a “Death Cycle,” the decreases of sea-ice between 1979 and 2012 becomes half-a-cycle, and perfectly normal and natural.
I had my own ideas about how nature engineers such a cycle, and our host was kind enough to print it here: http://wattsupwiththat.com/2014/01/09/author-of-its-own-demise-musings-on-the-amo/
Now we need, as Ray Boorman above rightly states, more of the “physical evidence” from the historical record, to back up the above paper’s claims. However I don’t think this paper should be mocked without serious consideration. Not that it will help me pay my heating bill this winter, but better understanding the AMO and other ocean cycles could result in better responses to the cards nature deals out, (far better than the idiotic responses idiotic governments are coming up with now.)
RE: Peter Taylor says:
August 21, 2014 at 2:22 am
I like your ideas, but you need to be aware you can’t just state that the jet stream influences the surface storm track, and there is no interaction back between the surface and the jet stream. Or, well, you can say it, but if you observe the arctic you might note a few things that could make you modify your statement.
One thing I’ve noticed is that storms seem to prefer open water to ice-covered areas. Just as hurricanes weaken over land, arctic lows weaken over ice-covered areas, likely because they lose their supply of rising warmth and moisture.
Just for the fun of it, observe the behavior of lows in the arctic as winter comes on, but before the ice has grown back. When the icecap is cold to the north, and the tundra is cold to the south, they seem to like to ripple along the area of open water between. That may be a case where the surface is effecting the higher levels of the atmosphere. Just an idea.
A lot of the ice-melt comes from warm water under the ice. If a shift in currents under the ice could change the areas that were ice-covered, I think the storm track might be changed as well.
Spam filter ate my last comment. I probably shouldn’t have used the word “idiotic.” Sorry.
Lewis P Buckingham on August 21, 2014 at 2:22 am
phlogiston says:
August 21, 2014 at 2:10 am
‘Note also no real role for CO2 which only gets a perfunctory nod.’
It would be interesting to know what the role of CO2 is in their opinion since they do cite ,
‘minor changes in the height of the NHISs and the amount of atmospheric CO2 can trigger the rapid climate transitions via a local positive atmosphere–ocean–sea-ice feedback in the North Atlantic’
Is this CO2 ‘trigger’ part of the model, or just the change in the ocean currents triggered by heightened ice sheet formation.
I did not read yet the full paper. So indeed its hard to be clear exactly what role they ascribe to co2. They may be playing the invert-cause-and-effect trick by confusing an increase in co2 responsive to increasing sea temperatures as some kind of driving role for co2. Or maybe not.
I’m guessing that since co2 was mentioned only once in the abstract and as the second item in a pair of factors after ice thickness, that it is maybe just a genuflection and the paper looks beyond co2 myopia in an enlightened way.
Caleb, went back and read your Epic of last January. The reason why ice floats and doesn’t sink an allows the sea to freeze from the bottom up. is that at 4oC as the water gets ready to crystalize into ice it expands and its density decreases. That means that ice floats without any worries about salt contents etc etc.
Link between Arctic atmospheric pressure and North Atlantic SST (AMO) shown
HERE in de-trended versions, appears logical and looks benign enough, but one shouldn’t be deceived by appearance (and few years of delay in the AMO). It may be hard for the science (settled or un-settled) to come to terms with and accept only possible explanation, beyond just calling it coincidence.
“Nothing in nature is by coincidence…Something appears to be coincidence only because of our lack of knowledge.” Baruch Spinoza
Model fantasy.
The truly sudden changes of temperature in climate history show that what we are experiencing is not close to extreme by historical standards. The climate fear machine tends to skip past this pesky evidence.
Small changes in initial conditions producing huge changes in the overall system are characteristic of chaotic systems. This paper seems to be pointing at the climate system as chaotic. Not surprising.
RE: Ken Calvert says:
August 21, 2014 at 3:35 am
We are a little off topic, but the point I was making in that essay was not that ice floats, but that cold water sinks. Therefore, in theory at least, the surface water would sink before it got a chance to freeze, replaced by upwelling warm water from beneath. (Obviously this doesn’t happen to a degree that prevents the formation of the icecap, in reality.)
The above is only true for salt water. Fresh water is strange stuff, for once you get to roughly 36 degrees (F) cold water rises. If you took the temperature of a pond just before it froze the water at the surface would be 32.1 degrees, the water an inch down 33 degrees, the water two inches down would be 34 degrees, and so on. It is counter-intuitive, but true: Cold water rises.
A really cool thing happens in the spring, when that layer of cold water on top of a pond or lake is first exposed after ice melts. That layer of cold water is just sitting there minding its own business on top, colder than the water beneath, however cold water only floats on top of warmer water when it is above 36 degrees. On a bright, sunny day in the spring, especially when there is little wind, that surface layer can be warmed above 36 degrees, whereupon it becomes denser than the water beneath, and quite abruptly the entire surface of a still lake sinks. Called “overturning,” it can actually bring bits of dead pond-weed from the bottom up to the surface when it happens. It is strange to see, if you happen to be out fishing on that day, and can either make the fishing great or ruin it, depending on what fish you are after.
Like I said, water is strange stuff.
Correction to above comment: ” …cold water only floats on top of warmer water when it is BELOW 36 degrees.”
@Caleb – Thanks for the additional insight. YOur comments are food for thought.
To me, this is the type of situation in which climate models are most useful – speculation leading toward new possible insights. No government policy implications involved, in fact, if you read carefully, no real implications about today’s climate at all. It’s nice to see inquiries into our planet’s climate that do not appear to involve “me too” climate change motives. I found the post quite interesting and as a layman, understandable. Thanks, Anthony.