Arguments
Are we heading into a new Ice Age?
What the science says...
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Worry about global warming impacts in the next 100 years, not an ice age in over 10,000 years. |
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Climate Myth...
We're heading into an ice age
"One day you'll wake up - or you won't wake up, rather - buried beneath nine stories of snow. It's all part of a dependable, predictable cycle, a natural cycle that returns like clockwork every 11,500 years. And since the last ice age ended almost exactly 11,500 years ago…" (Ice Age Now)
At a glance
In something like a Day after Tomorrow scenario, the idea that a new ice-age was just around the corner was the subject of a book, a DVD and a website created in 2002. The author was a retired architect, by the way. Fortunately for us, both the movie and the quote above are figments of someone's fertile imagination. But let's have a quick look at ice-ages and what makes them tick, after which we hope you will agree that the notion that another ice-age is just around the corner is nonsensical.
Ice-ages, also known as glacials, are cold periods that occur in a cyclic fashion within an Icehouse climate state. Earth's climate has been mostly of the Hothouse type (no Polar ice-sheets). However, on occasion it has cooled down into Icehouse, as has been the case in the last few million years. There are regular variations in Earth's orbit around the Sun, taking place over tens of thousands of years. These affect the amount of Solar radiation reaching our planet. During the Icehouse state, such variations can lower and raise planetary temperature sufficiently to trigger swings between cold glacials – when ice-sheets expand towards the Equator – and mild interglacials – when the ice retreats back polewards.
To give an idea of the time-scales involved, Europe and North America have seen glacials and interglacials come and go repeatedly over the last 2.5 million years, this being known as the Quaternary Period of geological time. The last glacial period started 115,000 years ago and the Last Glacial Maximum (LGM), when the greatest ice extent was reached, was around 22,000 years ago. The current interglacial – also known as the Holocene, commenced 11,700 years ago.
A general pattern may be seen here with a long cooling down towards Glacial Maximum but a relatively quick warming into an interglacial. The speed of the warming-up part of the cycle is due to climate feedbacks. Removal of pale, reflective snow and ice cover revealing the darker ground beneath allows more solar heat energy to be soaked up. Melting of permafrost releases carbon dioxide and methane. These and other feedbacks serve to amplify the warming effect, speeding it up.
However, our burning of fossil fuels has happened on such a vast scale that we have blown such factors apart. The atmospheric concentration of CO2 has risen well above the 180-280 ppm range typical of recent glacial-interglacial cycles. The current level, getting on for 420 ppm, is more typical of the mid-Pliocene. That was a geological epoch that happened around a million years before the start of the Quaternary. Mid-Pliocene ice-sheets were much smaller than those of the present day. Rather than being due another glaciation, we can expect a continued transition towards mid-Pliocene conditions.
Please use this form to provide feedback about this new "At a glance" section, which was updated on May 27, 2023 to improve its readability. Read a more technical version below or dig deeper via the tabs above!
Further details
Because our current interglacial (the Holocene) has already lasted approximately 12,000 years, it has led some to claim that a new ice age is imminent. Is this a valid claim? No.
To explore this topic further, it is necessary to understand what has caused the cyclic shifts between ice ages and interglacials during the Quaternary period (fig. 1). Such shifts are in part a response to regular changes in the Earth’s orbit and tilt, which affect the amount of summer sunlight reaching high northern latitudes and were described by the Milankovitch Cycles, first proposed in the early 20th Century by Serbian mathematician Milutin Milankovitch (1879-1958). For more about Milankovitch cycles this NASA page offers lots of graphics and explanations.
Figure 1: Temperature change through the late Quaternary from the Vostok ice-core, Antarctica (Petit et al. 2000). The timing of warmer interglacials is highlighted in green; our current interglacial, the Holocene, is the one on the far right of the graph.
When incoming sunlight declines in the high north, the rate of summer snow and ice-melt declines and the ice sheets begin to grow. When incoming sunlight increases, the opposite happens. So where are we in these cycles today? Changes in both the orbit and tilt of the Earth do indeed indicate that – were they singularly responsible for climate shifts - the Earth should be slowly cooling. However, recent research shows that is too simple. That's because we now have analyses of ice-cores going back 800,000 years or more. We have devised ways to use stable isotope ratios of various elements in things like fossils and we have developed many other proxy methods for telling us more about conditions in the relatively recent past that the Quaternary represents.
A number of irregularities in glacial-interglacial cycles have been determined, for example times when interglacials were skipped when orbital patterns suggest they should have happened. (Koehler and Van de Wal 2021). Such research has also been aimed at resolving the question of why Earth's 41,000 year obliquity cycle was a strong driver of glacial-interglacial transitions up until around one million years ago. Since then, glacials have instead typically lasted for much longer - around 100,000 years.
The importance of feedbacks within Earth's climate system has been increasingly recognised as the decades have gone by. A good example is the speed of transition from glacial to interglacial, which is relatively rapid because certain very effective climate feedbacks are involved. One such feedback involves albedo, defined as the ability of different bodies to absorb or reflect sunlight (e,g, Thackeray and Fletcher 2016).
Albedo is expressed on a scale of 0 (black body, absorbs everything) to 1 (white body, reflects everything. Fresh snow has a high albedo of as much as 0.9, whereas the muck revealed when old snow and ice cover melts has a much lower one in the range 0.2 to 0.4 – it can absorb lots more solar energy. So melting snow and ice leads to more heat energy retention, amplifying the warming (Fig. 2).
Fig. 2: Albedo feedback explained. Freshly-fallen snow is highly reflective of incoming sunshine, so that most of the solar energy is simply bounced back towards space. Bare sea ice can potentially absorb about half of the incoming energy, so if conditions become warmer, causing the snow to melt, there’s more energy retained on Earth. If the sea ice melts too, then almost all of the incoming solar energy is absorbed by the much darker surface of the sea. So an initial warming directly results in further warming. Graphic: John Mason.
Another feedback happens when permafrost gets thawed out, since the ground is then able to release previously trapped CO2 and methane. During a glacial, the extent of permafrost is vast, so as it thaws, the release of such gases occurs on an enormous scale – again, amplifying the warming.
Researchers have also modelled ice-sheet dynamics, investigating how the sheets behaved as they melted, for example. It has been found that the shorter-lived, lower latitude Northern Hemisphere ice-sheets that existed prior to one million years ago were much thinner and therefore easier to melt. So ice-sheet dynamics looks to have a role in the much longer freeze-ups of the past million years. This all goes to show that glacial periods arise through a whole lot of factors interacting with one another, of which orbital cycles are but one, albeit important, cog in the gearbox and are not necessarily able to drive the climate system from one state (glacial) to another (interglacial) in total isolation (e.g. Bintanja and Van de Wal 2008; Berends et al. 2021).
Talking of cogs in the gearbox, we are another – and a big one. Our intentional disturbance of carbon reservoir rocks – what we do when we seek, extract and burn the fossil fuels – is unique in the geological record. It's a one-off in the planet's 4.56 billion year long history and while the consequent overloading of atmospheric CO2 levels is still insufficient to take Earth back into a Hothouse state yet, it is perfectly adequate to prevent another glaciation any time soon.
Last updated on 27 May 2023 by John Mason. View Archives
Kevin @297, it is a shame that after the long wait you could only come up with a link to an article that I had previously linked to (@ 293, second link). It would have been nice if you had come up with something new. In this case the something new is a report of a new excavation of a late iron age, or possibly early Roman, vinyard (or possibly asparagus garden) in Hertfordshire.
It should be noted that viticulture probably survived in England throughout the dark ages, and certainly survived in England throughout the Little Ice Age, only ceasing after WWI, due to the commercial inability to compete with cheap French imports. It restarted in England following WWII. Given that, why do you regard the presence of vinyards in Roman Enland as proof it was warmer at that time than now rather than simply proof that it was not colder than the LIA (when grapes were also grown in England to make commercial wines)?
Also of interest is this study of modern viticulture in Baltic coastal regions. It shows this interesting map which shows commercial viticulture in Denmark and Sweden:
Again, given the northern distribution of commercial viticulture, which includes not only commerical vinyards in Denmark and Sweden, but vinyards being developed with the intention of becoming commercial in Scotland), why do you regard the presence of vinyards in more southerly regions in England as proof that the Roman Warm Period or Medieval Warm Period was warmer than current temperatures?
And finally, why do you regard evidence that it was warmer then than now in just one region of the world (evidence you have so far failed to produce) as proof that it was warmer than globally rather then now?
http://hockeyschtick.blogspot.com/2013/01/new-paper-finds-new-mexico-was-warmer.html
This paper shows temps from New Mexico area.
Your point? Global temperature reconstructions are estimated from a network of proxies from locations around the globe. If you think this one place is significant, then why do you deny the significance of other proxies, or those places where ice retreat is exposing bare rock for first time since HCO ?
Kevin @302, the first thing to notice is that the modern average is not necessarilly the average over the last few decades. Indeed, by convenction, in paleographic studies the present is 1950 unless otherwise stated. The article your denier propaganda site refers to is behind a paywall, so I am unable to determine which interval was used to determine the "modern average". I am able, however, that they determine an annual average temperature as a function of the reconstructed July temperature, and that their modern annual average of 11 C corresponds to a July temperature of 22 C. Further, the warmest temperature they show for July approximately 2000 years ago (see Table 2). The NCDC shows the twentieth Century average of July temperatures in New Mexico to be just under 23 C, so it appears the "modern average" is below the average over the 20th century. For comparison, the average over the last decade to be around 23.5 C (the equivalent of a mean annual temperature of 1.25 C above the twentieth century mean). The peak modern July temperature is 25 C, the equivalent of 2.5 C above the "modern average". In other words, it appears that the reconstruction does not show temperatures higher than that of the late twentieth century at all. (Treating studies showing warming greater than 1950, or in one case 1895 as being warmer than the late twentieth century has been a common denier trick.)
Second, if you look at the following graph you will see that warm and cold periods at different locations do not coordinate in time. Ergo simply picking out a location and saying it was warmer at some time in the past, and then a different location and saying it was warmer in the past does not tell you anything about whether the global average is warmer. To do that you need a multiproxy reconstruction. Oddly deniers have been unwilling to make multiproxy reconstruction despite all the purported examples of warmer temperatures in the past they claim to have found. Note that I say purported because what deniers claim, and what was actually found in the scientific study often differ widely (as shown above). I personally suspect that this unwillingness to make reconstructions is because the deniers know that any such attempt will give the game away.
There is one exception to the rule about deniers and reconstructions. Loehle (2007), and in particular as corrected by Loehle and McCulloch 2008 (Link for both papers). There are a few oddities about that reconstruction. First, many of the proxies used had previously been used by Moberg 2005, who Loehle cites as a source. Not all of Moberg's non-treering proxies were used, however, as shown in the graph above. One wonders why Loehle found it necessary to develop a methodology that excluded the Agassiz Ice Cap and Arabian Sea proxies, for example.
Of greater concern is that Loehle simply takes the average of all proxies to determine his reconstruction. The problem with that is found in this map of the location of his proxies (from Loehle and McCulloch 2008):
A quick count shows that 10 of his 18 proxies come from around the North Atlantic; another 4 come from China, and two each from Indonesia and South Africa. That is, they are heavily localized. Simply taking a mean of the values treats the North Atlantic (including Europe and North America) as though it constituted 56% of the Earth's surface; China as though it were 22%; Indonesia and South Africa as though they were 11% each; and the entire rest of the globe as though it did not exist.
What is even odder is that reconstructions using more widely dispersed proxies have shown where was warm, and where cool in the MWP. Curiously the distribution of proxies used by Loehle gives most weight to the area that was warmest, the next most weight to the next warmest area, and zero weight to any area that was known to be cool. It is an extraordinary coincidence.
And despite all these biases, it turns out that Loehle's reconstruction still shows a RWP and MWP cooler than the late twentieth century (note the instrumental record shown as a dashed line):
It is no wonder deniers don't like reconstructions.
And it is also no wonder that people who are interested in science want to see them; because if you haven't made the effort of making a reconstruction, you have not analyzed which intervals are warmer or cooler than others. At most you have attempted to pull wool over peoples eyes with an assortment of random facts.
Tom, I have access to the paper. Let me know if interested though it doesnt seem that interesting to global temp proxies for reasons you have outlined. Their weather data is based in 34-130 year records from stations within 100km, though for that diagram, it appears to be based on closest station at Mountainair, 160m higher than the study area. I dont think the authors put much stock on the absolute values but are instead focussed on the patterns - hot/dry v cool/wet, and particularly in the speed and severity of local climate change in 2nd century.
scaddenp @305, thankyou. Allowing that the mean for a portion of the state may not match the mean of the state, but that the anomalies are likely to be close, we can shift the state anomalies down by 0.9 C to give them the same mean as the local stations. In that cast, the 21st century average to date is 0.8 C greater than the mean for July, or an estimated 0.65 C greater for the annual average. The peak July averages are 1.9 C greater than the mean, for an estimated annual average 1.6 C greater than the mean.
On these figures, peak medieval warmth in a small part of New Mexico is may be just greater than the current warmth. That is not certain, however, as the NCDC data represents only a 117 year record, and as the additional 17 years required to match are almost certainly cooler than the mean, the differentials are underestimated.
I don't think anything else in my analysis requires correction.
On closer reading, the "current weather" in figure of (11degree) was chosen to be that of just the station at Mountainair, since the proxy record from C3 regression closely matched the values for that station.
One further word. It is fortunate that this area isnt a global proxy because the authors present evidence for a temperature rise of over 2 degrees in just 59 years during 2nd century. That would have been bad on a global scale! I wonder if Kevin has actually read the paper and how he feels about the way that the paper has been represented on his "skeptic" site?
The clear link between temperature and drought in the region doesnt bode well for its residents.
You've all got this backwards on the planning & ethics front. We developed skills over centuries to know that climate will typically go into a very annoying cold state every few thousand years & found that it can be ameliorated simply by burning old rotted vegetables we've found. Instead of leaving most of them for future humans to regulate their climate for a pleasant life, we greedy pigs are taking the whole lot now when it isn't needed. That's an "intergenerational evil" involving very very old rotted vegetables.
Hi there grindup,
I must be having a slow day... Could you spell out the argument for me?
Thanks.
Tzedakis et al. (2012), "Can We Predict the Duration of an Interglacial?" mentions the end of the current interglacial only insofar as saying "We should also be able to predict the duration of the current interglacial in the absence of anthropogenic interference," and "glacial inception is possible despite the subdued insolation forcing, if CO2 concentrations were 240±5ppmv (Tzedakis et al., 2012)." [Emphasis is mine.]
That latter citation is of "Determining the Natural Length of the Current Interglacial," which says "No glacial inception is projected to occur at the current atmospheric CO2 concentrations of 390 ppmv. ... The end of the current interglacial would occur within the next 1500 years, if atmospheric CO2 concentrations did not exceed 240±5 ppmv." [Emphasis is mine.]
Of course there is no chance at all of CO2 falling that low for the next several hundreds and probably not for the next several thousands of years (see Figure A in another post), so we're not getting a new "Ice Age" for a really, really long time.
jhnplmr (on an inappropriate thread) replied to my statement "your own calculations show only a 3 W/M^2 decrease of insolation in that one tiny region from the year 1950 to the year 3950--only .7%," with "0.7% of what?"
Insolation in the year 3950 is predicted to be .7% less than the insolation in 1950.
You also replied "Do you think it helpful to suddenly introduce 1950 into the discussion? I based my year zero on the graph on 1950 as it made it easier to transfer the data to the spreadsheet. You seem to want to consider a nebulous point between 1BC and 1AD as year zero. Let us call the present day 1950 and call it year zero it will save a lot of confusion. The solar minimum will occur in 2000 years time, 3950AD using your scale." Climatologists, among other scientists, use 1950 as the standard meaning of "present." So the year 0 in your NOAA data source means 1950. If you did not know that already, you would learned it if you had bothered to read the readme file that accompanies those data files.
You wrote you "also don't want us to slide into another glacial period." There is no risk of that anytime soon; see the papers I linked to in my April 26 comment.
312 Tom Dayton
"So the year 0 in your NOAA data source means 1950. If you did not know that already, you would learned it if you had bothered to read the readme file that accompanies those data files."
Which I had, I pointed out to you that I had used 1950 as my datum point for year zero. "I based my year zero on the graph on 1950 as it made it easier to transfer the data to the spreadsheet". It was you who introduced the 3950AD concept.
"You wrote you "also don't want us to slide into another glacial period." There is no risk of that anytime soon;"
I have pointed out this several times. You have mentioned my comments on this point yourself. You seem to have developed a habit of repeating my statements back to me as if I hadn't made them in the first place.
The difference between my position and most contributors on this board is that I consider the contribution of man-made warming beneficial as it has stopped us sliding into a prolonged cooler period, we started to do this 10,000 years ago until AGW stopped it in its tracks and stabilised the temperature. The other difference is that I consider trying to balance on the cusp of an interglacial period indefinitely is an impossible task but this is apparently what most of your contributors want!
You call your opponents "climate change deniers" but try to keep a changing climate constant!
jhnplmr,
I don't know why you think contributors here want to keep the planet at a constant temperature.
The future ice age (glacial, whatever you want to call it) is going to happen very slowly. The last one took tens of thousands of years from peak to trough, about 6 degrees average global temperature (more at the poles, less at the equator). The warm-up to the current interglacial took 5000 years.
We may achieve 6 degrees warming in as little as 200 years.
No one here wants to keep the global temperature constant. I think I can speak for most participants here by saying that we want change at the natural pace, not the rapid pace that may be caused by human activity.
We have geological evidence of what happens when the global temperature changes quickly, and that evidence recommends against making it happen.
jhnplmr@313
you acknowledge that human activiety has forced a "hockey blade" onto what would otherwise be an overal,comparatively slow decline in average global temperature: But given this acknowlegment of AGW, how can you conclude humans have "stabilized the temperature"? Also, I don't think the issue is "trying to balance on the cusp". The issue is rather that we've stopped the comparatively gradual natural cycle towards colder termperatures ("in it's tracks" as you put it) and forced a much faster rate of change in the other direction that is not, as you indicate "stabilized". Finally, I'm doubtful that any/ many of those that accept AGW think it is feasible to try to control the climate as you imply they do.
314 Barry
"The future ice age (glacial, whatever you want to call it) is going to happen very slowly. The last one took tens of thousands of years from peak to trough, about 6 degrees average global temperature (more at the poles, less at the equator). The warm-up to the current interglacial took 5000 years."
Well it won't happen by the next Jul 65N solar minimum in 2000 years time, but I agree with your general point that there is a slow descent into a glacial period yet a sharp rise, a sawtooth waveform. The Vostok temperatures took about 20,000 years to fall to a minimum in the last glacial period and about 5000 years to rise as you say.
"No one here wants to keep the global temperature constant. I think I can speak for most participants here by saying that we want change at the natural pace, not the rapid pace that may be caused by human activity."
You look upon the contribution of AGW as completely negative, I look upon AGW as stopping global temperatures from falling over the last 10,000 years. They started to fall in line with the falling insolation but then recovered and stabilised under AGW forcing. If you think that the present levels of CO2 are high at 393ppm then you should consider that in the age of dinosaurs they were 3000ppm. It is possible that the present levels will melt the ice caps and we will come out of the current ice age but I believe that AGW will dwindle as we run out of fossil fuels and then the falling insolation will cause a fall in temperatures.
We have already avoided the "natural pace", that happened some 6000 years ago when man started farming and keeping domestic animals. Besides, who wants to slide into a prolonged cooling period with advancing ice caps? Man's activities have stopped that scenario.
[DB] "If you think that the present levels of CO2 are high at 393ppm then you should consider that in the age of dinosaurs they were 3000ppm."
In this venue this is an example of an unsupported assertion, typically employed as a rhetorical device. Please avoid such in the future. If you wish to defend this rhetoric, please do so on a more appropriate thread and provide links to substantive sources published in the reputable literation which you feels support your position.
This is an evidence-based venue. "Beliefs" and opinions carry little weight here.
jhnplmr (from previous thread)... "My claims are not "fantastical", they are based on many sets of published data and the resultant graphs compiled from that data."
Your claim is that Milakovitch cycles vastly overwhelm the radiative forcing of atmospheric CO2 (100w/m^2 vs 2.3w/m^2). That is a "fantastical claim" because it clearly does not square with the vast body of research.
HK already pointed out that you're using an inappropriate metric that is not global. That is an elemental error, and when you present elemental errors that makes you look foolish. It's not meant as an insult. I make myself look foolish sometimes too. We all do.
You seem like a very nice person, John. All I'm saying is, stop believing you – as a non-expert – can disprove over a century of scientific research. Instead, you'd do yourself a greater service, if you're genuinely interested in this topic, to take the time to find out the answers to the questions you have.
jhnplmr said: "[Global temps] then recovered and stabilised under AGW forcing."
Um. I'm sorry but how do you come to the conclusion that global temperature has, in any way, stabilized?
315 Roger D
"But given this acknowlegment of AGW, how can you conclude humans have "stabilized the temperature"?
A glance at my graph, which Rob Honeycutt has promised to link for me, would convince you. The Vostok temperatures started to fall in line with the falling insolation 10,000 years ago but then recovered and stabilised at present levels. This recovery and stabilisation coincided with man clearing forests, starting farming and keeping domestic animals.
"Also, I don't think the issue is "trying to balance on the cusp".
Yet by trying to stop change that is precisely what you are trying to achieve, whether you realise it or not. You are trying to resist coming out of the current ice age, yet hopefully, trying not to slide into a cooling phase. That means that you want us to balance on the cusp of the current interglacial indefinitely.
"Finally, I'm doubtful that any/ many of those that accept AGW think it is feasible to try to control the climate as you imply they do."
I don't think that they are trying to control the climate, I think that the "climate control" that has occurred has been accidental, although beneficial to date.
What I am trying to do is to make you realise that trying to stabilise temperatures at their present levels in the continual changing conditions of an ice age is impractical as well as unnatural.
jhnplmr wrote "trying to stabilise temperatures at their present levels in the continual changing conditions of an ice age is impractical as well as unnatural." He also wrote "This will require enormous amounts of energy which is becoming increasingly scarce and expensive."
No, it is not at all impractical. Preventing cooling is trivial, as is empirically obvious. If by magic humans stopped all increases in greenhouse gases by ceasing use of fossil fuels, ceasing landscape changes, and so on, we could prevent a glacial period with a single chlorofluorocarbon factory for the entire world, as TonyW already told you. But as other people already have told you, our greenhouse gas increases to date with just the next couple of decades will be sufficient to prevent cooling, let alone glaciation, for thousands of years. Hare and Meinshausen produced a graph of the temperature consequence if we magically kept the greenhouse gas forcing at its 2005 level; it does not start going down.
jhnplmr:
I used data from the files bein1.dat and bein11.dat and created this graph showing the July insolation for 60oN and 70oN between 25,000 years ago and 25,000 years into the future. As you see, the insolation will decrease only marginally during the next 2-3000 years before it start increasing again.
AGW has not only stopped the long-term cooling trend that culminated with the Little Ice Age, but has already brought the global temperature back to the level of Holocene optimum 5-8000 years ago. A popular denier argument is that northern Europe and Arctic was considerably warmer at that time than today, but that only proves that the reason for the warming was regional, not global, although some of the feedbacks had a global impact. As this graph on RealClimate shows, the global difference between Holocene optimum and LIA was not more than 0.6-0.7oC while the Medieval warm period was just a speed bump on the long-term cooling trend.
So, the next ice age has been postponed for at least some tens of millennia, maybe several hundreds!
jhnplmr
"You look upon the contribution of AGW as completely negative"
Here you have assigned to me a standard contrarian talking point that completely misses the point I actually made.
No one is opposed to a slowly changing global climate. The concern is rooted in the pace of change. It wouldn't matter if the cause was anthorpogenic or natural. But it so happens that current human activity could cause a rapid change in global climate, the likes of which have caused massive disruptions to the biosphere in the gelological record.
If AGW caused global climate to change at the same rate as glacial transitions, no one would care.
I'm repeating this so you get it. It's not the change, it's the rate of change that is the concern. Our civilizations have flourished during a period of relative stability, the current interglacial. Now we are landlocked with huge populations dependent on hydrological and agriculatural infrastructure that are at risk from a changing climate. Large populations live and work near the coast and now the seas are rising. If they rose slowly we might be able to adapt with little pain. But if the climate changes rapidly, if extreme weather events become more numerous, if the sea eats into our coastlines displacing millions of people and destroying agricultural industries (the rice farms around the low-lying Mekong Delta feed millions in Asia), if floods drown our grain, drought starves our soil, more hurricanes wreck our homes and workplaces, and the changing climate uproots millians of people, we are in for a world of pain.
We don't want to stop the climate changing, we'd just prefer it changed at a natural pace.
Perhaps, jhnplmr, your gross overestimate of the effect of insolation decrease specifically at the 65N latitude is due to your misunderstanding of how glacial eras are started and ended. You seem to think that the relative decrease in insolation at that latitude is the sole trigger for glaciation. So you think glaciation will be triggered when the decrease from the previous insolation maximum nears the corresponding relative amount of decrease that happened at the initiation of the previous glaciation. But that is not how it works. Absolute, not just relative, temperature must be low enough to cause substantial increase in ice and snow cover, for the increased aledo to feed back, reducing the temperature, so the oceans start to absorb more CO2, feeding back to increase cooling, and so on.
If the temperature is high for any reason--hmmm, let's say high levels of CO2--ice and snow cannot increase enough to start that feedback. And if the oceans already are pretty close to their capacity for absorbing CO2, decreasing their temperature a smidge will not pull enough CO2 out of the atmosphere to make enough of a cooling feedback. And if there is a lot of CO2 in the atmosphere, the oceans will have to absorb far more than they did last time, for the absolute temperature to get low enough for the ice and snow to increase.
For some background, see the SkS post on Shakun's 2012 study of the CO2-temperature lag.
#317 Rob Honeycutt
"Your claim is that Milakovitch cycles vastly overwhelm the radiative forcing of atmospheric CO2 (100w/m^2 vs 2.3w/m^2). That is a "fantastical claim" because it clearly does not square with the vast body of research."
I went on to say that the temperature had remained relatively constant over the last 6000 years so the comparatively large fall in solar insolation was balanced by the relatively small rise in CO2 forcing. This implies that the two rates of forcing were equal despite their numerical inequality.
#318 Rob Honeycutt
"Um. I'm sorry but how do you come to the conclusion that global temperature has, in any way, stabilized?"
I sent you my graph showing that ice core temperatures over the last 6000 years had remained relatively constant, naturally there have been oscillations during this period, you can see them on the graph but the long term mean temperature has remained constant. This view is supported by this wikipedia extract Quaternary_glaciation:
"The present interglacial period (the last 10,000 to 15,000 years) has been fairly stable and warm, but the previous one was interrupted by numerous frigid spells lasting hundreds of years. If the previous period was more typical than the present one, the period of stable climate in which humans flourished—inventing agriculture and thus civilization—may have been possible only because of a highly unusual period of stable temperature."
Wiki goes on to cite a scientific paper AgOrigins.pdf which postulated that the development of modern agriculture was only possible due to the relatively stable climate during the present interglacial.
I might add that Vostok temperature over the last 50 years is only 0.2deg C warmer, last_50_yrs.html, a rise of 0.04deg C/decade.