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Are we heading into a new Ice Age?

What the science says...

Select a level... Basic Intermediate

Worry about global warming impacts in the next 100 years, not an ice age in over 10,000 years.

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)

According to ice cores from Antarctica, the past 400,000 years have been dominated by glacials, also known as ice ages, that last about 100,000. These glacials have been punctuated by interglacials, short warm periods which typically last 11,500 years. Figure 1 below shows how temperatures in Antarctica changed over this period. 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?

Figure 1: Temperature change at Vostok, Antarctica (Petit 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.

To answer this question, it is necessary to understand what has caused the shifts between ice ages and interglacials during this period. The cycle appears to be a response to changes in the Earth’s orbit and tilt, which affect the amount of summer sunlight reaching the northern hemisphere. When this amount declines, the rate of summer melt declines and the ice sheets begin to grow. In turn, this increases the amount of sunlight reflected back into space, increasing (or amplifying) the cooling trend. Eventually a new ice age emerges and lasts for about 100,000 years.

So what are today’s conditions like? Changes in both the orbit and tilt of the Earth do indeed indicate that the Earth should be cooling. However, two reasons explain why an ice age is unlikely:

  1. These two factors, orbit and tilt, are weak and are not acting within the same timescale – they are out of phase by about 10,000 years. This means that their combined effect would probably be too weak to trigger an ice age. You have to go back 430,000 years to find an interglacial with similar conditions, and this interglacial lasted about 30,000 years.
  2. The warming effect from CO2 and other greenhouse gases is greater than the cooling effect expected from natural factors. Without human interference, the Earth’s orbit and tilt, a slight decline in solar output since the 1950s and volcanic activity would have led to global cooling. Yet global temperatures are definitely on the rise.

It can therefore be concluded that with CO2 concentrations set to continue to rise, a return to ice age conditions seems very unlikely. Instead, temperatures are increasing and this increase may come at a considerable cost with few or no benefits.

Basic rebuttal written by Anne-Marie Blackburn

Update August 2015:

Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial

 

Last updated on 7 August 2015 by MichaelK. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page

Argument Feedback

Please use this form to let us know about suggested updates to this rebuttal.

Further reading

Tamino discusses predictions of future solar activity in Solar Cycle 24.

Acknowledgements

Many thanks to Sami Solanki for his invaluable advice and feedback as well as John Cross for his very helpful comments.

Further viewing

potholer54 published a video tackling this myth on June 27, 2020

 

Dave Borlace explains why we are not headed towards an ice age in this "Just have a think" video published in December 2019:

 

Comments

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Comments 1 to 50 out of 410:

  1. Hi John: Thanks for the thanks (although I will hasten to point out that all I did was offer some minor comments). I am especially delighted to me mentioned in the same acknowledgement as Dr. Solanki. I don't know if Dr. Solanki reads this blog, but if he does I would like to say that I have been reading his papers since I came across the Harold Jefferys Lecture that he did about 4 years ago. Very interesting work! Regards, John
  2. Re: "What if the sun did go through another Maunder Minimum?" Not IF but When. It's part of the long term climate cycle.
  3. Actually we can't head into an ice age at this point in time because we are already in one. Technically this is an interglacial period within the current ice age, ie. this is not "Earth Normal" climate, which is much hotter. Interglacial means "between glacations" and is about as unstable a climate as possible on this planet. That is assuming that it is an interglacial rather than the ice age ending (far worse for us). It also assumes that another glacation will occur. Looking at graphs of prior interglacials and glacations there is a particular constant: it warms slowly for thousands of years then cools rapidly (it looks very similar to a sawtooth inventory graph). Another fact is that CO2 has been very high when it suddenly became very cold (it did nothing to stop the glacations). So be afraid, be very afraid.
  4. Quietman: That's simply not true. The glacial cycles of the last 450'000 years have consisted of long periods of slow cooling, followed by rapid warming. (http://commons.wikimedia.org/wiki/Image:Ice_Age_Temperature.png - please note that present day is to the left). This rapid warming is believed to be started by small changes in solar forcing (see Milankovitch cycles: http://en.wikipedia.org/wiki/Milankovitch_cycles) which is enough to start positive feedback spirals: warmer -> more greenhouse gases -> even warmer... Since we've recently come out of an glacial period, temperature is basically is as it gets and should be slowly decreasing (on a long timescale) until we reach the next glacial period. But instead, anthropogenic releases of greenhouse gases have started the process of warming. The amount of released GHG so far is enough to keep the planet warming for a long time (but of course, with possible micro trends of cooling) and we've soon reached a level where positive feedback spirals kick in, meaning that temperature will continue to rise even if we would stop our own releases of GHG.
  5. should've been: "... is basically as high as it gets ..."
  6. Maunder minimum...solar radiation ( and I deliberately use that term) drops between .17 and .23 W/m2; the suggestion is that these changes are insufficient to account for glaciation periods, or , if they are responsible, the system must be pretty thermally unstable. What was perihelion at this time? Orbital eccenticities have FAR greater effects on the amount of radiation recieved (between 6 -7% depending on source)than solar variations. What other factors were different then as opposed to now? You cannot isolate one component of a system and use it to determine an end result. You have to include all factors. My general conclusions from what I have read and what I have tried to model, is that the 'normal' state of the earth is "cold". That the warm phases are the anomalies. I accept this is MY opinion, but given the choice between a longer, warmer interglacial period or an earlier decline into one, I think I would opt for the warmth; life ( of all kinds) flourishes better in warm climates
  7. Why am I not surprised that the snapshot used by "Image from Global Warming Art" is different from everyone elses? It's called cherry picking.
  8. To clarify, Look at the 5 million year chart instead. The highs and lows of 450 K years are only oscillations of a gradual warming trend for 5 million years.
  9. The O18 concentration in bottom-dwelling foramanifera is used as a proxy for surface climatic conditions: eg a warmer climate produces more dead bodies drifting to the sea bed which allows the organisms there to multiply and 'trap' more O18. There are some difficulties with this, namely, you need prior data on the level of O18 or C13 at 'ground zero'. Also how do you compute the time lags involved? Slide the graph around until there seems some kind of match to glacier records? How do you accomodate variations in surface species numbers due to local climate? Are 57 core samples sufficient to give a general trend? http://en.wikipedia.org/wiki/Paleothermometer
  10. "The difference in solar radiative forcing between Maunder Minimum levels and current solar activity is estimated between 0.17 W/m2 (Wang 2005) to 0.23 W/m2 (Krivova 2007)" Wang 2005: "The increase in cycle-averaged TSI since the Maunder minimum is estimated to be ~1 W/m2" (instead of 0.17 W/m2?) Krivova 2007: "[The model predicts] an increase in the solar total irradiance since the Maunder minimum of 1.3^+0.2_-0.4 W/m2" (instead of 0.23 W/m2?)
  11. I am a novice here but I have noticed a lot of technical jargon that I don't understand. "Al Gore" etc thinks it is going to get warm. The "skeptics" say it is going to get cold. I think that if I lived in the NH I would make sure I have got some warm clothes. The bottom line is that all our heat comes from the Sun. If it cools down so does Earth. Any variances due to different Ocean Currents, Magnetic Fields etc are only releasing stored energy from the Sun. At the end of the day this planet will be a Dead Rock circling a spent Sun. Lets hope it warms up, CO2 increases, Plants grow and life becomes comfortable for a while. The alternative is not nice.
  12. Nor is a seven metre rise in sea levels. Where are all the people who live within ten miles of the coast going to live? I doubt the dinosaurs worried too much about how much land they had available or how many times they were flooded out or how many died in forest fires or died of thirst.
  13. Samboc: Don't worry about the sun cooling down just yet, providing it doesn't do anything silly it will follow the normal sequence for its type and increase in luminosity ( by about 10% over the next billion years). And you're right: warmer is better as the paleorecord shows. The view that sea levels will rise to the levels predicted is based on assumptions, not facts. All that water has to come from somewhere- snow,glaciers, icecaps, thermal expansion et al. There are many unresolved factors such as land rebound, greater oceanic uptake of CO2 due to rising ocean volume decreasing the GG efect at the same time as ocean warming releases more CO2...and one can go on and on. We currently simply do not have the ability (or data)to fully understand and accurately model the climate process, so it is not reasonable to take action that would have severe economic and societal repercussions until that time arrives.
  14. Samboc Well said. Warm is better than cold.
  15. sandy Distance to the shore is irrelevant, height above sea level is relevant. But a small factor is left out when the alarmists talk about sea level rise and that is porosity and absorbtion. The Newark Basin in New Jersey is not very much higher than sea level and yet it was swamp lands during the mesozoic, not ocean bottom. Much of the coast that will flood is swamp land now. During the Mesozoic the midwestern US was an inland sea and remains lowland today. A catastrophic rise in sea level will most likely result in a return of the inland sea, something that alarmists fail to mention.
  16. On sea levels: In Australia a bed of semi-fossilised molluscs has been found above current sea level and dated at between 4500 to 6000 years old. This suggests current MSL has fallen over the last 6000yrs. since Australia is not very active tectonically. Wait til the creationists find that one! I can't remember the exact page but more detail is at John-Daly web site.
  17. Mizimi Plates literally float. They can shift, rise or fall. There are no true continents, what we see is a result of large pieces of lighter material breaking and mergeing or subducting. There is no real difference between sea floor and land other than elevation. So while Australia is relatively free from volcanism it is still subject to plate tectonics which are constantly active but change intensity and speed is cycles.
  18. QM: Yes, and it makes rather a nonsense of trying to assign mean sea level deviations to global warming. The Isles of Scilly is a group of around 50 islands some 45km south west of Cornwall, England. They have been inhabited since (at least) the bronze age, some 4000ya. At low tide, stone houses, roads and field perimeters are revealed, dating from that time, so either msl has risen a few metres in 4000yrs or the islands have sunk.....how to tell the difference?
  19. Mizimi I think Dr. Rhodes Fairbridge answered that question in his study of sea level cycles. It's referred to as the "Fairbridge curve" and I think wikipedia posted an explanation of how it works. I have not looked too deeply into it as I am more interested in his later "Solar Jerk" hypothesis.
  20. I heard about this on the radio last month, and this would prove that we are not the cause of climate change, and that industrialization is not harmful. Unfortunately, many people are still advocating global warming since they have their money on it. This seems to be the strategy for defense of these advocates: "if any part of the earth gets warmer during the industrialized age, industrialization is to blame. If the earth gets cooler, of course, industrialization is a bad thing anyway. Heads, I win. Tails, you lose!
  21. QM: have you read this, and if so, any comment? http://www.griffith.edu.au/conference/ics2007/pdf/ICS176.pdf How's the greehouse project?
  22. Mizimi Yes that was the first piece I saw that got me interested in the hypothesis. Mackey said the test period would be 2007-2011 and so far it's on the money. It's also the ONLY paper that predicted the cold snap, all the other articles and papers at that time said exactly the opposite. (I kept all the failed forcasts in a file to serve as bad examples for future generations). ps The green house is on hold until the ice melts. Right now I'm waiting on delivery of a larger snow blower that I can mount off the PTO on my tractor. Snow has been way too heavy for the gas fired one and too deep to plow, I have been using a bucket loader all winter to shovel it off and it's ruining my driveway. So I'm going to have to resurface it when everything finally melts. But I am planning to go with the blue plastic for the roof.
  23. John "We're heading into a new Little Ice Age" equals "It's the Sun". No different, same thread.
  24. SAME THREADS: "Are Sunspots Different" http://www.leif.org/EOS/2009EO300001.pdf "LONGER TERM SOLAR MINIMUM" http://icecap.us/images/uploads/SOLAR_MINIMUM.pdf
  25. [quote]The difference in solar radiative forcing between Maunder Minimum levels and current solar activity is estimated between 0.17 W/m2 (Wang 2005) to 0.23 W/m2 (Krivova 2007).[/quote] Solar radiative forcing is not the only change to account for. Solar minimum may also mean less solar wind and solar cosmic rays and geo-magnetic interaction. There is a correlation between solar cycles and the number of galactic cosmic rays, wich can cause more or less cloud formation that can increase the albedo reflecting a significative part of the total sun power. So when we talk about a solar cycle influence we should include also this issue.
  26. There are a number of issues that affect a possible ice age - the sun (and related wobble to the earth) being one. The others include the melting of the arctic ice cap. With the ice in place, the climate is arid; with the ice melted, there is a source for water vapor - necessary for precipitation to happen. During the last major ice age, a portion of the arctic ocean remained open, thereby providing for a source of water for the snowfalls that covered North America (Russia on the other hand escaped glaciation). The third issue to be considered is a change to ocean currents - specifically the interchange of waters from the North Atlantic and the Arctic waters. It was noted in a number of studies in the 1930's that the arctic ocean was showing signs of warming.
  27. OK. This is what I have to say. I am not skeptical about the warming trend caused by atmospheric changes. I do think the current discussion in the media is limited. If we were about to enter an ice age, we might want more greenhouse gases to counter an ice age trend independent of the atmosphere and likely caused by astronomical features. The above discussion says an ice age is unlikely becasue of the warming forces. I say that gets to my basic point. The discussion is too narrow. Do not say "it is getting warmer, we must cool the earth." Ask what will come next? What should we do if the future will be warmer? and also ask "What should do if the trend is for cooler weather?" Our changes to the atmosphere must be in response to what we know about the climate in the future. Warmer will be inconvenient. Ice will kill most of us. I don't know what will happen. I know we need a wider discussion. Our most important question is "So what?" We need more data and a more comprehensive picture with many more questions. The cureent discussion in public is limited.
    Response: The question "so what?" is addressed in the positives and negatives of global warming.
  28. At this point, given trends in atmospheric CO2, CH4, halocarbons, etc. "cooling" is not likely to be a problem for the foreseeable future. In fact, if we burn enough coal over the next century or two, it's entirely possible that there will still be enough additional CO2 in the atmosphere 50,000 years from now to prevent the next glacial cycle (Archer 2009). Cooling is not going to be a problem. Heat, sea level rise, and especially alterations to the hydrologic cycle will be.
  29. Ned. You are saying that the CO2 is good since it will keep the next ice age off our backs? High sea levels (if slow enough) just means rebuilding on higher ground. Most commercial buildings last less than 30- years. Water is the ultimate recyclable commodity. We can manage water if we have enough energy. Solve the energy problem and food and water will be sufficient for all.
  30. Don't be silly. If we're able to alter the climate enough to prevent the next glacial cycle 50,000 years in advance by accident, I suspect our descendants will be able to come up with a way of dealing with the next glaciation whether it happens 50,000 or 130,000 years from now. 21st century climate change will mean real hardship for many people, primarily due to alterations of patterns of rainfall and drought, plus the impacts of sea level rise on poorer countries where "just rebuild on higher ground" isn't necessarily an option. (Who's going to offer to take in a few tens of millions of Bangladeshis?) I don't think it's reasonable to impose that kind of hardship on actual people living in this century while patting yourself on the back for "preventing" a very slow glacial advance 50,000 years in the future.
  31. How about preventing an ice age in the next 50 years? Or 100? 50,000 wasy our number. I think you are nit picking.
    Response: Ice ages take thousands of years to develop. If you're that concerned about an impending ice age, just look to northern Canada. If there's a giant ice sheet slowly creeping down the North American continent, then you have reason to be concerned. But if glaciers are retreating worldwide and the Greenland and Antarctic ice sheets are losing ice mass at an accelerating rate, you can relax about the possibility of an upcoming ice age in your lifetime and the lifetime of your children and grandchildren.
  32. There isn't going to be an ice age in the next 50 or 100 years. If that was your concern, then you can rest easy tonight. And tomorrow you can wake up and start working on the real problem, which is preventing the potential catastrophe caused by too much warming rather than too much cooling.
  33. Well that makes your climate models easy. Are you familiar with the phrase "Dead Certain." Warmer may be inconvenient. Ice will kill most of the life on the planet. I think we should plan for every possible future and not pick only the ones we want to "solve." By the way - how do I get the nam "N/A" and how do I change it? I am having some degree of trouble here.
  34. "If you're that concerned about an impending ice age," Really, I am not. My point is that it is possible warming will be good if it prevents an ice age. We seem to be in agreement that CO2 is preventing an ice age. We differ if the CO2 is good or bad. Butr I do not know. I only see it as an important question to answer and have yet to see a satisfactory answer.
  35. I hate to be reiterating an old point but its all about rate. The transition into and out of ice age is extremely slow by human terms. (around 10,000 years). The rate of warming we are creating is by comparison very fast. Rates of change that overwhelm species capacity to adapt are the danger. Also, an ice age hardly kills most of the life on the planet. We have been in and out of ice ages right through quaternary period. They affect temperate zones mostly.
  36. N/A writes: I do not know. I only see it as an important question to answer and have yet to see a satisfactory answer. Nothing wrong with that. Spend some time browsing through the pages on this site -- there is a lot of information, and a lot of very careful discussion of the current peer-reviewed literature. A number of the commenters here are scientists working either within climate science or in related fields ... they can provide a very valuable perspective, too.
  37. Thanks for the thoughtful responses. I hope you are right about an ice age being so slow. But it is probably the only factor that would justify an increase in CO2 at this time. My background is that I am a retired science librarian with an earned PhD. That should qualify me to do compreshesive searches of the current literature and to understand what I find. I do question those who want to divide comment into either "warmer" or "denier." And I do want us to consider many other questions other than "Is it getting warmer?" I think it is getting warmer. I wonder if that is good or bad. And I am not convinced that it is bad.
  38. N/A - perhaps you should start with AR4, WG2 on the IPCC website. It is heavily referenced and details the impacts. Check the references, decide for yourself. The critical question is how fast can we adapt? The problem areas as I see it are densely-populated delta regions (Bangladesh, Nile, Niger etc), and regions prone to long-lasting drought. Disruptions to water cycle are unfortunately not that easy to predict.
  39. Im glad you brought this up. I have heard argument in the area of the 'mini ice age'. The melting of the northern ice cap along with the melting of ice in Greenland to flood the northern sea with fresh water. The fresh water from the melting ice is lighter, and so tends to hug the surface, forcing the Gulf Stream lower and in the end, further east sooner. The bulk of heat from the Gulf Stream is left to transfer toward Europe then, as Canada finds itself cooling... and entering a miniature ice age whose duration is limited to centuries at most. Your comments on this general theory would be considered a rare light in an area I consider shadowed, and confusing at best. Im sorry for the lack of references. It has been many many years since I read of this odd theory and references are lost to time.
  40. OK. I have said enough and I still do not know why I am listed as "N/A" But I have one last thought. What are the circumstances that would make more CO2 good? How do we know they arr not in the future? Best to ask many questions and be ready for anything that happens. Have a Swiss army knife of solutions - not one solution for one problem and only that one solution. Warmer will be inconvenient. Ice will kill most of us - if not all. I am gary4books.
    Response: The N/A is a website glitch, still trying to figure it out.
  41. Scinan, I think you are talking about the North Atlantic Current portion of the thermohaline circulation. It has happened in the distant past, so reasonably it has been speculated to be a risk in the near future. More research has revealed that it is extremely unlikely to happen in the near future. More information can be found by clicking on the links in the "More Information" section at the bottom of the page by climatologist Stefan Rahmstorf, "The Day After Tomorrow: Some Comments on the Movie." (For the quickest route to the information, skip straight to the bottom of that page.)
  42. Scinan, I should clarify that the risk of a shutdown of the North Atlantic Current (thereby causing a Northern Hemisphere ice age) is not an ice age threat that is reduced by increased CO2. To the contrary, the threat is increased by increased CO2. The point of my previous comment was that the probability of it happening is very small.
  43. gary4books might be right. both sides of the argument seem to be afraid of the possibility of change. EVERYTHING CHANGES! The biology of this planet is good at adapting to changes. "Positives and negatives of global warming" is a ridiculously one-sided attempt of trying to purport to have a balanced view. (of course there are more papers on scary outcomes than on positive outcomes - no one funds/reads rosy pictures) Even with all the evidence in the world nothing is going to make energy companies let go of the status quo in time to make a difference, nor will it make enough people CHOOSE to pay more for energy. I believe all of us in the scientific community need to stop trying to WIN the argument and start working out what we know and what we still need to know - then we will actually be able to move forward intelligently.
  44. mginaus writes: The biology of this planet is good at adapting to changes. That's true, but that adaptation process involves lots of losses (e.g., lots of species go extinct during time of abrupt change). Just because "life as a whole" survived the K-T impact doesn't mean we as one particular species should welcome catastrophic climate change. In addition, we have a huge investment ("sunk costs") in technological and cultural infrastructure built around a relatively stable climate. Here in the US (where I live), a small but long-term change in patterns of precipitation could be hugely expensive, dwarfing the trivial costs of things like the Iraq war or health care. Look at economic impacts of the 1993 or 2008 Midwest floods, or of similar drought years. The West Coast would (will?) incur immense costs if snowpack in the Sierras declines and the regional water infrastructure has to be completely reworked. Etc, etc, etc. As for willingness to change, we're going to have to change anyway, due to the conflict between increasing demand and decreasing availability of oil in the coming decades. Nuclear would help but can't replace oil by itself, at least not in the near future. Switching back to coal will incur huge health and environmental costs, and if continued for the long term it would drive the climate into absolutely disastrous conditions. mginaus concludes: I believe all of us in the scientific community need to [...] start working out what we know and what we still need to know - then we will actually be able to move forward intelligently. Hear, hear! I agree with that, more or less. I'd just add that we know enough already to justify starting making changes now (actually, we should have started 20 years ago...) -- I think John Cook and all the others who contribute to this site have done a great job of summarizing what we do know and what we still need to figure out.
  45. I am 17 years old and home schooled doing a project on climate change (trying to find all aspects for and against climate change and their effects) so i am not a scientist and do not understand everything that has been talked about on this page so bear with my questions please. How do we know that decreased insolation due to the larger projections of increased ppm CO2 in the atmosphere (i.e. GTon C) doesn't outweigh the resulting reduction of outgoing radiation thus causing global cooling?
  46. Erin, We know because of the measurements. The Earth is retaining more energy than it is radiating back to space despite the decreased insolation. I recommend reading up on the net forcing. Likewise, if decreased insolation outweighed the forcing of CO2, then we would see a cooling trend as opposed to a warming trend. Look no further than our temperature records. As a high school teacher, I commend the level at which you are approaching this topic. Keep it up.
  47. Thank you chudiburg. So I have another question. If you look back on the 100k year cycles in figure 4 they all peak at about where we are now and that peak is sharp. Why does the projection for the natural cycle in figure 4 predict the same estimated high temperature for the next 50,000 years if that has not happened in the past (at least in that figure)?
  48. Question: In the projections shown in Figure 4, it looks like there is an upper limit of about 4 degrees C for the temperature anomaly, even at 5000 Gigatonnes CO2 emission, and only about 2 degrees for 1000 Gton. However, the graph in Figure 2 shows that the temperature anomaly is projected to reach 4 degrees C in just under 100 years, on a more or less exponential path, with no sign of slowing down. Is there or is there not an upper limit to the temperature anomaly given these assumptions about the magnitude of CO2 emissions? How sure can we be that the projections shown are correct, given the chaotic nature of the planet's climate? Question 2: Can climate change result in positive feedback as ocean warming leads to the release of more CO2?
  49. Erin, If you click on the link for the source of figure 4, the author of the article addresses this. It has to do with expected low insolation variability for the time being. It is worth noting that this agrees with some predictions, but disagrees with others. The primary thing to take away from the study is that if we continue to release large amounts of CO2, then we could potentially delay the onset of glaciation indefinitely. McCloud, the 5000 gigatonnes CO2 emission is based on an estimate of how much we could potentially release if we burn all available fossil fuels. Thus, with the calculated forcing of that much CO2, we see an upper limit of 4 degrees warming. This study may not be the best source for looking at future temperature anomalies, however, because its purpose is to look at the potential for preventing the next glaciation. The scope of the study really isn't to make precise temperature anomaly predictions, but rather predict how much glaciation will be delayed under different emission scenarios. I would suggest clicking on the link to the study. As for your second question, yes there is potential for a positive feedback here. As water warms, its capacity for storing CO2 decreases, which will eventually lead to the oceans actually releasing CO2 as opposed to taking it in. I am fairly sure that most climate projections take this into account already.
  50. I need to do more reading on Broecker on this, and I tend to be at odds with some of the literature. However, some of it has given me some hunches. The PETM was a period 55 million years ago with no glaciation. The popular media presented this as a possible future state due to greenhouse forcing. I believe this must be a complete error. Moran et al. presents Arctic Ocean sediment core data that argues for a transition period between glaciations during the same time (between 45 and 35 million years ago (Ma)) when the southern ocean Antarctic Circumpolar Current (ACC) was being formed. Their graphic describes before and after the ACC formation as "Greenhouse" and "Icehouse" climate states respectively. One estimate of that formation using neodymium isotope ratios by Scher et al. estimates the opening of the Drake Passage based on ocean mixing to be around 41Ma. Data such as Zachos et al. shows how northern glaciations occured at a time after a robust southern ice cap has been built up and maintained. I hold a MS in Physiology, and have only done some independent study in geology and audited a course in Earth's Climate History and completed one in Physical Oceanography. Long before this, being acquainted with climate swings, I drew an analogy between a mechanism in my field, the excitable membrane action potential conduction mechanism seen most robustly in nerve and muscle tissues to the climate system. In the action potential mechanism, there is a rectification reaction that occurs because sodium and potassium ions are set in opposing gradients accross the plasma membrane, and have selective channel proteins that conduct the two ions with differing time courses so that the fast response sodium causes a change in voltage in one direction called depolarization while the slower response potassium channels cause a voltage change in the opposite direction by virtue of the fact of their opposite gradient. I draw my analogy to the climate system in this manner: Temperature is an analogue for voltage, a greenhouse stimulus that occurs in the atomosphere is an analogue for the sodium response, and an oceanic/cryospheric rectification mechanism is analogous to the potassium response. I of course cannot deny orbital forcing mechanisms that occur on 40Ka, 100Ka, and I believe 20Ka range changes, as has been mentioned here, but I believe another very important part of the equation is the ability of the earth to respond to heating changes, which shows the contrast in response pre-ACC formation, the greenhouse earth, and post-ACC icehouse earth. The time course of cryospheric/oceanic response is on the order of multi-century to millenial, putting it an order of magnitude smaller than the orbital forcings. There is evidence in the fossil record of events that may corroborate this idea. Terms to look for are "Iceberg Armada," "Heinrich Events," "Younger (and Older) Dryas," as well as the "8200 year before present event." I have a while to go to fully quantify this idea, but since the ocean stores about 1000 times as much heat as the atmosphere, it is not absurd to envision the domination of the atmosphere by oceanic responses. It is conceivable that a threshold for the response will be crossed where greatly increased motility of global major glacial masses could lead to a reversal of a warming trend that would include a reversal of the ability of the ocean to absorb CO2. By far the lion's share of ocean is very cold, fed by yearly calving of glaciers at both poles leading to what is known as North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW). These masses can represent a very adequate repository for resequestered CO2. I'm still in the "hunch" stage here, but I have posted some of my ideas at http://kayve.net/kayve

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