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Comments 51 to 65 out of 65:

51. exp137 at 01:00 AM on 12 January, 2012
    Not sure if I'm too late to get a response, but I have another question regarding the lag. I understand that cause for the lag at the initiation of warming, but it appears that in every case, as the climate shifts back to a glacial period, the temperature drops while the CO2 is still near it's peak concentration, with the temperature starting to fall very abruptly before the CO2 even begins to decline. Is there any explanation for that?
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52. muoncounter at 03:05 AM on 12 January, 2012
    exp137#51: Read through the original post, especially the mechanism given after Figure 2. The temperature change is initiated by orbital variation and reinforced by atmospheric gas concentrations. It's important to understand the time frame when you say "temperature starting to fall very abruptly". That is really not the case: glacial onsets are typically much slower than terminations (see Figure 1 and note the time scale). Terminations can take 100s-1000s of years; cooling to full glacial is much slower. The temperature graphs take on a 'sawtooth' shape. --source
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53. exp137 at 05:38 AM on 12 January, 2012
    muoncounter#52: I've read the article, but it focuses exclusively on the timing of the termination as opposed to the onset. I'm very comfortable with the reasoning behind the lag at the termination, but still curious about the lag at the onset. If you look at the last interglacial period (before the current one) in Fig. 1, the temperature has dropped rather significantly while the CO2 concentrations are still largely above 270ppm. While the cause of the glacial onset may indeed be orbital forcings, does this really mean that the imfluence of CO2 on the glacial-inerglacial cycle is essentially neglegible? Or is it that the high levels of CO2 essentially "soften the blow" of the orbital forcings, by slowing the onset of the next glacial period? Please don't assume I'm a skeptic or unfamiliar with the basics just because I'm asking questions, as I am currently a graduate student studying paleontology. This is an argument I've heard from skeptics in the past and I'd like to find a solid explanation, if one is known.
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54. scaddenp at 06:09 AM on 12 January, 2012
    exp137 - if two of the forcings go negative while GHG remain stable, then why would you expect temperature not to drop? Explicitly, when onset occurs, direct insolation from sun is dropping in NH (cooling), while CO2 concentration are possibly still rising - but very slowly as there is a considerable lag. On the other hand, ice albedo is a rapid feedback and the two together drop NH temperatures, freeze the methane sources, cool northern oceans which in turn leads eventually to lower CO2 and SH cooling as well.
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55. exp137 at 07:41 AM on 12 January, 2012
    scaddenp: I don't expect the temperatures to not drop at all, it's just that in the case of glacial cycles there doesn't appear to be much input from CO2 at all. This is a frequently used case by climate change skeptics that CO2 is not a climatic driver (or a very weak one at best) but instead simply follows temperature. I'm familiar with other cases in which CO2 does appear to drive temperature, such as the PETM and the Ordovician glaciation, but I just would like to understand this case better. I guess what I'm really looking for is a model that shows the relative influences of both insolation and CO2 during the Pleistocene glaciations. Because in this case, at least, it's obvious that changing solar input is the primary driver, but the role of CO2 isn't very clear.
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56. Bob Lacatena at 08:28 AM on 12 January, 2012
    exp137,

    ...there doesn't appear to be much input from CO2 at all.

    No? What about the fact that the glacial onset takes tens of thousands of years longer than a termination, because the orbital forcings/change in albedo are fighting against rather than in concert with CO₂ changes? Another factor, of course, is one of pure area... on a sphere (the earth), there is more area at a latitude closer to the equator than a pole, and the sunlight is more direct and seasonally constant towards the equator, with the upshot that albedo changes are stronger when the ice sheet is further south. This in turn means that when the ice begins to retreat (during a termination), the strength of change in albedo is greater and faster at the beginning of the change than at the end as compared to the reverse, the growth of ice from the pole southward, when the growth (in area) is slow at first with a greater affect near the end. A model for the relative influences cannot possibly be as simple as you'd like, but one problem is exactly that you are eyeballing graphs and using a rule-of-thumb approach to the problem. It's easy to come up with any inference that appeals to you with that approach. To go over the glacial onset... orbital forcings cause summers to be shorter and cooler. This leads to a very, very slow build up of the snow and ice extent in the northern hemisphere. Each year, the snow melts not quite as far back as it did the winter before. Each time this happens, the total amount of sunlight reflected back to space without warming the planet is greater. At first, the difference is small, because the change happens at a high latitude. The total change in area is small, and the angle of incidence and W/m² is less because of how the light strikes the earth at the poles. As the ice sheets extend further south, the effect becomes more and more severe. As temperatures drop due to this forcing, the amount of CO₂ (and H₂O) in the atmosphere falls, which causes temperatures to drop even further. There is no inconsistency in this.
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57. Bob Lacatena at 08:42 AM on 12 January, 2012
    exp137,

    ...in which CO2 does appear to drive temperature...

    This is a bad way to look at things. Talk of a "driver" is a trap, or rather a denier's canard. In a natural climate there are very few true forcings. Orbital forcings are one. Change in solar output is another. A massive injection of CO₂ due to, for example, volcanic or anthropogenic activity are also true forcings. A massive injection of dimming aerosols due to volcanic activity or a huge asteroid strike is yet another. These can be true "drivers." Outside of that, almost all of the possible influencing factors are interlocked. Atmospheric H₂O rises/falls quickly with temperature changes, and drags temperatures further up/down. CO₂ rises/falls similarly but less quickly, also dragging temperatures further up/down. Changes in temperatures can contract/expand ice sheets, which reduce/expand albedo, which drag temperatures further up/down. Virtually everything in the normal ebb and flow of climate is an interlinked feedback. Actual drivers vary, but the definition of the "driver" is more complex than simply "is this a driver?" The true "driver" somehow changed due do an event outside of the normal climate system... a quirk in the slow change in orbit, a massive flurry of volcanic activity, who-knows-what in the sun, or... a fossil-fuel based civilization. Albedo (snow/ice) is never a "driver"... orbital forcings cause that change, although orbital forcings don't themselves directly influence temperature, but rather the effects on snow/ice extent do so. CO₂ is a "driver" only when it is "un-naturally" changed (volcanoes, humans). Otherwise it is a feedback. But the reason for the change in no way changes the efficacy of the forcing. If a car runs you over, it doesn't matter if someone was behind the wheel driving it or not, it still has the same effect.
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58. scaddenp at 09:14 AM on 12 January, 2012
    My understanding of the role of CO2 in Pleistocene glaciations is twofold. 1/ One it is a slow feedback amplifying effect of solar (which would be too weak for the scale of temperature change by itself). 2/ It is part of the mechanism by which changes in NH also causes warming/cooling in the SH. Note that albedo is also major player in the feedback the amplifies the solar change. The relative importance of the forcings are discussed in Ch 6 of the IPCC WG1 report.
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59. Bob Lacatena at 09:36 AM on 12 January, 2012
    exp137, I should clarify something. I rather inadvisedly said "fighting against rather than in concert with CO2 changes." That didn't come out as I intended, and is incorrect. CO₂ is a feedback that enhances the temperature drop. The point I intended to make was rather that because CO₂ levels are high, the high temperatures that result help to hold back the expansion of the ice sheets. At the same time, because of the logarithmic nature of CO₂, a "draw down" of CO₂ levels and temperatures is much harder to enact than an increase. One must get more CO₂ out of the atmosphere, at the start of glaciation, to get the same change in temperature. As such the effects of CO₂ changes at the initial stages of glaciation are small/sluggish, the opposite of the result of changes in CO₂ at the start of a glacial termination. This makes it look like CO₂ provides a bigger "kick" to a glacial termination (it does, at the start, as opposed to glaciation where that kick is harder at the end).
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60. Bob Lacatena at 09:49 AM on 12 January, 2012
    exp137, Okay, one more point. I'm actually really glad that you brought this up, because it highlights a very, very important aspect of CO₂, and that is that because of the logarithmic nature (i.e. double CO₂ to get the same incremental increase in temperature, so 2x = 1, 4x = 2, 8x = 3, and so on) the effects of increasing CO₂ are more evident when you raise it from a low value, and less evident when you first drop it from a high value. This has huge implications on the idea of adding it, and somehow later trying to reduce it and return temperatures to normal. What this inevitably means is that it is easier to raise temperatures by increasing CO₂ than it is to drop them by decreasing CO₂. As such, the impact of CO₂ at the start of a glaciation is far less evident than the impact on the start of a glacial termination. Also... it is going to be a whole lot easier for man to raise temperatures by increasing CO₂ than to draw them down by extracting it. The bang from the first X gigatons added to the atmosphere is far greater than the bang from the first X gigatons somehow subtracted (assuming we can come up with a cost effective way of doing so).
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61. Rob Painting at 10:30 AM on 12 January, 2012
    Sphaerica - You have the makings of a post/rebuttal there. Maybe a few graphics to explain how it fits together. I only mention this because it is a legitimate area of confusion, and perhaps we haven't adequately explained the process.
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62. exp137 at 11:05 AM on 12 January, 2012
    Thanks for the responses everyone. This was seriously informative, and needed. I agree with Rob Painting here, that it would be great to put this together into a post to fill the gap left by focusing mainly on the lag on the warming end of the cycle. From what I've seen, every time you leave something not explicitly explained, a skeptic will use this as a point of attack against the entire concept. Again, thank you very much for your quick and detailed responses.
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63. Daniel Bailey at 11:30 AM on 12 January, 2012

    "every time you leave something not explicitly explained, a skeptic will use this as a point of attack against the entire concept"

    Those aren't skeptics. Those are fake-skeptics.
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64. muoncounter at 13:14 PM on 12 January, 2012
    It turns out the mechanisms for glacial onset are still uncertain. Timmerman et al 2010: Compared to the rapid glacial terminations, the buildup of glacial ice sheets in the Northern Hemisphere took tens of thousands of years. During the buildup phase, the growing ice sheets were subject to major orbitally induced summer insolation changes, without experiencing complete disintegration. The reason for this behavior still remains elusive. But why another lag post? See the existing 'CO2 lags temperature' rebuttals; both basic and intermediate are quite good - they generated 330 comments.
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65. Rob Painting at 14:08 PM on 12 January, 2012
    Muon - Sphaerica's comments helped clear exp137's confusion, whereas our current rebuttals did not. There is almost always room for improvement, and that's no slight on all our current rebuttals. I don't think that the number of comments is of that much value either - sometimes specific topics are contentious because they decimate well-worn fake-skeptic myths and said fake-skeptics swarm to defend it. The more valuable rebuttals, to my mind, are those that so thoroughly discredit a myth it never really gets going in the first place But YMMV.
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