I find this paper (PDF) interesting, but it still does not address the temperature/CO2 800 year time lag seen in ice core records. h/t to Leif Svalgaard – Anthony
Fossil soils constrain ancient climate sensitivity
Dana L. Royer 1
Department of Earth and Environmental Sciences, Wesleyan University, Middletown, CT 06459
Global temperatures have covaried with atmospheric carbon dioxide (CO2) over the last 450 million years of Earth’s history (1). Critically, ancient greenhouse periods provide some of the most pertinent information for anticipating how the Earth will respond to the current anthropogenic loading of greenhouse gases. Paleo-CO2 can be inferred either by proxy or by the modeling of the long-term carbon cycle.
For much of the geologic past, estimates of CO2 are consistent across methods (1). One exception is the paleosol carbonate proxy, whose CO2 estimates are often more than twice as high as coeval estimates from other methods (1). This discrepancy has led some to question the validity of the other methods and has hindered attempts to understand the linkages between paleo-CO2 and other parts of the Earth system. In this issue of PNAS, Breecker and colleagues (2) break important new ground for resolving this conflict.
The paleosol carbonate proxy for atmospheric CO2 is based on the analysis of carbonate nodules that precipitate in soils in seasonally dry to dry climates. These nodules incorporate carbon from two sources: atmospheric CO2 that diffuses directly into the soil and in situ CO2 from biological respiration. Because the stable carbon isotopic composition of these two sources is distinct, the concentration of atmospheric CO2 can be inferred if the concentration of soil CO2 and the isotopic compositions of the two sources are known (3). Atmospheric CO2 estimates scale directly with soil CO2 concentration: If the soil term is wrong by a factor of two, the inferred atmospheric CO2 will be off by a factor of two.
Estimates of soil CO2 concentration for fossil soils have been based on measurements taken during the growing season in equivalent living soils. However, Breecker et al. (2, 4) demonstrate convincingly that the window of active carbonate formation is restricted to the warmer and dryer parts of the growing season. Carbonate formation is simply not thermodynamically favorable during cooler and wetter seasons. Critically, biological productivity and respiration are low during these dry periods. As a result, soil CO2 concentration during the critical window of active carbonate formation has been overestimated in most soils by a factor of two or more (2).
What does this mean? CO2 estimates from the paleosol carbonate proxy can be cut in half (or more). Doing so snaps the paleosol-based estimates in line with most other approaches (2) (Fig. 1B) and produces the most precise view to date of Earth’s CO2 history. We are now better equipped to answer some important, basic questions. For example, what is the quantitative relationship between CO2 and temperature? That is, for every doubling of CO2, what is the long-term (103–104 years) equilibrium response of global temperature (termed here climate sensitivity)?
Most assessments of climate sensitivity for the present day hover around 3°C per CO2 doubling (5), although if the longterm waxing and waning of continental ice sheets are considered it is probably closer to 6°C (6). Less is known about climate sensitivity during ancient greenhouse periods, simply because having poles draped in forest instead of ice represents a profound rearrangement of climate feedbacks.
Records of CO2 and temperature are now sufficiently robust for placing firm minimum constraints on climate sensitivity during parts of the Cretaceous and early Paleogene (125–40 Mya), a well-known globally warm interval. Indeed, owing to the logarithmic relationship between CO2 and temperature, the geologic record is ideally suited for establishing minimum thresholds. This is because, to accommodate a declining sensitivity, other boundary conditions of the Earth system need to shift exponentially, for example, unreasonable oscillations in atmospheric CO2. Policywise, establishing a basement value for climate sensitivity is a critical step for addressing our current climate crisis (5).
With few exceptions, CO2 during the Cretaceous and early Paleogene was<1,000 ppm (2) (Fig. 1B). Global mean surface temperature is very difficult to establish for these ancient periods. However, temperature change in the tropics today scales at roughly two-thirds the global change (5, 6).
If we assume a similar relationship in the past and a climate sensitivity of 3°C perCO2 doubling, a rise in atmospheric CO2 to 1,000 ppm results in a 3.6°C warming in the tropics (relative to a 280-ppm baseline).
Given that tropical sea surface temperatures range from 27° to 29°C today, tropical temperatures exceeding 30.6°–32.6°C (red band in Fig. 1A) during the Cretaceous and early Paleogene likely correspond to a climate sensitivity >3°C. This threshold was commonly surpassed during the Cretaceous and early Paleogene (Fig. 1A). For times when CO2 was <1,000 ppm, the tropical temperature threshold for a 3°C climate sensitivity would shift to correspondingly cooler values.
Further, there is abundant evidence for flatter latitudinal temperature gradients during greenhouse periods (7, 8), meaning, again, that the tropical temperature threshold used here is probably a maximum. Together, it is clear that during the Cretaceous and Paleogene climate sensitivity commonly exceeded 3°C per CO2 doubling.
Although further work is needed, the geologic evidence (2) (Fig. 1) is most consistent with long-term, future climate change being more severe than presently anticipated (5). Also, global climate models tuned to ancient greenhouse periods commonly have emergent climate sensitivities of <3°C and they fail to simulate the shallow latitudinal temperature gradients (9). Thus even for times with little ice, there are important positive feedbacks that are presently not captured adequately in climate models. Processes for warming the high latitudes without a change in CO2 include more vigorous heat transport (10, 11), more widespread stratospheric clouds in the high latitudes (12), and climate feedbacks from polar forests (13). and their study highlights the value of a clearly resolved paleo-CO2 record. However, a limitation is that they uniformly apply a “best guess” value of 2,500 ppm for soil CO2 concentration.
They recognize this as an oversimplification and is an area for future work. Better modeling of the term, perhaps through independent proxy (14), may result in a further tightening of the paleo-CO2 record.
1. Royer DL (2006) CO2-forced climate thresholds during the Phanerozoic. Geochim Cosmochim Acta 70:5665– 5675.
2. Breecker DO, Sharp ZD, McFadden LD (2010) Atmospheric CO2 concentrations during ancient greenhouse climates were similar to those predicted for 2100 A.D. Proc Natl Acad Sci USA 107:576–580.
3. Cerling TE (1991) Carbon dioxide in the atmosphere: Evidence from Cenozoic and Mesozoic paleosols. Am J Sci 291:377–400.
4. Breecker DO, Sharp ZD, McFadden LD (2009) Seasonal bias in the formation and stable isotopic composition of pedogenic carbonate in modern soils from central New Mexico, USA. Geol Soc Am Bull 121:630–640.
5. IPCC (2007) Climate Change 2007: The Physical Science Basis, Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (Cambridge Univ Press, Cambridge, UK).
6. Hansen J, et al. (2008) Target atmospheric CO2: Where should humanity aim? Open Atmospheric Sci J 2: 217–231.
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8. Bijl PK, et al. (2009) Early Palaeogene temperature evolution of the southwest Pacific Ocean. Nature 461: 776–779.
9. Shellito CJ, Sloan LC, Huber M (2003) Climate model sensitivity to atmospheric CO2 levels in the Early-Middle Paleogene. Palaeogeogr Palaeoclimatol Palaeoecol 193: 113–123.
10. Korty RL, Emanuel KA, Scott JR (2008) Tropical cycloneinduced upper-ocean mixing and climate: Application to equable climates. J Clim 21:638–654.
11. Ufnar DF, González LA, Ludvigson GA, Brenner RL, Witzke BJ (2004) Evidence for increased latent heat transport during the Cretaceous (Albian) greenhouse warming. Geology 32:1049–1052.
12. Abbot DS, Tziperman E (2008) Sea ice, high-latitude convection, and equable climates. Geophys Res Lett 35:L03702.
13. Beerling DJ, Nicholas Hewitt C, Pyle JA, Raven JA (2007) Critical issues in trace gas biogeochemistry and global change. Philos Trans R Soc Lond A 365:1629–1642.
14. Retallack GJ (2009) Refining a pedogenic-carbonate CO2 paleobarometer to quantify a middle Miocene greenhouse spike. Palaeogeogr Palaeoclimatol Palaeoecol 281:57–65.
15. Bice KL, et al. (2006) A multiple proxy and model study of Cretaceous upper ocean temperatures and atmospheric CO2 concentration. Paleoceanography 21: PA2002.
16. Bornemann A, et al. (2008) Isotopic evidence for glaciation during the Cretaceous supergreenhouse. Science 319:189–192.
17. Forster A, Schouten S, Baas M, Sinninghe Damsté JS (2007) Mid-Cretaceous (Albian Santonian) sea surface temperature record of the tropical Atlantic Ocean. Geology 35:919–922.
18. Forster A, Schouten S, Moriya K, Wilson PA, Sinninghe Damsté JS (2007) Tropical warming and intermittent cooling during the Cenomanian/Turonian oceanic anoxic event 2: Sea surface temperature records from the equatorial Atlantic. Paleoceanography 22:PA1219.
19. Moriya K, Wilson PA, Friedrich O, Erbacher J, Kawahata H (2007) Testing for ice sheets during the mid-Cretaceous greenhouse using glassy foraminiferal calcite from the mid-Cenomanian tropics on Demerara Rise. Geology 35:615–618.
20. Norris RD, Bice KL, Magno EA, Wilson PA (2002) Jiggling the tropical thermostat in the Cretaceous hothouse. Geology 30:299–302.
21. Pearson PN, et al. (2001) Warm tropical sea surface temperatures in the Late Cretaceous and Eocene epochs. Nature 413:481–487.
22. Pearson PN, et al. (2007) Stable warm tropical climate through the Eocene Epoch. Geology 35:211–214.
23. Schouten S, et al. (2003) Extremely high sea-surface temperatures at low latitudes during the middle Cretaceous as revealed by archaeal membrane lipids. Geology 31:1069–1072.
24. Tripati A, et al. (2003) Tropical sea-surface temperature reconstruction for the early Paleogene using Mg/Ca ratios of planktonic foraminifera. Paleoceanography 18:1101.
25. Wagner T, et al. (2008) Rapid warming and salinity changes of Cretaceous surface waters in the subtropical North Atlantic. Geology 36:203–206.
26. Wilson PA, Norris RD (2001) Warm tropical ocean surface and global anoxia during the mid-Cretaceous period. Nature 412:425–429.
27. Wilson PA, Norris RD, Cooper MJ (2002) Testing the Cretaceous greenhouse hypothesis using glassy foraminiferal calcite from the core of the Turonian tropics on Demerara Rise. Geology 30:607–610.
28. Wilson PA, Opdyke BN (1996) Equatorial sea-surface temperatures for the Maastrichtian revealed through remarkable preservation of metastable carbonate. Geology 24:555–558.
29. Sexton PF, Wilson PA, Pearson PN (2006) Microstructural and geochemical perspectives on planktic foraminiferal preservation: “glassy” versus “frosty”. Geochem Geophys Geosyst 7:Q12P19.
30. Pagani M, Lemarchand D, Spivack A, Gaillardet J (2005) A critical evaluation of the boron isotope-pH proxy: The accuracy of ancient ocean pH estimates. Geochim Cosmochim Acta 69:953–961.
There is a lot of data selection and confusing graphs in this paper. That is the signature of all of Royer’s papers.
The pedogenic carbonates/paleosols/fossil soils CO2 estimates have always produced widely varying estimates (Royer changes the name each time he uses them).
Look at the data in the chart for around 65 to 70 million years ago. The revised fossil soils CO2 estimates range from Zero to 750 ppm with the majority being lower than 280 ppm. Previously, these estimates ranged from Zero to 1,500 ppm at this time period (2,950 ppm at 75 million years ago).
They should just quit using this method, it is not reliable. Any CO2 estimate that produces Zeros and unrealistically low estimates consistently should be discontinued (there will be at least 12 estimates below 100 ppm with this newer method now).
It is no wonder that Royer wants to restate these CO2 estimates, they do not match the temperature record at 3.0C per doubling.
Here is all of the CO2 estimates over the whole record from Fossil Soils/Pedogenic Carbonates as used in the IPCC AR4 (I believe they were collated by Royer).
http://img708.imageshack.us/img708/4313/co2fossilsoilsar4.png
as documented here: figure6.1top
ftp://ftp.ncdc.noaa.gov/pub/data/paleo/ipcc2007/
This paper says, “If we assume a similar relationship in the past and a climate sensitivity of 3°C perCO2 doubling, a rise in atmospheric CO2 to 1,000 ppm results in a 3.6°C warming in the tropics (relative to a 280-ppm baseline).” Note the word “assume”. But Dr. Jeffrey Glassman, in
THE ACQUITTAL OF CARBON DIOXIDE
http://www.rocketscientistsjournal.com/2006/10/co2_acquittal.html
has shown that the 800 – 1000 year time lag in the Vostok ice cores and the overall shape of the temperature and CO2 curves is entirely consistent with temperature being the driver of CO2 concentration, and that there is no evidence in the Vostok cores that CO2 drives temperature at all. I have looked for a reasoned refutation of Glassman’s thesis on the web and have not found one.
Leif Svalgard: “The paleo data is not fine-grained enough to show what follows what. The data does show that a much higher CO2-content was rather normal, and that the SST was ~5 degrees higher than today.”
And that proves – what?
A “printable” left angle bracket is made with these four characters in succession:
&
lt
;
For a right angle bracket, use “gt”
As the time baseline extend back, the 800 years of precede or lag becomes invisible due to the error margin in the time domain. Until the recent ice core data is explained – this paper has a fundamental problem – one could look at the paleo-data and rationally identify CO2 as a lagging indicator of warming. As one could do today.
Still seems chicken or egg stuff to me….. Warming first then CO2…. Or warm because of CO2.
No very clarifying…
Am I at WUWT?:
If we assume a similar relationship in the past and a climate sensitivity of 3°C perCO2 doubling, a rise in atmospheric CO2 to 1,000 ppm results in a 3.6°C warming in the tropics (relative to a 280-ppm baseline).
The Holy Church of Global Warming.
For your glossary:
PNAS= Proceedings of the National Academy of Sciences
Carl Chapman (23:08:23) :
Taking your feedback exercise a stage further consider 1880 with a temperature anomoly of -0.2C on http://www.cru.uea.ac.uk/cru/data/temperature/nhshgl.gif and 0.5C in 2000 both are points at a local peak of temperature so not a cherry pick. Assume CO2 grew from 280 ppm (pre industrial?) in 1880 to 360ppm in 2000 according to http://www.esrl.noaa.gov/gmd/ccgg/trends/#mlo_full What value of x,r do you calculate. I make x to be about 0.48 and r=1.92 (assuming the flat last 10 years means no more warming from pre 2000 addition of anthropic CO2). I assumed warming due to CO2 alone (before feedbacks) as ((1/ln(2))*ln(C2000/C1880) = 0.363 so doubling CO2 would give 1 deg C as you used. C2000 is CO2ppm in 2000.
Lonnie Schubert (04:46:46) : Thanks for that link to Scotese. His hot/cold flip flop certainly doesn’t fit with linear temp/CO2
anna v (00:39:31)
The absurd cartoon you link to bears no resemblance to the way that anybody thinks that the climate works.
Once again starting with the assumption that CO2 drives temperature than try to make the record suggest a large sensitivity. Very telling is the repeated use of the idea of degrees change per doubling as if a second doubling would have an equal effect.
I am very disappointed, the built in bias is so large I have trouble taking it seriously and they may have something of interest buried here
anna v (22:08:14) :
I wonder if there is somebody who has calculated how much CO2 will boil out of the oceans…and wayne (02:48:21) :
My estimates suggest that warming the oceans from 27-29 DegC range to 30.6°–32.6°C range could reduce the CO2 solubility by a relative 10% to 15%.
How much CO2 is that? Well, it looks to me like it could pretty much account for the CO2 range shown in the diagram B. In other words, warming the surface & intermediate ocean levels (from any source) could account for all of that CO2 without AGW forcing.
Details:
I started with the CO2 – Water solubility per Glassman to get an approximation of the CO2 lost as the water warms.
This table shows the grams of CO2 in 100 g water at various temperatures. The percentage columns show the remaining CO2 relative to the 27, 28 or 29 Deg CO2. So the interpolated CO2 value for 32.6 would be about 86% versus a 27 Deg starting point, and would be 91% vs a 29 Deg baseline. For the sensitivity of this estimate, I suggest 10% to 15% loss is a reasonable approximation.
DegC__g/100g_________ vs 27___vs 28___vs29
27_____0.137 100.0%
28_____0.133 97.1% 100.0%
29_____0.129 94.6% 97.4% 100.0%
30_____0.126 92.0% 94.7% 97.3%
32.6____0.118 interpolated 86.2% 88.8% 91.2%
35_____0.111 80.9% 83.3% 85.5%
How much CO2 is that relative to estimated Ocean and Atmospheric material balances?
From Holmen, 2000 we get estimated CO2 in Surface + Intermediate Ocean to the order of 7600 Gt CO2 (as Carbon)
and for atmospheric CO2 of 380 ppm, the estimate is about 750 Gt CO2 (as Carbon)
If 10 to 15% of the 7600 Gt is released to the atmosphere, that would be an additional 760 to 1100 Gt CO2, which could change the
380 ppm (at 750Gt)
to 760 ppm (at 750+760 Gt)
up to about 960 ppm (at 750 + 1100 Gt).
To my eye, that CO2 range up to 960 ppm pretty much accounts for the ‘dotted line’ area for Atmospheric CO2 (ppm) as shown in graph B.
JT (06:40:08)
The only response I have seen is here:
Does this prove that CO2 doesn’t cause global warming? The answer is no. The reason has to do with the fact that the warmings take about 5000 years to be complete. The lag is only 800 years. All that the lag shows is that CO2 did not cause the first 800 years of warming, out of the 5000 year trend. The other 4200 years of warming could in fact have been caused by CO2, as far as we can tell from this ice core data.
http://www.realclimate.org/index.php/archives/2004/12/co2-in-ice-cores/
Like I said earlier, this line of thinking still seems to be an answer in search of a problem.
RE: J.Hansford (07:03:51) : “Still seems chicken or egg stuff to me….. Warming first then CO2…. Or warm because of CO2.”
As water vapor [‘dihydrogen monoxide’] is *the* primary greenhouse gas in the atmosphere, it might be more instructive to attempt to track or estimate past changes in the H2O concentration of the atmosphere and perhaps more importantly, the stratosphere…
http://www.scientificamerican.com/article.cfm?id=is-water-vapor-in-the-stratosphere-slowing-global-warming
Dr. Royer references his own paper (Royer, 2006) for this claim. Veizer’s and Shaviv’s reply to Royer’s previous assertion of Phanerozoic and temperature covariance (Royer et al., 2004) pointed out that this covariance was obtained through a pH adjustment to oxygen isotope data; which brought the temperatures into line with the pale-CO2 levels. The pH adjustment was derived from CO2. So, Royer used CO2 to calibrate temperatures to CO2…
Then Royer suggests that the deviation of paleosol (pedogenic carbonate) derived CO2 from other methods of CO2 derivation, somehow brings those other methods into question…
In the case of Royer’s 2004 paper on Paleocene-Eocene Thermal Maximum (PETM) CO2 levels, he showed that pedogenic carbonates actually yielded CO2 levels that were 4-5 times as large as those derived from fossil plant stomata indices. Yet Royer insists that pedogenic carbonates yield “the most precise view to date of Earth’s CO2 history.” Plant SI data can actually be calibrated to the modern instrumental record and can be empirically tested; yet Royer would reject these data in favor of the data that supports his hypothesis of Phanerozoic covariance of CO2 and temperature.
This leads Royer to the absurd conclusion that “for every doubling of CO2” is “is probably closer to 6°C”; rather than the maximum of 2°C that can be derived from CO2 and temperature data since 1880. And the 2°C can only be achieved if every bot of warming since 1880 is the result of an enhanced greenhouse effect from anthropogenic CO2.
What could possibly lead Dr. Royer to continually torture the data until it says what he wants it to say?
Maybe it has something to do with him being an environmental activist… Ask a Professor: Dana Royer, Earth and Environmental Science.
@ur momisugly John Wright (01:08:34) :
Just a semantic niggle and I don’t know if it’s relevant here (although I think it does reveal a certain mindset): a model can take observed facts into account and perhaps give a new slant on them, but I don’t see how it can “capture” anything, let alone “important positive feedbacks”.
The contrary of what you just said is really the logical failure at the heart of the GCMs – so I think it is very important that we do not fall into the same bad way of thinking. Careful use of observed data is very useful for supporting/refuting theories (like feedbacks), but in and of itself can’t make theories. As discussed on another thread recently, this is where Economics (specifically Econometrics) seems to be a much better evolved and mature discipline than climate science.
You can tease correlations out of data sets all day long, but those correlations have to then be underpinned by a theory that can then be proven or falsified through observation or experimentation. All of that must be done *before* you even think about plugging that relationship into a more complex model and consider it to have any predictive power based on solid science.
It might just be a “semantic niggle”, but it is important.
It appears to me that the paleosol carbonate proxy, is proved to be useless. The chart offered above shows this proxy is all over the place in results and the author says the results need adjustments to make them fit in. Shades of Mann and Jones, we don’t need more of this. BS (bad science) poor premis both in chemistry and physics.
From Robert:
“Imagine a vase on the top of a stand. As long as you leave it alone, nothing. But push on it, even with one finger, it will tip over and then fall. Positive feedback takes over, in the form of gravity. You can see how gravity works by measuring how long the object takes to fall, just as you can measure the greenhouse effect of CO2 in the climate record, even though in both cases the initial “nudge” is different from the force being measured.”
—————
Reply:
So Robert, you’re saying it took 800 years for the CO2 vase to fall, yet in the meantime the contents within the vase were flowing out, causing the forcing?
Your analogy doesn’t hold water.
Because not only is there an 800-year lag of CO2 w/ warming, but also a similar lag following cooling. You’re saying there’s something that nudges the vase so it stands upright?
Again, your analogy doesn’t hold water.
People here keep trashing the paper because it doesn’t give a 5 minute sound bite proof that AGW/GHG exists/doesn’t exist.
Quick point of fact: Prior to the paper, there was an issue where a particular paleoproxy for CO2 levels did not jive with other proxies. This paper provides a correction that eliminates that problem with a meaningful, logical reason for it. It then points out how, once that particular detail is adjusted, other inferences (which were previously hard to reconcile) can be made.
That’s all. No more, no less. It’s one guy, one scientist, whose job is to work with paleo-data (and he’d have that job with or without AGW, people, he doesn’t need a climate crisis to keep his post). He did something useful, which incrementally increases our knowledge of the climate.
That’s all! If it doesn’t say what you want it to say… so what?
The fact that you all want him to talk about CO2 being a driver or a feedback, or providing proof about time lags, and all that other nonsense is like standing up in the middle of a theater and complaining that you want caviar instead of popcorn. It’s silly, embarrassing, and unproductive.
Read the article, learn, and move on.
Carl Chapman (23:08:23) :
son of mulder (07:59:50) :
And further to the above 2 posts imagine halving the amount from 280 ppm of atmospheric CO2 and halving it again. Keep doing this for 8 times and you get down to approx 1 part CO2 and 273 deg K. I’m sure at some point other factors would cut in but drawing the graph of temp vs CO2 ppm the go forward effect of CO2 and feedbacks gets less and less per unit of added CO2. It would take something quite dramatic to change the go forward graph into a runaway. What?
Tamara (05:49:36) : Not to worry, humans are a tiny spec of the respiring biomass on this planet, even with all our excesses we contribute about 3% of the CO2, or 2ppm/yr. If we disappeared CO2 would not change significantly.
This study is interesting by finding a way to cut previous CO2 in half, making modern CO2 levels more significant, clever. That seems to be the sole innovation, the rest is the usual CAGW/IPCC slop.
I am biased against AGW, so I choose to believe the following:
The Interglacial and other paleo-warmings are completely unrelated to CO2 with respect to causation or feedback. Most CO2 production on earth is due to respiration of organisms: bacteria, fungi, etc. When it warms for non-CO2 reasons, these organisms thrive and covert C and O2 to CO2 at a higher rate—so atmospheric CO2 rises, lagging warming, and then falls, lagging cooling.
Of course my “belief” is not science, even if it is based on “scientific conjecture”. Neither is the warmist belief in positive feedbacks of other types causing rising CO2 which then accelerates warming. In fact, compared to other parts of science, climatology is in its infancy and no self-respecting scientist should hold to any particular “beliefs”. My god, we cannot even measure the current “global temperature” with accuracy, yet some are pretending that paleo-proxies for temperature and atmospheric CO2, etc. are sufficient for scientific conclusions about climate sensitivity to rising CO2. Shame on these people for calling their work “science”.
KW
Speculation.
There’s a massive logical problem with speculations/conclusions of this nature. If positive feedback takes over, then how can the increase ever possibly stop? The world would continue heating, or at least hit a high point and then stay there. That’s the entire nature of a positive feedback.
I see this sort of statement all over the place, but have yet to see anyone propose a mechanism that would explain how historically we’ve had far higher CO2 levels, and yet temperatures do not continue increasing, or even hit a maximum and remain there – no, temperatures fluctuate, and return to far colder conditions.
Anyone who propounds CO2 as a positive feedback that is robust enough to overcome existing negative feedbacks, must also be able to explain logically how the system can then cycle back to colder temperatures in the face of their tipping point CO2 positive feedback.
Otherwise, it seems to me that the very idea of a positive feedback CO2 ‘tipping point’ is pretty much disproved right on the face of it. (which was, I believe, the whole point of Rabe’s perpetual motion machine – grin)
@ur momisugly JER0ME (00:30:05) : Over at unreal climate they have fully explained the temperature/CO2 800 year time lag:
==========================
That’s the same logic problem with the ‘real climate’ explanation too – if CO2 dun-it, then whatever allowed the process to stop and temperatures to cool again, in the face of highly elevated CO2 levels?