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Schmittner et al. (2011) on Climate Sensitivity - the Good, the Bad, and the Ugly

Posted on 27 November 2011 by dana1981

A new paper in Science from Schmittner et al. (2011) attempts to constrain climate sensitivity based on temperature reconstructions of the Last Glacial Maximum (LGM) approximately 20,000 years ago:

"combining extensive sea and land surface temperature reconstructions from the Last Glacial Maximum with climate model simulations we estimate a lower median (2.3 K) and reduced uncertainty (1.7–2.6 K 66% probability)."

This estimate is significantly narrower and a bit lower than the IPCC-estimated 66% probability range for equilibrium climate sensitivity of 2 to 4.5°C for doubled atmospheric CO2, and is illustrated in Figure 1.

schmittner sensitivity

Figure 1: Marginal posterior probability distributions for equilibrium climate sensitivity to doubled atmospheric CO2 (ECS2xC) from Schmittner et al. (2011), estimated from land and ocean, land only, and ocean only temperature reconstructions.

Concerns About the Study

There are some unusual aspects about this study which require further investigation before the conclusions of the study can be accepted, as the authors themselves point out.  For example, the study uses a relatively new global mean surface temperature reconstruction for the LGM of just 2.2°C cooler than interglacial temperatures in the locations where they have proxy data, or 2.6°C from the global model average.  This is significantly lower than most paleoclimate estimates, which generally put the LGM in the range of 4 to 7°C cooler than current temperatures.  For comparison, in their study also using the LGM to constrain climate sensitivity, Hansen and Sato (2011) used a mean surface temperature change of 5°C, consistent with the body of literature (Figure 2).

Fig 2

Figure 2: Climate forcings during the ice age 20 ky ago relative to the pre-industrial Holocene from Hansen and Sato (2011)

Since the radiative forcing associated with doubled CO2 is 3.7 Watts per square meter (W/m2), Hansen and Sato's result implies a fast-feedback climate sensitivity of 2.8°C, which is slightly outside the Schmittner et al. 66% probability range (at the upper end of their 90% probability range).  In fact, as Urban explains, the main reason Schmittner et al. arrive at a lower climate sensitivity estimate than previous studies is due to their lower LGM temperature reconstruction:

"our LGM temperature reconstruction is quite different from what has been commonly assumed, and our study may prove inconsistent with other evidence that we have not yet considered. This is something that will have to be sorted out by further debate and research...our new temperature reconstruction explains a lot of the difference between our climate sensitivity estimate and previous estimates."

In an interview with New Scientist on this paper, Gavin Schmidt said:

"The model estimate of the cooling during the Last Glacial Maximum is a clear underestimate...A different model would give a cooler Last Glacial Maximum, and thus a larger sensitivity."

As Figure 1 shows, the Schmittner et al. global climate sensitivity estimate is dominated by the ocean data, which is based on from the Multiproxy Approach for the Reconstruction of the Glacial Ocean (MARGO) project, about which Richard Alley noted:

"MARGO made a solid effort, which indicates very small temperature changes. But, there are other ways to do it, and indeed, [Schmittner et al.] coauthor Alan Mix has published independent papers indicating that the temperature changes were larger in some regions than indicated by MARGO.   David Lea and others have also obtained larger temperature shifts….

In short, the MARGO data for the ocean show very small temperature change from the ice age to today, and thus lead to the low climate sensitivity, but they disagree with some independent estimates showing larger temperature change.  They also lead to disagreement with the pollen-based land temperature data.  Furthermore, they lead to an answer that disagrees with many other lines of evidence for climate sensitivity."

Another concern regarding the study is in the model they used - the University of Victoria (UVic) climate model.  UVic is an admittedly simple model compared to other global climate models, as co-author Nathan Urban discussed in an interview with Planet 3.0:

"UVic isn’t the most complex model either.  It has a simplified atmosphere, which is an advantage and disadvantage.  The disadvantage is that it has a very approximate representation of atmospheric processes.  The advantage is that this makes the simulations run faster.  It is less computationally expensive."

A number of other climate scientists interviewed for a BBC article also expressed reservations about the study's assumptions and results.  For example, the climate sensitivity in transitioning from a cold to warm period may be different than that in transitioning from a warm to a hot period, as Andrey Ganopolski noted:

"There is evidence the relationship between CO2 and surface temperatures is likely to be different [during] very cold periods than warmer."

This is particularly true since the LGM only experienced fast feedbacks, whereas due to the rapid rate of the current climate change, slower feedbacks may be triggered on century timescales.

The authors themselves have their own concerns, as is the case with any good scientific study, and expressed a number of caveats about their findings.  For example:

"Two things are immediately apparent from these [Figure 1] curves.  First, the sea surface temperature data support lower climate sensitivities and the land surface temperature data support higher sensitivities.  There isn’t a great deal of overlap between these curves, so this suggests a possible inconsistency between the land and ocean analyses.  Second, when we combine the land and ocean data, the ocean data dominate the result (the black and blue curves are very similar), “overruling” what the land data have to say.  I think this is, at least in part, because there are more ocean data than land data."

"There are many hypotheses for what’s going on here.  There could be something wrong with the land data, or the ocean data.  There could be something wrong with the climate model’s simulation of land temperatures, or ocean temperatures.  The magnitudes of the temperatures could be biased in some way.  Or, more subtly, they could be unbiased, on average, but the model and observations could disagree on the cold and warm spots are, as I alluded to earlier.  Or something even more complicated could be going on.

Until the above questions are resolved, it’s premature to conclude that we have disproven high climate sensitivities, just because our statistical analysis assigns them low probabilities."

Setting these concerns aside for the moment, what would the Schmittner et al. results mean, if correct?

The Good News

The climate denialists have of course focused on the good news aspect of this paper - that it claims to rule out the 'long tail' of high climate sensitivity.  Most individual studies estimating climate sensitivity are unable to rule out very high sensitivity values (Figure 3).

Figure 3: Probability distribution of climate sensitivity to a doubling of atmospheric CO2

However, Annan and Hargreaves (2009) used a Bayesian statistical approach to investigate various probabilistic estimates of climate sensitivity, and concluded that

"the long fat tail that is characteristic of all recent estimates of climate sensitivity simply disappears, with an upper 95% probability limit...easily shown to lie close to 4°C, and certainly well below 6°C."

So Schmittner et al. would not be the first study to find low probability of very high values of (fast feedback) climate sensitivity.  Nevertheless, their conclusion that high sensitivity models do not simulate LGM changes well is good news:

"models with ECS2xC > 4.5 K overestimate the cooling almost everywhere, particularly at low latitudes. High sensitivity models (ECS2xC > 6.3 K) show a runaway effect resulting in a completely ice-covered planet."

The Bad News

For those true skeptics among us who look at the entire study, unfortunately it contains substantial bad news.  Firstly, in addition to ruling out very high equlibrium climate sensitivity values, it would also rule out very low values:

"Models with ECS2xC < 1.3 K underestimate the cooling at the LGM almost everywhere, particularly at mid latitudes and over Antarctica"

In other words, Schmittner et al. find equilibrium sensitivities of less than 1.3°C just as unrealistic as sensitivities greater than 4.5°C.  The low sensitivity arguments made by the likes of Spencer, Lindzen, Christy, Monckton, etc., which are the climate denialist "endgame", proclaim that climate sensitivity is indeed less than 1.3°C for doubled CO2.  According to Schmittner et al., they're wrong.  Somehow the climate denialists glossed over this aspect in their reporting on the paper.

It's worth briefly noting here that when confronted with the fact that paleoclimate data are inconsistent with their asserted low climate sensitivity values, the "skeptics" suddenly find the proxy data and models unreliable.  For example, Pielke Sr.:

"I do not find the glacial and interglacial periods as useful comparisons with the current climate since when we study them with models"

But once the proxy data and models support a conclusion they want to believe - climate sensitivity is not extremely high - suddenly the supposed "skeptics" (including Pielke's 'colleagues') are willing to accept the results entirely uncritically.  Just another of those denialist self-contradictions to add to the list.

Secondly, as noted above, Schmittner et al. have assumed that the difference between a glacial maximum and interglacial temperature is a mere 2.6°C.  The global average surface temperature has already warmed 0.8°C over the past century.  During the LGM, the surface was covered with huge ice sheets, plant life was different, and sea levels were 120 meters lower. As Schmittner notes:

"Very small changes in temperature cause huge changes in certain regions, so even if we get a smaller temperature rise than we expected, the knock-on effects would still be severe."

and in a Science Daily interview:

"Hence, drastic changes over land can be expected.  However, our study implies that we still have time to prevent that from happening, if we make a concerted effort to change course soon".

The Ugly News

In short, if Schmittner et al. are correct and such a small temperature change can cause such a drastic climate change, then we may be in for a rude awakening in the very near future, because their smaller glacial-interglacial difference would imply a quicker climate response a global temperature change, as illustrated in Figure 4.

schmittner vs IPCC

Figure 4: IPCC and Schmittner et al. CO2-caused warming based on business-as-usual (BAU) emissions (defined as IPCC Scenario A2) and their equilibrium climate sensitivity best estimates, assuming transient sensitivity is ~67% of equilibrium sensitivity (solid lines) vs. their best estimates for the average global surface temperature change between the LGM and current interglacial (dashed lines).

As Figure 4 illustrates, although the Schmittner et al. best estimate for climate sensitivity results in approximately 20% less warming than the IPCC best estimate, we also achieve their estimated temperature change between glacial and interglacial periods (the dashed lines) much sooner.  The dashed lines represent the temperature changes between glacial and interglacial periods in the Schmittner (blue) and IPCC (red) analyses.  If Schmittner et al. are correct, we are on pace to cause a temperature change of the magnitude of an glacial-interglacial transition - and thus likely similarly dramatic climate changes - within approximately the next century.*

* - Note that this calculation and Figure 4 exclude warming caused by non-CO2 greenhouse gases (GHGs) whose warming effects are currently approximately offset by aerosols, but this offset probably won't continue in the future as GHG emissions continue to rise and aerosol emissions likely fall due to efforts to achieve clean air.  Thus our CO2-caused warming estimates are likely conservative, underestimating total future global warming.

Schmittner Take-Home

To summarize,

  • Schmittner et al. believe they have found low probabilities for both very high and very low equilibrium climate sensitivities, and their best-fit model sensitivity is 2.4°C for doubled CO2
  • There are some concerns about the Schmittner et al. methodology, such as their use of the simple and outlying UVic climate model, and their estimate that the temperature change between interglacial periods and the LGM was just 2.6°C
  • If Schmittner et al. are right about climate sensitivity and LGM temperature change, then if we continue with business-as-usual GHG emissions, we will match the amount of warming between glacial and interglacial periods within roughly the next century.  Some of the differences between glacial and interglacial periods include 120 meter sea level rise, and a completely different global landscape - very dramatic climate changes.

In short, we should not over-emphasize the results of Schmittner et al., as the authors themselves warn.  Their results are roughly consistent with other estimates of climate sensitivity (Figure 5).

Various estimates of climate sensitivity

Figure 5: Distributions and ranges for climate sensitivity from different lines of evidence. The circle indicates the most likely value. The thin colored bars indicate very likely value (more than 90% probability). The thicker colored bars indicate likely values (more than 66% probability). Dashed lines indicate no robust constraint on an upper bound. The IPCC likely range (2 to 4.5°C) and most likely value (3°C) are indicated by the vertical grey bar and black line, respectively (Source: Knutti and Hegerl 2008)

In fact if Schmittner et al. are totally correct, we may be in for some rapid climate changes in the relatively near future, as we approach the amount of warming that separates a glacial from an interglacial period.

Note: this is the intermediate rebuttal to 'Schmittner finds low climate sensitivity'

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Comments 51 to 77 out of 77:

  1. @50, Royer et al, 2007 has shown that climate sensitivity between 1.6 and 5.5 degrees C has been a feature of the Earth's climate for the last 540 million years. More recently, Park and Royer, 2011 refines that result. As they state in their abstract:
    "As a result, our experiment maintains an agreement with ΔT2x estimates based on numerical climate models and late Cenozoic paleoclimate. For a climate sensitivity ΔT2x that is uniform throughout the Phanerozoic, the most probable value is 3° to 4 °C. GEOCARBSULF fits the proxy-CO2 data equally well, and with far more parameter choices, if ΔT2x is amplified by at least a factor of two during the glacial intervals of the Paleozoic (260-340 Ma) and Cenozoic (0-40 Ma), relative to non-glacial intervals of Earth history. For glacial amplification of two, the empirical PDFs for glacial climate sensitivity predict ΔT2x(g)>2.0 °C with ∼99 percent probability, ΔT2x(g)>3.4 °C with ∼95 percent probability, and ΔT2x(g)>4.4 °C with ∼90 percent probability. The most probable values are ΔT2x(g) = 6° to 8 °C. This result supports the notion that the response of Earth's present-day surface temperature will be amplified by the millennial and longer-term waxing and waning of ice sheets."
    Note that they are discussing the slow-feedback climate sensitivity, ie, the climate sensitivity with the Earth is allowed to adjust by changes of vegetation, and the melting of ice sheets etc. In contrast Schmittner et al discuss fast-feedback climate sensitivity. For comparison, Hansen has recently found a fast-feedback climate sensitivity of 2.8 degrees C per doubling, and a slow feedback climate sensitivity of 6 degrees C per doubling of CO2. Applying the same ratio to Schmittner et al' fast-feedback climate sensitivity from their best fitting model (2.4 degrees C per doubling of CO2) would yield a slow-feedback climate sensitivity of 5.14 degrees C per doubling. Most of the response of the slow-feedback climate sensitivity is due to melting ice sheets, so that in non-glacial worlds the slow and fast feedback sensitivities approximately equal each other (best estimate 3 to 4 degrees C ). Applying Hansen's ratio to the glacial slow-feedback sensitivity suggests a glacial fast-feedback as derived from Park and Royer in the range of 2.8 to 3.7 degrees C. That is a little rough, of course, but suggests that slow-feedback climate sensitivities are approximately constant across a wide range of geographical configurations and temperature ranges. To that it should be added that in discussing Schmittner et al, Real Climate report that Hargreaves and Annan find model simulations of the LGM show short-feedback climate sensitivity that is 80-90% of that found for a doubling of CO2 from preindustrial conditions across a range of models. So, some difference, but small. More importantly, and as discussed in my post @48, because the equilibrium warming ratio is a consequence of evaporation, either directly, or due to increased humidity and hence reduced lapse rates, in a cooler world (and hence a world with less evaporation) we would expect the warming ratio to be smaller. Indeed, there is some evidence of this in Sutton et al, 2007 which show the warming ration declining to 1 near the poles in models, and (less clearly) in observations. Hence, while I do think there will be some change in the Warming ratio in the LGM, it will be in a direction that makes my point (1) above more significant, and my point (2) above less significant.
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  2. #51 Tom : "In contrast Schmittner et al discuss fast-feedback climate sensitivity" This point is not clear for me. Their analysis deal with "annual mean surface temperature (sea surface temperature over oceans and near surface air temperature over land) change between the LGM and modern" (legend, figure 1). So in my mind, that is a 10 or 12 ka change between two equilibrium states, including what you call "slow feed-back".
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  3. Most importantly, I think the central point is not how well UVic model deals with evaporation, lapse rate, etc. – it is very unlikely in my mind that a particular model can be considered as more or less realist in its simulation, see here for example (figure 8.14) the still important divergences among IPCC AOGCM for WV, lapse rate, cloud, etc. The land/ocean warming ratio is a starting point from the reconstructed temperature (new proxy data set), not an utlimate result from the model runs. The model just try to reproduced the temperature and it seems to me that you reason as if the inverse was true. If the new proxy reconstruction is correct, then the land-ocean ratio will have to be reproduced by any model, no matter its complexity (RCM, EMIC, AOGCM, etc.). What you suggest in fact is that the proxy results are probably false, because most models produce a land/ocean warming ratio incompatible with the new proxy-based temperature.
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  4. @52, they allow change in albedo due to changes in ice sheet as a forcing in the model. Because the change is ice sheet is treated as a forcing, it is not treated as a feedback. Hence there are no (or at least no large) slow feedbacks in their model, from which it follows that they estimate fast-feedback climate sensitivity.
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  5. @53, no! What I suggest is that one of the model warming ratios, the land proxy temperatures, the sea proxy temperatures, or both, are in error because of the discrepancy between the Warming ratio found by Schmittner et al, and that from models in Sutton et al. I further suggest based on the additional evidence of warming seas north of Iceland according to Schmittner et al's reconstructions that an underestimate of sea surface temperatures is at least one component of the puzzle, so that once the issue is cleared by further studies a higher estimate of climate sensitivity is likely to be one of the results. Further, regarding models, the UVic model is described as follows:
    "A globally-averaged lapse rate is used to reduce the model’s apparent sea level temperature in calculating the following: the outgoing longwave radiation; the surface air temperature (SAT) dependent planetary co-albedo through the calculation of the areal fraction of terrestrial snow/ice; the saturation specific humidity to determine the amount of precipitation; whether the precipitation will fall as rain or snow"
    "The other major simplification to the atmosphere is the parametrization of atmospheric heat and moisture transport by diffusion, although moisture advection by the winds is also included as an option (Section 4)."
    Ray Pierrehumbert comments on this at Real Climate:
    "What is more severe, in my view, is that the energy balance model cannot represent the geographic distribution of lapse rate, relative humidity or clouds. In the interview over on Planet 3, Nathan Urban clearly doesn't understand the full limitations of the model even though he is one of the authors of the paper. It's more than just failing to represent the albedo effects of clouds -- the model doesn't represent the geographical variation of cloud infrared effects either, or the way these change with climate. Given that clouds are known to be the primary source of uncertainty in climate sensitivity, how much confidence can you place in a study based on a model that doesn't even attempt to simulate clouds?"
    So, this is not just another example of GCM's disagreeing about climate parameters. This is a case the model not allowing the relevant variables that determine the warming ratio to be set by physics within the model. That is a fair enough choice given budget constraints, but it does have consequences. Further, with regard to the use of other models, James Annan writes:
    "Jules has also been looking at some of these data recently, particularly in comparison to the PMIP2 experiments - that is, simulations of the last glacial maximum by several state of the art climate models, most of which also mostly contributed to the CMIP3/IPCC AR4 database of modern/future projections. One telling point is that several of the PMIP2 models actually appear to fit the data better than Schmittner's best model, even though these were not specifically tuned to fit the data. Moreoever, these models are all clearly colder, in terms of global mean temperature anomaly, than the -3C value obtained in this latest paper. We haven't done a thorough analysis of this yet but I think it is safe to say that there is a significant bias in the Schmittner fit and that the LGM was really more than 3 degrees colder than the present. The implication of this for climate sensitivity is not immediate (since there are also well-known forcing biases in the PMIP2 simulations), but this line of argument also seems to suggest that it may be reasonable to nudge the Schmittner et al values up a bit."
    So initial indications are that use of an ensemble of AOGCMs would have resulted in a higher climate sensitivity than found in the paper. So, I think in this case we can consider AOGCMs, and certainly and ensemble of AOGCM's to be more realistic in this case (because UVic ignores physics for relevant processes) and that it does make a difference, and is likely to have biased Schmittner et al's results low. Again, this is not a flaw in Schmittner et al's study, but a constraint on it. I'm sure they would have preferred to use an ensemble of AOGCM's if somebody had ponied up the cash. Nor is it a conclusive argument that they are wrong. But it is certainly grounds for caution with regards to their result, and suggests that when all the smoke clears, they will be low estimate of the LGM climate sensitivity.
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  6. dana1981 writes: "neal - it does appear that the models are related. For example, the UVic sea ice module was included in CCCMA, and the UVic model is extensively used in developing and testing the CCCma model. But it's not a critical point, and if it exaggerates the relationship between the two (which is unintentional, if so), I don't have a problem with removing that section." Thank you for removing that section, but what you say above is also false. I am a CCCma scientist and have extensive experience with the CCCma models, and well as with the UVic model and many other models. These models share some common features, as do all climate models -- but beyond that they have been developed and tested independently of one another. This is not a big deal in terms of what you're debating in your post, but I correct you all the same.
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  7. Jfyre11 @56, Dana's claim quoted by you represents IMO a summary of the information in this paper quoted as reference 14 in Schmittner et al 2011. That paper that:
    "In fact, our model has been used, and is still being used, as a tool to examine the sensitivity of a particular process or subcomponent model across a wide range of parameters, in order to streamline the process of improving certain components of the CCCma coupled AOGCM. The complexity of the CCCma AOGCM is such that relatively few ‘production runs’ can be conducted, leaving systematic parameter sensitivity analyses to be conducted with the University of Victoria (UVic) ESCM."
    "In fact, our model has been used, and is still being used, as a tool with which to examine the sensitivity of a particular process or subcomponent model across a wide range of parameters, in order to streamline the process of improving certain components of the CCCma coupled AOGCM."
    "The sophisticated sea-ice model was built and tested within the context of the ESCM and has since been included in the Canadian Centre for Climate Modelling and Analysis (CCCma) coupled Atmosphere-Ocean General Circulation Model (AOGCM)."
    These claims, IMO, support Dana's claims about the relationship between the CCCma and the UVic model. Are those claims false, or merely dated?
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  8. Tom Curtis @57 OK, I can certainly see from these statements (from the UVic group) where Dana is coming from. The first two statements were more aspirational than anything. The third statement is dated but true. I'd forgotten that early-on the UVic group helped with some aspects of the CCCma sea-ice model. To say though that the UVic model is "extensively used in developing and testing the CCCma model" is not reality. I hope this is helpful.
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  9. Jfyre11 @58, thankyou. For my part is certainly is helpful. I know the authors here at SkS always try to get the facts straight, and appreciate any correction when we fail.
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  10. dana: yes, I overlooked that, and I apologize. I guess for a blog you don't have to justify the choice of your number (67%). I'd still be curious. Will it be constant with time?
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  11. #53 Tom : Hard for me to understand why we disagree about proxy/model discrepancies... I think I'm OK with your point : "one of the model warming ratios, the land proxy temperatures, the sea proxy temperatures, or both, are in error because of the discrepancy between the Warming ratio found by Schmittner et al, and that from models in Sutton et al." For the Pierrehumbert critics about cloudiness, does he mean that AOGCM correctly simulate the cloud? It would be a great surprise for me (see CFMIP last considerations about that, still large divergence among models in CMIP5). Of course UVic model have major simplifications (the authors acknowledged), but as AOGCM models have major uncertainties in cloud simulation, I don't see clearly whose would be the more robust for telling us with a reasonable precision the cumulus / stratus latitudinal amounts 10, 14 or 20 ky ago!! Inapprehensible for me, we would correctly realize in past climates with much more unknowns (like total aerosol load) what we poorly realize with present climate, far better observed?
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  12. @61, I'm sure Pierrehumbert means no such thing. The difference between AOGM's and the UVic model, however, is that over ice clouds are definitely warming agents. So, when modeling the LGM, we can be sure that whatever discrepancy is caused by the lack of clouds in the UVic model will be increased. In contrast, with AOGCM's, they may well not capture cloud effects accurately, but there is no reason to think their distortion will change between industrial and LGM conditions. Further, I am sure that Pierrehumbert would agree that this style of study would be best done with an ensemble of AOGCMs if funding permitted, to avoid distortions that arise from any given model. In this context, it should be noted that Schmittner et al acknowledge that the UVic model performs poorly over the Antarctic ice sheet. That is not reason for confidence in its performance over the continental ice sheets of the LGM. However, this is beside the point for me. It is not the failure to represent clouds, but the fixed lapse rate and the lack of a hydrological cycle which is relevant to the issue of land/ocean warming ratio. Pierrehumbert, or course, has broader concerns.
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  13. Dr. Schmittner - I chose 67% simply as a very rough estimate (equilibrium sensitivity is ~3°C for 2xCO2 and transient is ~2°C for 2xCO2, according to the IPCC), but I don't think the ratio is constant with time (among other things I believe it depends on the magnitude of the net forcing). Figure 4 is merely meant as a rough approximation, but it should be reasonably close to reality.
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  14. Does anyone else find it encouraging (in some slight way), that this paper is being touted by skeptics and folks like Pat Michaels for "only" showing 2.3 C best estimated climate sensitivity? It seems to me a sign that we're moving from "it's not happening" and "it's not C02" to "it's not that bad". I guess I'll take any hope I can get...
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  15. Utahn when I see the skeptics tout this paper I wonder what the hell had we talked about for years. But you know, they don't miss any chance of saying that the IPCC is wrong, even if it is irrelevant.
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  16. If the denialists weren't constantly contradicting themselves, then it would be encouraging. But tomorrow they'll be back to touting 1°C sensitivity. However, it does seem there has been some movement from "it's not happening" to "it's not CO2" and "it's not bad", probably mostly as a result of the BEST study (which is ironic, since BEST's results were nothing new). There's still plenty of "it's not CO2" denial out there though.
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  17. @61, Ray Pierrehumbert has directly adressed your question himself at RealClimate:
    "In the response by raypierre- I agree about the problems with simple energy balance model and its lack of spatial representation, but it’s tough to fault the authors for the lack of cloud detail, since the science is not up to the task of solving that problem (and doing so would be outside the scope of the paper; very few paleoclimate papers that tackle the sensitivity issue do much with clouds). [Response: I can't agree with this assessment. General circulation models do simulate clouds, and the clouds they simulate are a big part of the nature of their response to both doubled CO2 and to LGM forcing. However, because of the various unknowns in the cloud process, the models give quite different climate sensitivities, accounting for much of the IPCC spread. So, the key thing in evaluating climate sensitivity is to use the LGM as a test of how well the models are doing clouds, using the LGM, and then see what happens in the same model when you project to the future. You cannot do that in a model which doesn't have the dynamics needed to simulate changes in clouds. --raypierre]"
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  18. "If the denialists weren't constantly contradicting themselves, then it would be encouraging. But tomorrow they'll be back to touting 1°C sensitivity". Sadly, I'm afraid you're right, but I allow myself slight hope.
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  19. Can someone explain how the Pliocene was 3-5C hotter with a CO2 conc. of 350ppm and a cooler sun if CS isn't higher than 3oC?
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  20. Ranyl, it's my understanding that temps in the Pliocene had time enough to equilibrate with forcings/feedbacks (such as CO2). Thus the use of the Pliocene as the nearest comp for today's CO2 levels, as the conditions of that world point to what is in store for this world, once temps again equilibrate. Assuming we freeze emissions at zero and hold them there, the warming "in the pipeline" will eventually reach that of the Pliocene. However, a zero-emissions state is something which we show no sign of doing. As such, a few more decades of BAU will lock-in a level of change unseen in this world for eons. With only the mythical CCS existing to save us from this petard of our own devising (powered by cold fusion, no doubt).
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  21. #69 ranyl In this recent discussion , Koenig et al 2011 suggest that "the mean annual global temperature for the Pliocene is 15.5 °C compared to 13.3°C for pre-industrial." And the IPCC AR4 states : The Mid-Pliocene (about 3.3 to 3.0 Ma) is the most recent time in Earth’s history when mean global temperatures were substantially warmer for a sustained period (estimated by GCMs to be about 2°C to 3°C above pre-industrial temperatures; Chandler et al., 1994; Sloan et al., 1996; Haywood et al., 2000; Jiang et al., 2005), providing an accessible example of a world that is similar in many respects to what models estimate could be the Earth of the late 21st century. The Pliocene is also recent enough that the continents and ocean basins had nearly reached their present geographic configuration. Taken together, the average of the warmest times during the middle Pliocene presents a view of the equilibrium state of a globally warmer world, in which atmospheric CO2 concentrations (estimated to be between 360 to 400 ppm) were likely higher than pre-industrial values (Raymo and Rau, 1992; Raymo et al., 1996), and in which geologic evidence and isotopes agree that sea level was at least 15 to 25 m above modern levels (Dowsett and Cronin, 1990; Shackleton et al., 1995), with correspondingly reduced ice sheets and lower continental aridity (Guo et al., 2004). So, a 2-3 K (rather than your 3-5K) equilibrium warming for a 360-400 ppm (rather than your 350 ppm) seems in the range of modelled climate sensitivity. But initial conditions of Pliocene were probably different for ice, vegetation, oceanic circulation, etc. and as it has been discussed above, we're not sure that climate sensitivity for 2xCO2 should be a constant value for different climates.
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  22. Pagani, Lunt and Seki results suggest a 100year (60% of equilibrium as per Hansen 2006 but Hansen now says 80% so would be higher again) in a range of 3oC to 7.9oC most likely 5.25oC for the Pliocene when CO2 levels were between 330-400ppm (IPCC too high for Ealry Pliocene) (13-15).The Schneiders when summarising all the recent Pliocene findings shows that a SCS of 1.5-4.5oC is very likely to be too low to account for the temperatures seen (16). The Pliocene is an appropriate past analogy for the future, the CO2 were the same as now at ~330-400ppm and the general pattern of the earth’s continents were very similar to today however the global temperatures were ~3-5oC hotter than pre-industrial times, the North Pole was ~10-14C hotter, the Greenland, WAIS and marine based EAIS ice sheets weren’t present and sea levels were ~20-25m higher (13-17). Therefore at the present as CO2 levels are 390ppm, it seems reasonable to suggest that the earth will eventually heat up by at least 3C and that sea levels will rise somewhere between 20-25m. Although the full sea level rise will take some time, 1-2m by 2100 is likely by 2100 and 5m in a century has occurred (ref (10) in 9). 13. Lunt D.J. et al, “Earth system sensitivity inferred from Pliocene modelling and data”, Nature Geoscience, VOL 3 , JANUARY 2010 14. Pagani M et al, “High Earth-system climate sensitivity determined from Pliocene carbon dioxide concentrations” Nature Geoscience VOL 3 , JANUARY 2010 15. Seki O. et al, “Alkenone and boron-based Pliocene pCO2 records” Earth Planet Sci Lett (2010) 16. Schneider B. “Global warmth with little extra CO2”, Nature Geoscience, VOL 3, pg. 6-7, 2010
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  23. #72 ranyl : Thank you for the references. I didn’t find Schneider’s paper for free, any link ? The distinction by Lunt et al 2010 between what they called Charney sensitivity (CS), fast feedbacks, and Earth System sensitivty (ESS), slow feedbacks, is interesting. The robustness of the application to mid-Pliocene warm period depends ultimately on the robustness of the estimation of boundary conditions they considered (CO2 orography, vegetation, icesheet) and of the reconstructed temperature change (for checking model fiability), all derived from PRISM data. So, to be continued ( here the page for selected publications around PRISM). Anyway, this leave me with a question : when models compare LGM and Holocene, should we consider they estimate CS or ESS ? As far as I undestand, climate of the mid-Holocene is considered as stable, so I guess it is ESS.
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  24. @73, they can be estimating either Charney Sensitivity or Earth System Sensitivity, depending on which forcings they consider. If they consider changes in albedo as a forcing, they measure Charney Sensitivity. If they consider changes in Albedo as a feedback, they measure Earth Sytem Sensitivity. Thus Hansen and Sato give CS as 3 degrees per doubling, but ESS as 6 degrees per doubling. I consider the latter a very suspect number for prediction of future events. With the retreat of ice towards the poles, albedo effects and hence ESS will become weaker. That is partly compensated for by an increased strength in the WV feedback with increased temperature, but none-the-less, a higher ESS in glaciated conditions than non-glaciated conditions is a persistent feature of the Earth's climate system according to Park and Royer, 2011. They find a best estimate ESS of 3-6 degrees per doubling of CO2 for non-glaciated conditions, but 6-8 degrees C for glaciated conditions. I should not that WV is a fast feedback, so technically this would indicate that CS increases with increasing temperature, while ESS decreases with decreasing temperature until the Earth reaches a non-glaciated state.
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  25. Sorry I don't have a link to a free copy. By SCS I mean Charney, the ESS of the early Pliocene (before Greenland Ice Sheet), is very high, taking 350ppm as the mean, that is 3-5C for only 1/4 of a doubling of CO2 for ESS. Tom, Hansen and Sato also make the early Pliocene only 2C warmer due to their taking deep tropical ocean values and estimating global temperature from that. So in terms of ice and cliamtic shift effects, although the CS is only 3oC in their terms, is akin to one of 6oC if their Pliocene had matched the most commonly reported values of 3-5C hotter. Laslty far from the present being a time of low albedo amplification surely it is a time of high albedo amplification and rapid amplification as the sea ice turns to black ocean, and as I'm sure your aware losing the summer ice (as all the arctic sun is in the summer) tends to accelerate N Polar warming by 3 to 4x, so we do reside at time were things are on a knife edge, or on the point of of saddle transformation to an ice free Northern Hemisphere. Also hasn't Hansen jsut announced that he thinks the CS is actually higher than his recent estimates in the AGU meeting? We are in a bit of pickle and need a total transformation of everything ot get out of it smiling.
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  26. ranyl @75, interesting about Hansen and Sato. With regard to albedo, yes we currently have a stronger albedo related feedback than periods with no polar glaciation. But we have significantly weaker albedo feedback than during glacials. That is both because a melt back of 1 degree latitude removes far less area of ice now than it did during a glacial, and because the ice, being at a lower latitude, reflected more sunlight in the glacial.
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  27. Hi Tom, Fair point about the ice a lower latitude in view of amount of land per unit of solar input due the earth's curvature but that isn't that great an effect from North to South Canada is it? The length of time in the sun is pretty similiar due to very long summer day in the poles, winter ice is irrelevant almost for albedo effects. Also during the Last Glacial the sea in the Pacific was cut of from the Arctic so was ice free I suspect and due to the gulf stream can't see it getting a massive amount larger in the North Atlantic in the summer either and the albedo effect on land is a much lower shift in effect (0.9-0.4-5) compared to (0.9-0.10) for open water. Then of course there are the extra accelerants of tree line moving northwards and the warming effects of the Arcitc ocean that were never present in most previous interglacials. Also Land based Ice sheet take ages to melt away compared to sea ice (although surface melt ponds can be siginificant), so sea ice loss at the pole in summer is like a turbo albedo effect is it not? I just can't help feeling that losing the arctic sea ice in summer (recently predicted for 2015) is going to rapidly increase warming, change all the NH weather systems to some degree. Of course Greenland and WIAS are also suspectable and from warming in the tropic was all the hot water baths the underneath of the below sea level ice sheets (especially in WAIS, which may accelerate the albedo effect. For short-term CS it seems worrying to realise that within 95% probability it could be upto 6oC with Ice present if the pliocene and lots of other paleo data is correct like the Royer paper suggests, that means 350ppm still means a 95% range of temperature rise of 1.8-3C roughly, and getting to 350ppm is going to take a lot lot lot more radical approach to anything that is being suggested by any international agreements. The evidence suggests it is time for a transformational scale change away from fossil fuels to carbon sequestration and every once of carbon counts as even total reforestation only draws down 20ppm! "The drawdown generated in northern-temperate- and tropicalafforestation(~20 ppm) simulations is more than double the drawdown produced by boreal afforestation (9 ppm)." Small temperature benefits provided by realistic afforestation efforts Vivek K. Arora1* and Alvaro Montenegro NATURE GEOSCIENCE VOL 4 AUGUST 2011 514
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