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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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How sensitive is our climate?

What the science says...

Select a level... Basic Intermediate Advanced

Net positive feedback is confirmed by many different lines of evidence.

Climate Myth...

Climate sensitivity is low

"His [Dr Spencer's] latest research demonstrates that – in the short term, at any rate – the temperature feedbacks that the IPCC imagines will greatly amplify any initial warming caused by CO2 are net-negative, attenuating the warming they are supposed to enhance. His best estimate is that the warming in response to a doubling of CO2 concentration, which may happen this century unless the usual suspects get away with shutting down the economies of the West, will be a harmless 1 Fahrenheit degree, not the 6 F predicted by the IPCC." (Christopher Monckton)


Climate sensitivity is the estimate of how much the earth's climate will warm in response to the increased greenhouse effect if we double the amount of carbon dioxide in the atmosphere.  This includes feedbacks which can either amplify or dampen that warming.  This is very important because if it is low, as some climate 'skeptics' argue, then the planet will warm slowly and we will have more time to react and adapt.  If sensitivity is high, then we could be in for a very bad time indeed.

There are two ways of working out what climate sensitivity is. The first method is by modelling:

Climate models have predicted the least temperature rise would be on average 1.65°C (2.97°F) , but upper estimates vary a lot, averaging 5.2°C (9.36°F). Current best estimates are for a rise of around 3°C (5.4°F), with a likely maximum of 4.5°C (8.1°F).

The second method calculates climate sensitivity directly from physical evidence, by looking at climate changes in the distant past:

adapted fig 3a

Various paleoclimate-based equilibrium climate sensitivity estimates from a range of geologic eras.  Adapted from PALEOSENS (2012) Figure 3a by John Cook.

These calculations use data from sources like ice cores to work out how much additional heat the doubling of greenhouse gases will produce.  These estimates are very consistent, finding between 2 and 4.5°C global surface warming in response to doubled carbon dioxide.

It’s all a matter of degree

All the models and evidence confirm a minimum warming close to 2°C for a doubling of atmospheric CO2 with a most likely value of 3°C and the potential to warm 4.5°C or even more. Even such a small rise would signal many damaging and highly disruptive changes to the environment. In this light, the arguments against reducing greenhouse gas emissions because of climate sensitivity are a form of gambling. A minority claim the climate is less sensitive than we think, the implication being we don’t need to do anything much about it. Others suggest that because we can't tell for sure, we should wait and see.

In truth, nobody knows for sure quite how much the temperature will rise, but rise it will. Inaction or complacency heightens risk, gambling with the entire ecology of the planet, and the welfare of everyone on it.

Basic rebuttal written by GPWayne

Update July 2015:

Here is the relevant lecture-video from Denial101x - Making Sense of Climate Science Denial

Last updated on 5 July 2015 by skeptickev. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page

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Further reading

Tamino posts a useful article Uncertain Sensitivity that looks at how positive feedbacks are calculated, explaining why the probability distribution of climate sensitivity has such a long tail.

There have been a number of critiques of Schwartz' paper:


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Comments 201 to 201 out of 201:

  1. RW1 @198:
    I admit I have not yet verified if what he's claiming is correct or not, but you have neither verified what the IPCC is claiming the 3.7 W/m^2 represents from the model simulations. I've looked all through the IPCC 2007 report, I don't find this information - they seem to be really ambiguous about where exactly the 3.7 W/m^2 is derived from. I've also looked all over the internet and cannot find verification either way.
    Let me reiterate what I first pointed out to you @209 on the "A Swift Kick in the Ice Thread" where this dicussion started; ie, that the IPCC explicitly claims that the radiative forcing from doubling CO2 is 3.7 w/m^2, and that "radiative forcing" is the change in net irradiance at the top of the atmosphere. To be quite clear, an increase in incoming radiation or a decrease in outgoing radiation both increase the radiative forcing, so a reduction in Outgoing Long-wave Radiation increases radiative forcing. Therefore, by simple logic, if the IPCC claims that doubling CO2 will increase radiative forcing by 3.7 w/m^2, then it is also claiming that doubling CO2 will reduce OLR by 3.7 w/m^2. The only way it does not have this implication is if changing CO2 levels in Earth's atmosphere could some how change the Sun's level of activity. So, what did the IPCC say in these mysteriously hard to find passages for which I have already provided you a link? The definition of Radiative Forcing:
    The definition of RF from the TAR and earlier IPCC assessment reports is retained. Ramaswamy et al. (2001) define it as ‘the change in net (down minus up) irradiance (solar plus longwave; in W m–2) at the tropopause after allowing for stratospheric temperatures to readjust to radiative equilibrium, but with surface and tropospheric temperatures and state held fixed at the unperturbed values’. Radiative forcing is used to assess and compare the anthropogenic and natural drivers of climate change. The concept arose from early studies of the climate response to changes in solar insolation and CO2, using simple radiative-convective models. However, it has proven to be particularly applicable for the assessment of the climate impact of LLGHGs (Ramaswamy et al., 2001). Radiative forcing can be related through a linear relationship to the global mean equilibrium temperature change at the surface (ΔTs): ΔTs = λRF, where λ is the climate sensitivity parameter (e.g., Ramaswamy et al., 2001).
    That was from section 2.2 of WG1 concealed under the obscure title of "The Concept of Radiative Forcing". The effect of CO2:
    The simple formulae for RF of the LLGHG quoted in Ramaswamy et al. (2001) are still valid. These formulae are based on global RF calculations where clouds, stratospheric adjustment and solar absorption are included, and give an RF of +3.7 W m–2 for a doubling in the CO2 mixing ratio. (The formula used for the CO2 RF calculation in this chapter is the IPCC (1990) expression as revised in the TAR. Note that for CO2, RF increases logarithmically with mixing ratio.) Collins et al. (2006) performed a comparison of five detailed line-by-line models and 20 GCM radiation schemes. The spread of line-by-line model results were consistent with the ±10% uncertainty estimate for the LLGHG RFs adopted in Ramaswamy et al. (2001) and a similar ±10% for the 90% confidence interval is adopted here. However, it is also important to note that these relatively small uncertainties are not always achievable when incorporating the LLGHG forcings into GCMs. For example, both Collins et al. (2006) and Forster and Taylor (2006) found that GCM radiation schemes could have inaccuracies of around 20% in their total LLGHG RF (see also Sections 2.3.2 and 10.2).
    That was carefully concealed in section 2.3.1 of WG1, titled "Atmospheric Carbon Dioxide". So what was it you wrote? That you've "...looked all through the IPCC 2007 report, I don't find this information - they seem to be really ambiguous about where exactly the 3.7 W/m^2 is derived from"? Really, you've looked all over, but never managed to look at the specific pages you were explicitly linked to? And specific mention of the types of models used, with references to three scientific papers that include the equations is being "really ambigous about where exactly the 3.7 w/m^2 is derived from"? Don't be absurd. Apparently you have also looked "all over the internet" with similar lack of success. But, again, without looking at the page that scaddenp explicitly linked you to. On that page you would have found a detailed discussion of all the issues raised here, along with images from a textbook, including the three @192 above showing the detailed mechanism used in calculating spectra in LBL models, and comparing LBL model results with reality. You would even find the actual formula (as if that would do you any good): And if that was not enough to clarify, you could always have looked up the actual textbook (as I have previously suggested). If that was not enough, you could also followed my link @192 above to SoD's seven part discussion of climate models and atmospheric physics in which he step by step builds an open code radiative transfer model. That is, of course, if your diligent search of the net had not already found it by noticing all seven posts in the "Recent Posts" section of SoD, or finding them in the "Atmospheric Physics" category (again, such careful concealment of information). If that was not enough, you have had, for over a hundred posts now, the opportunity to double check one well known radiative transfer model (Modtran) for internal consistency, as I linked you to that before the discussion came to this thread. Of course, that would be difficult and time consuming, just as it was difficult and time consuming for all those scientists who developed multiple models, and fact checked them against literally hundreds of thousands of observations, only to have their work dismissed by a electrical engineer who thinks his word is better than their about what the output of their models actually represents. And his acolyte. This whole discussion has become a waste of time. Clearly you will not do even basic research, and will not think about the outcomes of what research you do. I have long believed you are a troll, but have persisted in the discussion on the basis that interested readers may also have been confused by George White. Well for anyone who can think, it is diamond clear by now that George White's claim about the 3.7 w/m^2 radiative forcing from doubling CO2 is simply an error, and an error that anyone half way knowledgeable on the subject could not make. If you are still confused, it is because you want to be - you do not want to know the truth.
  2. Tom @201, Devastating. Your last sentence also nails it.
  3. "You also may be not making a mistake, and I have simply misunderstood you. It is true that the presence of evapo/transpiration and convection, by making energy transfer more efficient, cool the surface compared to the temperature it would be if all energy transfers in the atmosphere were radiative (about 70 degrees C). So in that respect, the fact that evapo/transpiration carries energy into the atmosphere, a portion of which does eventually escape to space does mean the surface is cooler than it otherwise would have been." Having re-read this of yours, it is NOT what I meant. What I was saying is that kinetic energy (evaporation & transpiration) transferred from the surface into the atmosphere has be returned to the surface in equal and opposite amounts - mostly in the form of precipitation, weather, etc. Any amount of it radiated into the atmosphere that ultimately leaves at the top of the atmosphere, results in less kinetic energy returned to the surface in the form of colder precipitation mostly, which cools the surface, resulting in the surface emitting an equally opposite amount less than it would otherwise.
  4. @ Tom Curtis (201) Ditto to what Albatross said. Devastating. Game, set, match to TC. Though I'm undecided if ending your comment with a simple "QED" would've been over the top or a masterstroke coup de grâce. The Yooper
  5. Tom (RE: 201), I'm well aware of the IPCC definition of 'radiative forcing' and the passages you're citing, and I know exactly what they are claiming. In a more general sense of the term, technically all the 3.7 W/m^2 is 'radiatively forced'. Show me the detailed output data of the radiative transfer models used that corroborates that the 3.7 W/m^2 number claimed by the IPCC is the downward emitted amount and not the incremental absorption or reduction in total transmittance. If it's agreed that only half the incremental absorption affects the surface, and the model simulations take this effect into account, then the incremental absorption should be 7.4 W/m^2. Show me this. I don't see this information in any of the sources provided by your or anyone else here. You can lecture me all you want about not being interested in the truth or call me a troll, but simply declaring these things correct on the basis of authority or majority goes against science and logic. One way or another I'm going to get to the bottom of this.
  6. Word salad.
  7. Another reference and explanation:
  8. Brief quote below: see original and thread for more: "... Unfortunately, you can't read this off MODTRAN very well. There are two reasons for this. One is that it depends on the latitude. The second is that it depends on the altitude of the sensor. Part of the problem is the appropriate definition of a forcing. I describe it, with references, in msg #1 of "Estimating the impact of CO2 on global mean temperature". ... The reason you get a difference at higher altitude is that the atmospheric temperature profile in this calculator is held fixed, and so the calculator actually has stratospheric warming as a response to an increase temperature offset. What happens in reality is that the stratosphere cools.... The upshot is that to get a sensible value for the forcing response to doubled CO2, you should really take the lower altitude sensor. Also, you can't have a tropical atmosphere over the whole planet. The value you get will be somewhere between the tropical atmosphere and the standard 1976 atmosphere; and you also need to consider clear sky and cloud as well. All told, the MODTRAN calculator will get you into the right ball park; but it can't serve as a refutation of the forcing for doubled CO2, which is about 3.7 W/m2 to 10% accuracy or better."
  9. Again, technically all the incremental absorption, whatever it may actually be, is 'radiatively forced' - it's just that half of it is 'forced' in the same general direction it was already going.
  10. PS, as you've made it clear you don't know how to find this on your own -- here's how:"3.7w" By contrast, if you just searched for IPCC 3.7w you would get about 13,400 results -- many of them copypasted denial stuff, johndaly, wattsup, and so on. While there's a pony in there somewhere, the site-limited search finds it fast.
  11. RW1 opines, "Show me the detailed output data of the radiative transfer models used that corroborates that the 3.7 W/m^2 number claimed by the IPCC is the downward emitted amount and not the incremental absorption or reduction in total transmittance." Que?! Gregory, Jonathan, Mark Webb, 2008: Tropospheric Adjustment Induces a Cloud Component in CO2 Forcing. J. Climate, 21, 58–71. Forster, P. M., and J. M. Gregory, 2006: The climate sensitivity and its components diagnosed from Earth Radiation Budget data. J. Climate, 19, 39–52 Myhre, G., E. J. Highwood, K. P. Shine, and F. Stordal, 1998: New estimates of radiative forcing due to well mixed greenhouse gases. Geophys. Res. Lett., 25, 2715–2718. Forster and Gregory (2006) is especially helpful. "One way or another I'm going to get to the bottom of this." Wow-- I can't wait for the revelations. Back to earth though--you really are way behind in the game. You can indeed get to the bottom of this" by a) actually, listening to others who are sincerely trying to guide you, b) actually then reading the pertinent literature and allowing the content to resonate, c) being willing to learn fro others, and d) not assuming something nefarious is going on. For goodness' sakes even Spencer and Lindzen et al. do not dispute the 3.7 W/m number or what it represents. Either you are a brilliant soon-to-be Nobel physicist laureate or you are a D-K. Please do not try and insult others by trying to claim otherwise, you have been called on your game. You have been wasting everyone's time for a while now-- enough is enough. Do you perhaps also have issues with the Stefan-Boltzmann constant that you need to get to the bottom of?
  12. It may amuse (warning, facepalm risk) to see the same issue raised here:
  13. Hank @212, that is actually a very informative and helpful post for anyone not clear on the greenhouse effect. I heartily recommend it, something I could not say about almost all of Judith Curry's other posts. Of course, the crazies still come out in the comments ...
  14. Tom, I agree; same for Spencer's comparable effort at
  15. A curious digression on this topic - Dr. Roger Pielke Sr recently posted something on his blog, attempting to redefine the term Climate Sensitivity as: "Climate Sensitivity is the response of the statistics of weather (e.g. extreme events such as droughts, land falling hurricanes, etc), and other climate system components (e.g. alterations in the pH of the oceans, changes in the spatial distribution of malaria carrying mosquitos, etc) to a climate forcing (e.g. added CO2, land use change, solar output changes, etc). This more accurate definition of climate sensitivity is what should be discussed rather than the dubious use of a global annual average surface temperature anomaly for this purpose." Redefining a term used in all of climate science? I wonder why measuring the temperature response of the climate to a particular amount of radiative forcing wasn't working for him? This is a clear example of the Moving the Goalposts fallacy, often a sign that the original argument has been lost.
  16. A curious digression on this topic - Dr. Roger Pielke Sr recently posted something on his blog, attempting to redefine the term Climate Sensitivity I wonder what's next, redefining the laws of physics to fit a specific outcome?
  17. > what's next, redefining the laws of physics ....? Chuckle. Yep.
  18. hat tip to:
    Response: Please provide some context for links. Link-only comments will be deleted.
  20. wups, lost the comment part, sorry. Suggesting a look at this site, which is trying a grade-school-level approach (well, for a very scientifically literate grade school population). Worth a look given the amount of confusion shown in the comments. "... ... we explain what a greenhouse gas does. The two spectra are crucial to the understanding of the role of greenhouse gases in the atmosphere..... ... ... The next simulated spectra are those for 380 ppmv and 760 ppmv of CO2 respectively looking down from an altitude of 70 km and hopefully show the slight broadening of the 'well' that is crucial to the understanding of why more CO2 leads to a little more warming, even though such warming might not be measureable...."
  21. The prior suggestions are 1) poptech redefining the laws of physics, a notorious outlier shows his stuff everywhere 2) Isaac Held has a blog, finally. If you don't know his name, read some of his papers and look for his rare posts at other climate blogs about his work. Very good news to see him start writing more for the public in this blog form. 3) Stoat on Spencer on climate sensitivity: Spencer thinks he can't possibly be wrong, and given that assumption, what else can explain why he's so alone?
  22. oh, and on the first link -- I point it out as one that needs to be looked at carefully on the question of climate sensitivity. The 'discussion' page is going to be very attractive to self-identified skeptics; it may be in the same ballpark as Spencer's "yes Virginia" or Curry's "Sky Dragon" attempts to explain the basic science in a way that will draw in people who don't want to believe there could be a problem. It may be well presented stuff with a denier core -- hard to say without reading every bit: But this doesn't look good (sigh) they're "Teaching the Controversy" (TM Doonesbury) on ocean pH, leaving out the rates of change and the observed results so far:
  23. For your amusement - this is now an active topic on Jo Nova's site. One of her readers did a blog post for her on this very thread, claiming that efficacy was a "fudge factor" allowing made-up numbers. Discussion ensued...
  24. Gilles wrote: "I am completely ready to admit that CO2 contributes to warm the atmosphere , on very simple arguments of radiative transfer. My only questions are quantitative." This is the best place to have such questions answered; I am sure there are plenty of knowledgeable posters here who would be happy to answer them for you.
  25. A brief follow up: I've spent quite a bit of time emailing around the climate science community on this. None of the scientists I've communicated with seem to know much about it, and appear to have more or less just accepted the number with little (if any) thought. I'm still pursuing the issue with one of them in particular though. Meanwhile, GW has given me the details on the simulations he's done and I see no indication that he's incorrectly interpreting the results as claimed here. For example, this is plot of the clear sky absorption spectra he used, where each gas is represented by a different color. The Y axis is the amount of emitted surface power absorbed by the atmosphere. You can clearly see that the line by line transmittance is 1 minus the value. If the half up/half down effect was included, the maximum value would be 0.5 and not 1.0, because even if 100% is absorbed, half is emitted to space anyway: Click
  26. Here is another showing which gases are most responsible for absorption at various wavelengths: Click
  27. Here is the composite absorption with the emitted energy spectrum (grey line), which gives 255K. You can see that where the wavelengths are completely saturated (i.e. the 15u band of CO2), the transmittance is zero. If the halving effect was included, the maximum for the saturated bands would be only 0.5 and not zero: Click
  28. RW1 immagine a light source and a absorbing medium. A transmission experiment will give you the fraction of light passing through the medium along the line connecting the source and the detector. On the contrary, the light eventually re-emitted by the medium has no preferential directions. You should not expect the transmittance to saturate at 0.5.
  29. Riccardo (RE: 229), "You should not expect the transmittance to saturate at 0.5" I don't. The point is if the half up/half down effect was included in the spectral data and Modtran simulation output, the maximum transmittance would be 0.5 since even if absorption is 100% half escapes to space anyway. That transmittance in the saturated lines is 1.0 means it represents the total absorption - not the downward emitted half.
  30. RW1 modtrans does take into account emission; infact, you can see light coming from the saturated bands. On the contrary, transmittance measurements or calculations don't.
  31. Riccardo (RE: 231), "RW1 modtrans does take into account emission; infact, you can see light coming from the saturated bands. On the contrary, transmittance measurements or calculations don't." OK, show me where or how it does this. Is it your contention that the reduction in transmittance from 2xCO2 is 7.4 W/m^2?
  32. RW1 any textbook will explain you this point. As for the contention, I did not quote any number, just explaining the meaning of transmittance with which apparently you're not familiar.
  33. Riccardo (RE: 233), Obviously you haven't followed the discussion in this thread. The definition of transmittance, in the context of this discussion, is the amount of surface emitted LW that passes straight through to space as if the atmosphere wasn't even there. The claim is this reduces by 7.4 W/m^2 when CO2 is doubled, because the referenced 3.7 W/m^2 from 2xCO2 supposedly already includes the effects of half up/half down.
  34. RW1 it's typical of many to show up transmittance spectra and draw conclusions from them. I was just trying to show how one should look at this kind of spectra. It is essential for a proper understanding of radiation in the atmosphere. Never mind, those are pretty standard concepts. You will easily find them whenever you think it's appropiate.
  35. "basically if there is some large negative feedback which makes the sensitivity too low, it would have prevented the planet from transitioning from ice ages to interglacial periods" Doesn't the above assume that the negative feedback is linearly related to T? Is that a safe assumption if so?
  36. MajorKoko - Feedbacks are amplifications (positive) or dampenings (negative) of a forcing change. So they're related to changes in T, not T itself. That said, there are definitely phase changes (melt of clathrates, lack of summer ice in the Arctic, etc.) where feedback levels can be expected to change. As to the original statement: "basically if there is some large negative feedback which makes the sensitivity too low, it would have prevented the planet from transitioning from ice ages to interglacial periods" The Milankovitch cycle forcing change between ice age and interglacial is estimated to be on the order of 3.4 W/m^2, for a direct forcing change of ~1C. Global temperature changes for those cycles, however, are on on the order of 5-6C or so. So a short term sensitivity of ~3C for a doubling of CO2 (3.7 W/m^2) with additional long term feedbacks (ice melt, vegetative changes, CO2 temperature/solubility changes from deep ocean, etc) matches the feedback amplification seen in the ice age cycle.
  37. In the Pielke Sr thread, dana1981 says: "I think I'm most disappointed that we never got an answer regarding the discontinuity between low climate sensitivity arguments and the paleoclimate record. I've never seen any low sensitivity proponent answer this question, and unfortunately it seems Dr. Pielke was unable to answer it as well." Also a long time ago I promised scaddenp I would address low sensitivity, and this is a start. The portion of Knutti and Hegerl (2008) that goes with "Advanced" figure 4 (click on Advanced tab above) (Various Estimates of Climate Sensitivity) is shown to the left.

    I labeled the 3 paleo sensitivity estimates in question. The problem arises from the red squares in the first column "similar climate to base state". The key question is how well can the dissimilarity be accounted for in the models. Specifically, the 8C rise from the last glacial came from combination of Milankovitch forcing, dust feedback, CO2 feedback, and other feedbacks that are modeled and equate to a 3C (best estimate) for 3.7 W/m2 of forcing. However, the leftmost red square is red because there are lots of unknowns compared to the present. There are many complications for modeling. In Claquin et al posit one of the factors in ice age transitions have an added factor, namely dust, that adds long term positive feedback. Less dust means higher SST but also less fertilization so less algae and more CO2 all adding to the warming. In short, there is a higher sensitivity for glacial to interglacial compared to today. Here is a general complication. A large sensitivity difference also arises from ice and snow albedo changes. During the ice age the ice and snow reflect a lot more sunlight and as it melts the surface albedo decreases as a positive feedback. The feedback is obviously higher than for the present climate which has a lot less snow and ice. The problem in determining the difference comes from highly nonlinear responses to Milankovitch forcing compared to today's CO2 forcing. Here's just one example: The modeling attempts to account for numerous differences from the modern climate including THC and sea ice, poleward heat transport and temperature gradient, precipitation changes, etc. All of these will be radically different with 3.7 W/m2 of CO2 forcing. Most point to a much larger feedback from Milankovitch forcing due to seasonal, geographic, and ice age climate differences.

  38. Just a quick comment - the question about glacial dust would certainly be something for carbon-cycle models to worry about but for a climate model, what matters is how much CO2 eventually ended up in the atmosphere. This is a known (from gas bubble) so model doesnt need to calculate it. Its tricky to see how uncertainties from glacial aerosols could lead to lower sensitivity given that the rise in CO2 is known. Albedo feedbacks would be different last glacial termination (they are so in the models), but can be reasonably estimated. (area covered by ice).
  39. Continuing further on this, I note that you have focused on the paleoclimate measures of sensitivity, though they are in broad agreement with the other measures of sensitivity. Schmidt at RC commented recently on this too. "It's certainly conceivable that climate sensitivity is a function of base climate and surely is at some level. How large that dependency is unclear. But you need to distinguish between estimates of sensitivity derived from comparing older climates to today, and estimates of variability within an overall different base climate. Comparing the LGM or Pliocene to today is the former, looking at the variations during an ice age would be the latter. There have been a couple of papers indicating that sensitivity at the LGM is different to today (Hargreaves - not sure of the year - for instance), but in each case the differences (while clear), are small (around 10 to 20%). - gavin" See here. You have commented previously that you thought climate sensitivity was low (hence no "C"AGW). What science did you examine that led you to that conclusion? At the moment, it looks you are trying to find science to back an a priori determination that sensitivity is low.
  40. I'm trying to understand the relationship between climate sensitivity and C02:temp feedback. Assuming that CS is 3C for the radiative forcing resulting from doubling atmospheric C02: 1)Over what time period is this realized? 2)Is this the limit of the temp:C02 feedback or is this just the first order effect? 3)Wouldn't the C02:temp feedback limit be dependent on the amount of C02 already in the atmosphere? 4)If the radiative forcing came from a non-C02 source, wouldn't the temperature rise be larger, as there'd be more 'room' for the feedback to occur?
  41. 240, Tristan, 1) No one knows for sure, because it's never been doubled this quickly before. The models give some insights, but this is hard to pin down. We're also pretty early in the process, so it's hard to even estimate it at the current rate of warming. We haven't hit any step-changes yet, and the system is sluggish. What we do know is that no matter how slowly it seems to happen, it is happening, and it is going to continue well beyond the point where we stop raising CO2 levels. 2) To my knowledge, this is the "Charney sensitivity" or "equilibrium sensitivity", meaning the final, end result sensitivity after everything has stabilized. Also note that while 3C is an easy working number, the assumed range is 2C to 4.5C, and it may even be lower (unlikely) or higher (also unlikely, but more possible than lower than 2). This is in contrast to the "transient sensitivity" we would see within 20 years of doubling CO2 levels, which would see all fast feedbacks come into play, but not some slower ones. An excellent paper to consider in studying this is Hansen and Sato (2011). They talk exactly about these issues in a fairly clear fashion, and compare current positions to what can be inferred from previous similar changes in climate. There are, really, I think (in my mind, not officially) three levels of feedbacks... very fast, slow, and very slow. Very fast includes humidity and cloud changes that happen quickly. Slow feedbacks involve things like albedo and CO2 feedbacks that require major ice melt and fast ecosystem changes. Then very slow feedbacks require even longer term things (the point where oceans warm enough to release rather than absorb atmospheric CO2, and major, large-scale ecosystem changes occur that in turn change albedo and release or absorb more CO2). But I think the hoped for answer is that 3C is all of these effects combined. [I will confess that someone else may be able to give you a more direct and perhaps different answer than this one... this is what I understand, but I could be wrong here. Hansen and Sato 2011 in particular talk about fast and slow feedbacks on other time scales.] The sad reality, though, is that we won't know if 3C is the accurate estimate of the final feedback result until 1,000 years pass. 3) That's why it's expressed in terms of a doubling of current concentrations, and not based on the incremental amount added. 4) Yes and no. There are logically slight differences in feedbacks depending on the source of a temperature increase, but overall feedbacks are driven by temperature change, regardless of the cause in temperature change. Refer to this chapter on efficacy (i.e. how one forcing differs from another) in the IPCC AR4 report. There would be more "room" for CO2 feedbacks, because the same amount of CO2 released would be proportionally larger to a lower starting level. But at the same time we'd have pumped less CO2 into the oceans to release there. More importantly, the CO2 feedback is only one of many. Other feedbacks (water vapor, albedo changes, etc.) are in aggregate probably more important. So that difference wouldn't amount to that much.
  42. Thanks Sphaerica, your response was just what the doctor ordered! xox
  43. You may be interested in Professor Shaviv's writings about climate sensitivity. He explains why he comes up with a lower number From: "Prof. Nir J. Shaviv, who is a member of the Racah Institute of Physics in the Hebrew University of Jerusalem. According to PhysicaPlus: "...his research interests cover a wide range of topics in astrophysics, most are related to the application of fluid dynamics, radiation transfer or high energy physics to a wide range of objects - from stars and compact objects to galaxies and the early universe. His studies on the possible relationships between cosmic rays intensity and the Earth's climate, and the Milky Way's Spiral Arms and Ice Age Epochs on Earth were widely echoed in the scientific literature, as well as in the general press." Chris Shaker
  44. cjshaker I'm a bit surprised to see this old and debunked Shaviv paper pop up again. Honestly, I do not find it that much interesting.
  45. A quote from the Climate-time-lag.html article: "How long does the climate take to return to equilibrium? The lag is a function of climate sensitivity. The more sensitive climate is, the longer the lag. Hansen 2005 estimates the climate lag time is between 25 to 50 years." While reading through Lacis et al regarding CO2 as a control knob, I noticed this diagram

    Taking less than 10 years to cool to equilibrium suggests a short lag. That is for full removal of CO2, etc and I don't know if the time constant would be different for a change in CO2. But if the lag time is much shorter than the 25 to 50 years suggested above, then climate sensitivity is also lower than estimated by Hansen.

  46. Depends what you call equilibrium Eric - the temperature units on the Lacis diagram are pretty large, and it clearly hasn't reached perfect equilibrium even after >50 years. Why you suggest it supports a 10 year equilibrium mystifies me. From the diagram the change has reduced to being relatively slight after ~25 years, but equatorial regions are still cooling after 50 years.
  47. Eric (skeptic) @245, the rapidity with which a system adjusts to a new equilibrium depends not just on thermal inertia, but also on the magnitude of the disequilibrium. With the enhanced greenhouse effect, the disequilibrium is small, being approximately 1 W/m^2. This is because of both the small initial perturbation and the fact that the full effects of positive feedbacks are not felt until the system approaches the equilibrium temperature. In contrast, in the model analyzed by Lacis et al, the initial perturbation is around 30 W/m^2. Consequently the system adjusts towards equilibrium much faster because of the much larger disequilibrium. Even so, as skywatcher @246 points out, the system has still not reached equilibrium after 50 years. Further, Lacis et al state that they use the Q-flux ocean model with a 250 meter mixed layer depth. Had they used a model with deep diffusion, time to equilibrium would have been significantly extended (by a few centuries, I suspect), but the early changes of the system would have been unaffected. It just would have taken longer to close the last 0.1 W/m^2 of disequilibrium.
  48. 245, Eric, 246, skywatcher, 247, Tom, I'd also point out that Lacis et al is dealing exclusively with fast feedbacks (like water vapor and clouds). As we are now seeing, things like the ice albedo feedback take a comparatively long time to develop, as would carbon feedbacks that result from methane release or major ecosystem changes. The point is, we are still, fortunately, talking about things in terms of a climate that could have a quick return to the old equilibrium if CO2 could somehow be drawn down. This will not necessarily be the case in the longer term, when those slower feedbacks begin to kick in, and so the reverse will consequently be just as slow (along with the fast feedbacks that go with the slow feedbacks instead of with the initial forcing). Beyond this, I am very, very concerned about how much CO2 the ocean has absorbed. In many past scenarios the ocean was the source of, not a damper on, added CO2. In this case the ocean is acting to hold down atmospheric CO2 levels by soaking up some of the excess. Even after we completely stop emitting CO2, where will it go? It can't go into the ocean, because it already is (and that is in balance). It has to go into biomatter, either on land or in the ocean, and in some way be sequestered, but the mechanics of it I would have to believe will take a very, very long time. And even after any part of it is drawn out of the atmosphere, the ocean will certainly respond by trying to maintain an equilibrium and so transfer it from the ocean to the atmosphere. If slow CO2 feedbacks involve things like the transition of huge swaths of the Amazon to savanna, or other ecosystems to desert, then this puts more CO2 into the atmosphere/ocean. But how does it then get back into biomatter? Temperatures must drop for rain forest or prairie to again take hold where savanna/desert has appeared, so that vegetation can then grow and put the carbon into other forms. But how does this happen until temperatures first drop? And how long will this reverse process take? The bottom line is that we're not getting anywhere near any of the fast-acting we-cut-atmospheric-CO2-back-to-285ppm scenarios any time soon, probably not for several hundreds of years, which will be more than long enough for us to see at least some if not many of the "slow feedbacks" take hold and therefore hard to reverse. [This with the understanding that "slow feedbacks" in past climate change events are going to be relatively fast in this case because we are pumping the CO2 into the atmosphere so abruptly and quickly as compared to increases due to most natural processes in the past.]
  49. Sphaerica, we're never getting back to 285 anytime soon because there is an exponential decay from whatever level we are at. Q1: is it necessary to return to 285 (it may not be possible anyway)? Is a higher level ok? Q2: What about storage in the deep ocean, would that help the recovery prospects?
  50. Eric (skeptic), the highest levels (per the Antarctica ice cores) achieved at any point in the past 800,000 years was 298.7 ppm. Humanity has seen that in the rear-view mirror long ago. To an uncertain future with great temerity we go.

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