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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...

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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 of the utmost importance. Why? Because it is the factor that determines how much the planet will warm up due to our greenhouse gas emissions. The first calculation of climate sensitivity was done by Swedish scientist Svante Arrhenius in 1896. He worked out that a doubling of the concentration of CO2 in air would cause a warming of 4-6oC. However, CO2 emissions at the time were miniscule compared to today's. Arrhenius could not have foreseen the 44,250,000,000 tons we emitted in 2019 alone, through energy/industry plus land use change, according to the IPCC Sixth Assessment Report (AR6) of 2022.

Our CO2 emissions build up in our atmosphere trapping more heat, but the effect is not instant. Temperatures take some time to fully respond. All natural systems always head towards physical equilibrium but that takes time. The absolute climate sensitivity value is therefore termed 'equilibrium climate sensitivity' to emphasise this.

Climate sensitivity has always been expressed as a range. The latest estimate, according to AR6, has a 'very likely' range of 2-5oC. Narrowing it down even further is difficult for a number of reasons. Let's look at some of them.

To understand the future, we need to look at what has already happened on Earth. For that, we have the observational data going back to just before Arrhenius' time and we also have the geological record, something we understand in ever more detail.

For the future, we also need to take feedbacks into account. Feedbacks are the responses of other parts of the climate system to rising temperatures. For example, as the world warms up. more water vapour enters the atmosphere due to enhanced evaporation. Since water vapour is a potent greenhouse gas, that pushes the system further in the warming direction. We know that happens, not only from basic physics but because we can see it happening. Some other feedbacks happen at a slower pace, such as CO2 and methane release as permafrost melts. We know that's happening, but we've yet to get a full handle on it.

Other factors serve to speed up or slow down the rate of warming from year to year. The El Nino-La Nina Southern Oscillation, an irregular cycle that raises or lowers global temperatures, is one well-known example. Significant volcanic activity occurs on an irregular basis but can sometimes have major impacts. A very large explosive eruption can load the atmosphere with aerosols such as tiny droplets of sulphuric acid and these have a cooling effect, albeit only for a few years.

These examples alone show why climate change is always discussed in multi-decadal terms. When you stand back from all that noise and look at the bigger picture, the trend-line is relentlessly heading upwards. Since 1880, global temperatures have already gone up by more than 1oC - almost 2oF, thus making a mockery of the 2010 Monckton quote in the orange box above.

That amount of temperature rise in just over a century suggests that the climate is highly sensitive to human CO2 emissions. So far, we have increased the atmospheric concentration of CO2 by 50%, from 280 to 420 ppm, since 1880. Furthermore, since 1981, temperature has risen by around 0.18oC per decade. So we're bearing down on the IPCC 'very likely' range of 2-5oC with a vengeance.

Please use this form to provide feedback about this new "At a glance" section. Read a more technical version below or dig deeper via the tabs above!

Further details

Climate sensitivity is the estimate of how much the earth's climate will warm in response to the increased greenhouse effect if we manage, against all the good advice, to double the amount of carbon dioxide in the atmosphere. This includes feedbacks that can either amplify or dampen the warming. If climate sensitivity is low, as some climate 'skeptics' claim (without evidence), 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. Feeling lucky? Let's explore.

Sensitivity is expressed as the range of temperature increases that we can expect to find ourselves within, once the system has come to equilibrium with that CO2 doubling: it is therefore often referred to as Equilibrium Climate Sensitivity, hereafter referred to as ECS.

There are two ways of working out the value of climate sensitivity, used in combination. One involves modelling, the other calculates the figure directly from physical evidence, by looking at climate changes in the distant past, as recorded for example in ice-cores, in marine sediments and numerous other data-sources.

The first modern estimates of climate sensitivity came from climate models. In the 1979 Charney report, available here, two models from Suki Manabe and Jim Hansen estimated a sensitivity range between 1.5 to 4.5°C. Not bad, as we will see. Since then further attempts at modelling this value have arrived at broadly similar figures, although the maximum values in some cases have been high outliers compared to modern estimates. For example Knutti et al. 2006 entered different sensitivities into their models and then compared the models with observed seasonal responses to get a climate sensitivity range of 1.5 to 6.5°C - with 3 to 3.5°C most likely.

Studies that calculate climate sensitivity directly from empirical observations, independent of models, began a little more recently. Lorius et al. 1990 examined Vostok ice core data and calculated a range of 3 to 4°C. Hansen et al. 1993 looked at the last 20,000 years when the last ice age ended and empirically calculated a climate sensitivity of 3 ± 1°C. Other studies have resulted in similar values although given the amount of recent warming, some of their lower bounds are probably too low. More recent studies have generated values that are more broadly consistent with modelling and indicative of a high level of understanding of the processes involved.

More recently, and based on multiple lines of evidence, according to the IPCC Sixth Assessment Report (2021), the "best estimate of ECS is 3°C, the likely range is 2.5°C to 4°C, and the very likely range is 2°C to 5°C. It is virtually certain that ECS is larger than 1.5°C". This is unsurprising since just a 50% rise in CO2 concentrations since 1880, mostly in the past few decades, has already produced over 1°C of warming. Substantial advances have been made since the Fifth Assessment Report in quantifying ECS, "based on feedback process understanding, the instrumental record, paleoclimates and emergent constraints". Although all the lines of evidence rule out ECS values below 1.5°C, it is not yet possible to rule out ECS values above 5°C. Therefore, in the strictly-defined IPCC terminology, the 5°C upper end of the very likely range is assessed to have medium confidence and the other bounds have high confidence.

 IPCC AR6 assessments that equilibrium climate sensitivity (ECS) is likely in the range 2.5°C to 4.0°C.

Fig. 1: Left: schematic likelihood distribution consistent with the IPCC AR6 assessments that equilibrium climate sensitivity (ECS) is likely in the range 2.5°C to 4.0°C, and very likely between 2.0°C and 5.0°C. ECS values outside the assessed very likely range are designated low-likelihood outcomes in this example (light grey). Middle and right-hand columns: additional risks due to climate change for 2020 to 2090. Source: IPCC AR6 WGI Chapter 6 Figure 1-16.

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°C or even more. These are not small rises: they would signal many damaging and highly disruptive changes to the environment (fig. 1). In this light, the arguments against reducing greenhouse gas emissions because of "low" climate sensitivity are a form of gambling. A minority claim the climate is less sensitive than we think, the implication being that as a consequence, 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. Both such stances are nothing short of stupid. Inaction or complacency in the face of the evidence outlined above severely heightens risk. It is gambling with the entire future ecology of the planet and the welfare of everyone on it, on the rapidly diminishing off-chance of being right.

Last updated on 12 November 2023 by John Mason. View Archives

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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:

Denial101x videos

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

Additional video from the MOOC

Expert interview with Steve Sherwood


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Comments 251 to 275 out of 356:

  1. 249, Eric, Q1: My opinion only, but no, I don't think a return to 285 is necessary, just desirable (but impossible). I do think a return to 350 is required, but also impossible at the current rate of action (meaning that by the time we start, we'll be lucky to hold it to 500 at this point, and as I described, I think getting it down once its up will be almost impossible). Q2: How do you propose that the carbon get into the deep ocean? There are natural processes that work on huge, huge ("global") scales, but not nearly so quickly as to naturally drawn down both atmospheric and ocean CO2 levels to a reasonable degree (although I could be wrong on this... this is where an ocean expert like Doug Mackie should step in. Perhaps he knows better. But otherwise, how does someone suck all of that carbon out of the air and oceans and put it in a form that will sink to the bottom and stay there? It is interesting to note two things. The first is that the 300+ gigatonnes of carbon that man has burned in fossil fuels so far (and we're still not done) took nature hundreds of millions of years to sequester underground. It took a mere 100 years to release it, but there is no reasonable way to match nature's feat and put it back. It can go into the atmosphere, ocean or biomass, but not very easily back into the ground. The second point is that I recently did a back of the envelope calculation, trying to figure out how much land would be needed to plant giant sequoia redwoods that could suck up the carbon and turn it into biomatter (trees). The answer, with some very optimistic fudging, was that 75% of the arable and agricultural land on earth needed to be covered with redwoods in order to drawn atmospheric CO2 back down to 285 ppm in the course of 100 years from today, assuming we planted those trees right now and also instantly stopped burning more fossil fuels. This of course presumes that all of humanity moves to deserts and other places of the earth so that we can exclusively grow food crops on the remaining agricultural/arable land. Of course, since that represents only 25% of the total, we must also assume that food production will drop by 75%. This implies that the population of the earth (currently 7 billion) must also drop as a consequence -- so that "solution" implies: 1) Plant sequoias immediately on 75% of the arable land on earth (completely ignoring the fact that sequoias will not grow just anywhere, and in fact could only grow in very specific environments) 2) Move the entire human population off of such land 3) 5 billion people must die (because there won't be enough land to produce enough food to feed 7 billion, but instead only 2 billion).
  2. (Redirected from the Cloud Feedback thread here) RW1 - Quite frankly, this issue on total sensitivity has been explained to you, at length, in multiple threads here. You have yet to demonstrate any tendencies to incorporate the science you have been shown. For new readers: A doubling of CO2 would add 3.7 W/m^2 to the top of atmosphere (TOA) forcing of the climate. This should by all measures (and by that, I mean spectroscopic effects as integrated through the depth of the atmosphere - very basic physics) result in ~1.1°C warming directly. That works out to ~0.3°C/W/m^2. The 3°C warming estimated from numerous estimates is in the range of roughly 2-4.5°C, most likely estimate of 3°C, indicating a TOA forcing of roughly 10 W/m^2. That's an additional 6.3 W/m^2 forcing from feedbacks. CO2 represents roughly 1/3 of the current greenhouse effect - increases in water vapor will easily (well within the uncertainty ranges) account for the additional 2/3. In regards to cloud feedback (the change in forcing with temperature, not the initial value as RW1 emphasises), please read the opening post on that thread. The best estimates, best data, on that topic indicate that cloud feedback with temperature is slightly positive, with a range of uncertainty that does include (at low probability given the data) very slightly negative. Certainly not enough to overwhelm the increase in CO2 and temperature dependent absolute humidity.
  3. KR, You're very good at 'explaining' things to me and making declarations I'm wrong, but this is not offering anything to the discussion here or on the other thread.

    [DB] Actually, KR and others have more than amply explained things to you, including the specific points where you stray from accepted understandings into error. 

    Thus, it is your intransigence (amounting to agenda) that is standing in the way of the discussions.

  4. RW1 - I don't think anyone knows how to help you. People go so far and then you seem to point blank refuse to get it. "I dont understand". Left to a 14 year girl to do your homework IIRC at one point. It seems you believe one thing and when an argument takes you to the point when that belief is challenged, then you shut down. It still seems to me that you are stuck in the "back radiation cant warm the surface" mindset, and discussion cant go forward till you get that.
  5. RW1 - Two items. First: The appropriate numbers for TOA forcing with feedbacks given a 3°C climate sensitivity is ~10 W/m^2, resulting in the aforementioned 3°C rise in surface temperatures due to changes in total Earth emissivity and the surface temperature required to emit another 10 W/m^2 at TOA. Your numbers are wrong. Second: I wrote my most recent post for the general public, the readers of this thread. You have not shown any indications that you recognize evidence contrary to your preconceptions. As per the Debunking Handbook, Worldview Backfire , "...outreaches should be directed towards the undecided majority rather than the unswayable minority."
  6. KR, "Your numbers are wrong." Are you trying to say the surface does not have to receive +16.6 W/m^2 in order to warm by 3C?
  7. RW1 - You continue to mix TOA forcings with surface flux. Please note that an effective TOA emissivity of ~0.612, as measured and calculated, means that a TOA forcing of ~10 W/m^2 leads to 1/0.612 = ~1.64, or a required ~16.4 W/m^2 increase at the surface to increase emissions by ~10 W/m^2 at TOA to eliminate the imbalance. And that is strictly due to the emissivity of the Earth wrt. a blackbody. Your posts continue to interchange TOA with surface forcings, neglecting the effective emissivity to space (as per multiple threads), continue to invoke inappropriate "halving" of absorptions, and IMO represent errors. Nonsense statements such as "+6 W/m^2 (+1.1C) from 2xCO2 (3.7 W/m^2 directly from the CO2 'forcing' and the remaining 2.3 W/m^2 from the current average opacity of the atmosphere" do not aid your position (I have absolutely no idea where you got 2.3 from, for example). Your insistence on these issues demonstrate either (a) a lack of comprehension, or (b) an unwillingness to let data influence your position. Enough said. You have repeatedly demonstrated either a lack of knowledge or unwillingness to examine the evidence. Readers - if you wish to follow these conversations further, I would suggest the Climate Sensitivity or Lindzen and Choi threads, where this issue is discussed at great length. Personally, I feel no desire to rehash these topics...
  8. RW1 - Well, then, my apologies, I had not found that clear from your posts, which is probably my mistake. Mea culpa. So: a 10 W/m^2 TOA forcing results in a ~16.4 W/m^2 surface change. Of that 10 W/m^2 3.7 W/m^2 is direct CO2 forcing (assuming a CO2 doubling), and the remaining 6.3 W/m^2 is (as predicted by a 3°C sensitivity, with caveats due to uncertainties - 2-4.5°C) due to feedbacks. I would suggest viewing the detailed discussions at Water Vapor vs CO2 as a “Greenhouse” Gas and in particular Clouds and Water Vapor – Part Four by Science of Doom. Note that models of outgoing longwave radiation (OLR) match very well with observations, indicating that the models are pretty accurate. Water vapor represents ~2/3 of the greenhouse effect (in conjunction with clouds), and since water vapor is temperature dependent, it is a feedback, not a forcing. In addition, see Philipona et al 2005 ("...enhancing the forcing and temperature rise by about a factor of three") and others - water vapor is a strong positive feedback, as shown by the data.

    [DB] Please note that RW1 has never retracted this statement:

    I appreciate that you seem to be interested in helping me, but I'm not really interested in being helped per say. I'm a staunch skeptic of AGW, so my purpose here is to present contradictory evidence and logic that disputes the theory. That's what I'm doing.

    By his own admission he is here to not learn.

  9. Skywater @261 I believe that is the starting point in understanding how energenic the atmosphere realy is And without the van allen belts to protect us from the solar winds and cosmic rays (charged partical) life how we see it would not exist. That said it's my belief that what we are seeing regarding man made climate change is a result of or com's and detecting that use electromagnetic radiation from the ground and space and is very sensitive to this process and would create changes and hot spots through processes like this. There's a picture halfway down that shows hot spots expanssion and a bulging at the equator that is interesting. And as you know heat and preasure differences drive the weather Just putting it together to for a bigger picture
  10. DB, you should probably tack that comment on to every post that RW1 makes, so that no one makes the mistake of engaging him/her in discussion. It will also act as a standing demonstration of exactly what denialism is--a one-way street where the denialist presents the Truth and refuses to admit error. I've asked "wrongologist" Kathryn Schulz to target the global warming issue through interviews with a number of opinion-makers and scientists. I hope she ends up doing so. It's a very rich area for her--not just in the exploration of what happens when people who are committed to alternative theories come up against hard evidence against them, but also in statisticians' and scientists' relationship with modeling, in people who are paid to be publicly wrong, and in how scientists deal with being wrong.
  11. For the record, the 16.6 W/m^2 RW1 keeps throwing around is from his own entirely incorrect logic that there is a direct relationship between the radiation leaving the surface and the radiation emitted at TOA, and that this requires a linear "gain" and therefore the creation of non-existent energy. It basically comes from a completely flawed understanding of the system, I think because he is trying to translate it entirely into his own misapplied EE concepts of "gain," "feedback" and control theory rather than by understanding climate science and the actual system under discussion. The clearest (convoluted) path to understanding his logic is here, but in a nutshell, 16.6 W/m2 is a ridiculous constraint of his own devising, and there is no arguing with it, because he can't get past the mumbled incantations and heavy incense needed for his magical spells.
  12. I love Kathryn Schulz! Her talks aren't as good as her book though.
  13. jmorpuss, none of those links give any hint of any way by which the phenomena discussed could have any material effect on climate - by orders of magnitude. As such I cannot see how you can link this in any way to the idea that climate sensitivity is low.
  14. John Russell at 04:50 AM on 16 December, 2011: I certainly did not feel like I was being banned. I am moving to this thread so as to not be off topic. The warming since 1950 has been around 1 degree F or .55 degrees C. But how much of that is due to CO2? I read 75% in one article and 50% in another. Lets say 75% of the warming from 1950 is due to CO2 and the rest is due to land use changes, black carbon on snow, methane, etc. So .75 degrees F seems like a fair estimate of the warming from 1950 caused by CO2. First off - does that seem about right to you? I am not a climate scientist - so what I see from this data is that we seem to be on target for about the amount of warming you would expect from physics - but just the direct warming - no amplification effect. Doesn't that imply about 1.5 degrees F of warming to 2100 (or about .83C) from just CO2. Of course, there would be temperature increase also due to the other 25% non-carbon causes. If you just extend the trend line - doesn't it look like we will get about 1.2 degrees C by 2100? When I look at the data, I see the direct warming from CO2 - but no indirect warming from CO2. That is my main problem with a CS of 3 degrees C - it just doesn't add up for me.
  15. Richard Arrett, among many other problems, you're ignoring the possibility that more than 100% of the warming since 1950 is due to CO2. Impossible! I hear you cry. But not, actually, as it is quite likely that aerosols are offsetting the non-CO2 warming effects and some of the CO2 warming effects. It will be "interesting" in a Chinese curse sort of a way, when China and India sort out their pollution issues. Another issue is you keep discussing equilibrium sensitivity values when you should be discussing transient sensitivity values, which are closer to 2C per doubling. We don't expect to see equilibrium sensitivity-sized changes instantaneously.
  16. Richard#265: "The warming since 1950... " Let's set the record straight. Warming of ~0.7 C since 1970; approx 0.18 C per decade. CO2 in 1970 = ~325 ppm, now = 395 ppm (see Mauna Loa). The 'percentage caused by CO2' is a meaningless hairsplit at this cursory level of analysis; it's a system of forcings and feedbacks. But if you want more detail, you can find it here. The sensitivity of 3C per doubling of CO2 is a straightforward calculation; you can find it many places here or even on wikipedia (search 'radiative forcing'). But in very rough terms, if a 22% increase in CO2 results in 0.7 degrees; four 1/2 times that in CO2 gets you in the ballpark of 3C. The point is this: A 2 or 3 C increase in global temperature has effects that are not what you should want to risk.
  17. A question to all - is it really necessary to rehash the previous discussion on the Lindzen and Choi thread, where RW1 spent considerable time pushing the same hypotheses, and where he was pointed at the same facts that he's being pointed at (and ignoring) now? RW1 - the same objections to your unbased claims still hold. What's the term? Debunked a thousand times (DATT)? Readers - Take a look at the Lindzen and Choi "Working out climate sensitivity from satellite measurements" thread if you have any questions about this discussion. Personally, I don't have the patience to discuss this again, as RW1 has stated: "I appreciate that you seem to be interested in helping me, but I'm not really interested in being helped per say. I'm a staunch skeptic of AGW, so my purpose here is to present contradictory evidence and logic that disputes the theory. That's what I'm doing." Those are not the words of someone willing to discuss the data, the facts. Rather, the words of someone who just wants to argue. DNFTT - Do Not Feed The Troll.

    [DB] "What's the term? Debunked a thousand times (DATT)?"

    Very close.  PRATT - Point Refuted A Thousand Times.  Silver Star to you, circle gets the square.


    A large number of off-topic comments by RW1 and responses to him were deleted after this, as they belonged more properly on the 2nd law of thermodynamics contradicts greenhouse theory thread (or others) and were thus off-topic here.

  18. RW1, clearly you do not understand. What determines the strength of the greenhouse effect is the difference in the energy as it is radiated to space from energy radiated from the atmosphere, and the energy that would have been radiated from the surface in the same spectral band with no atmosphere. Therefore, the thing which determines the strength of the greenhouse effect is the temperature of the gas or cloud top which radiates to space relative to surface temperature, not the temperature of any intervening cloud top or gas which may absorb IR radiation in the same spectral band. The consequence of this is that in the band of strongest absorption (and emission by CO2), it contributes almost all of the greenhouse effect when compared to low lying cloud or water vapour. It will still contribute most of the greenhouse effect over medium level cloud. Over high cloud, the effect will be similar, depending on the cloud. Consider the spectrum below taken over the Sahara. It was taken on a day with high humidity as can be seen by the strong H20 signal in the spectrum. From the temperature difference between the black body curve of the surface (320 K) and the water vapour bands (280 K) we can tell that the effective altitude of emission to space from water vapour was about 6 Km, approximately the boundary layer between midlevel (alto) clouds and high level (cirrus) clouds. The effective altitude of emission to space of the CO2 is approximately 15 km is this case, which is also approximately the altitude of the tropopause. The important regions of the chart for this discussion are the yellow and green areas. The yellow area is approximately the area of overlap between the absorption/emission spectrum of the water vapour and the CO2. The absorption band of water vapour weakens in that area, so that in the absence of cloud and CO2 the H2O absorption/emission band would in fact slope up to 320 K within the yellow area, so that the yellow area overstates the greenhouse effect of water vapour in the area of overlap. Of course, solid cloud tops of that temperature would result in a spectrum following the 280 K black body line except for a trough in the green area, and an ozone peak from the stratosphere. Importantly, that means that in either case, the green area represents a contribution to the greenhouse effect which exists regardless of the presence or absence of H2O, either as water vapour or clouds. More importantly, in the complete absence of water vapour or clouds, the CO2 trough would have occupied almost the entire area of both the yellow and green zones. In this situation, the most accurate description is that in the area of overlap, CO2 contributes the entire greenhouse effect, as CO2 would have contributed all of that effect regardless of whether H2O was present or not. There are circumstances in which CO2 actually counters the greenhouse effect of very high clouds. There are also circumstances in the Antarctic winter in which CO2 contributes an anti-greenhouse warming effect. Consequently it is wrong to assume that the globally averaged effect of CO2 with water vapour present is the same as its globally averaged effect without water vapour present. But none-the-less, CO2 contributes a substantial part of the greenhouse effect in the spectral band where CO2 and H2O emissions overlap, and therefore best estimates place the contribution of CO2 to the greenhouse effect at about 20%, with a further 5% coming from other well mixed green house gases (O3, CH4, NO2, etc). Therefore your persistent claim that CO2 contributes 10% or less of the greenhouse effect is just false. Finally, I note on rereading your posts that you continually claim that half of absorbed radiation is emitted upward to space, while half is emitted downward to Earth. For a layer of atmosphere thin enough to have approximately the same temperature through its entire thickness, this is true, but it is not true of the atmosphere as a whole as can be easily determined by looking at the diagram @273. That you should think so suggests you continue to use a single slab model of the atmosphere to guide your thinking. Such models are false representations of the atmosphere,and only used in climate science as instructional tools to introduce more complex models with (in typical cases) around 20 layers. Using a single slab model reduces the atmosphere to a 2 dimensional shell, making my comment @275 exactly correct. Please take due note of the Trenberth et al diagram, and of the explanation above, and allow some reality into your model of the atmosphere.

    [DB] Note:  This comment was inadvertently deleted and has been reinstated.  Apologies.

  19. Hello First of all, hats off to this website for its clarity and ease-of use. It has been a great source of information for me. I am developing a high-school physics chapter where I show the back-and-forth debate on AGW. Here is a skeptic's graph that they say shows that observed temps/CO2 levels of the last 100+ years indicate a climate sensitivity of about 1.85. Can you please tell me how the skeptics have their data/graph/conclusions wrong? (please remember that the audience is going to be high-schoolers). the graph can be seen more clearly at I am not here to debate the issue, just to get information and to move on to the next topic. I thank you in advance.
  20. 269, SirNubWub, First, for a high school text I suggest you use log2 rather than natural logs. If you do so, then your constants will actually be the climate sensitivity (3, 1.85 instead of 4.7, 2.73). For the discrepancy, the short answers: 1) The model as described is far too simplistic. For instance, it presumes that the only influence on climate in the past 100 years has been CO2. More specifically, it ignores the opposing anthropogenic negative forcing of aerosols. Unfortunately, as we work for cleaner air we are reducing the aerosols without reducing CO2 emissions. And if we were to stop abruptly, the added aerosols would quickly fall out of the atmosphere while the CO2 would stay active for hundreds/thousands of years. See this page of the IPCC AR4 report and more specifically this diagram. 2) The model presented only measures transient, not equilibrium climate sensitivity. The first is what you get from fairly fast feedbacks, while the latter is what you get if you wait long enough for the system to stabilize (which includes all ice sheet melting, ocean warming, ecosystem transitions, permafrost methane releases, etc.). Transient climate sensitivity is estimated to be about 2˚C per doubling, and equilibrium sensitivity about 3˚C, so your 1.85˚C/doubling number is pretty close, especially after you consider the negative influence of aerosols. Sadly, the ice on Earth is far from finished melting, the carbon cycle is far from equalizing, and the oceans are far from absorbing as much heat as they can. 3) The model presumes that warming is instantaneous. Honestly, very few times in the history of the earth has a forcing of this magnitude been applied this quickly. It is very hard to predict how long it will take for the forcing imbalance to raise the planet to new equilibrium temperatures, even without considering the slow (equilibrium) feedbacks.
  21. It should also be pointed out that the IPCC quotes 3C/doubling as the most likely sensitivity. The value of 3.25 used above is a slight misrepresentation that overstates the mismatch. It is the average of the upper and lower extremes (2 and 4.5 respectively) cited by the IPCC, but the probability distribution is not symmetrical between these extremes.
  22. Stephen, I didn't even notice that the graph had upped it to 3.25. SirNubWub, I'd almost think you were a denier-in-disguise, playing hard-to-notice tricks with your audience. The number is 3 (or better yet, the range between 2 and 4.5). And transient sensitivity is around 2. And there is absolutely nothing at all behind any such denial argument (and please don't call them skeptics, because they aren't, if they were skeptical they would have researched the issue well enough to figure this out for themselves and not bother to make such a specious argument).
  23. SirNubWub @269, that is a strange graph. In addition to the errors noted by Stephen Baines and Sphaerica, I notice that the line marked "Global Warming Models" is almost certainly mislabeled. I draw your attention to the comparison of actual temperatures (HadCRUT3) and model results from the IPCC AR4 below: As you can see, the model results for actual forcings (Red line, graph a) very closely follow the observations (black line). Therefore in a graph such as you show, the model results would be shown by a scatter plot that overlapped with the observed data through out its entire range, and which had nearly the same mean for most of it. Fairly obviously the line labelled "Model Range"is therefore not the model range at all. Rather it is a simple plot based on a projected response function of temperature to CO2 of 3 degrees per doubling. As noted by Sphaerica, that response function is not the climate sensitivity. I note, however, that the IPCC definition of Transient Climate Response states:
    "The transient climate response is the change in the global surface temperature, averaged over a 20-year period, centred at the time of atmospheric carbon dioxide doubling, that is, at year 70 in a 1% yr–1 compound carbon dioxide increase experiment with a global coupled climate model. It is a measure of the strength and rapidity of the surface temperature response to greenhouse gas forcing."
    In other words, the Transient Climate Response is not the immediate response of temperature in the year of the change of CO2 concentration. As such, and contrary to Sphaerica, the graph plotted is not a graph of the Transient Climate Response. To plot that you would need to plot CO2 concentration against lagged temperature. Even then you would still face confounding factors in the effects of other Green House Gases, increases in solar radiation, and most particularly, changes in Aerosol Optical Depth. It is dubious, therefore, that such a plot will give anything better than a vague approximation of the Transient Climate Response. However, if you were to plot the IPCC TCR on the graph for comparison, the IPCC AR4 value for the TCR for a doubling of CO2 is between 1 and 3 degrees C:
    "Agreement among models for projected transient climate change has also improved since the TAR. The range of transient climate responses (defined as the global average surface air temperature averaged over a 20-year period centred at the time of CO2 doubling in a 1% yr–1 increase experiment) among models is smaller than the range in the equilibrium climate sensitivity. This parameter is now better constrained by multi-model ensembles and comparisons with observations; it is very likely to be greater than 1°C and very unlikely to be greater than 3°C. The transient climate response is related to sensitivity in a nonlinear way such that high sensitivities are not immediately manifested in the short-term response. Transient climate response is strongly affected by the rate of ocean heat uptake. Although the ocean models have improved, systematic model biases and limited ocean temperature data to evaluate transient ocean heat uptake affect the accuracy of current estimates. {8.3, 8.6, 9.4, 9.6, 10.5}"
    (Note, this quote is from the Working Group 1 technical summary. The numbers at the end are relevant section of the WG1 report for further details.) In looking at the graph I noticed that the measurement range of CO2 was from 287.50 - 388 ppmv. The temperature range was 0.8 degrees C. That strongly suggests the temperature data was taken from Gisstemp, and that the time interval of the graph was from 1880 to approx 2009. This information, and the information about who produced the graph should always be included in any graph used for teaching (or science in general). I also notice from the large number of temperature values at the lowest CO2 value that the plot was made against the most recent ice core value of CO2 concentration prior to the availability of Mauna Loa data. Using that method instead of plotting against a smoothed value introduces further inaccuracies to the graph. There is some possibility that values other than ice core values where used prior to 1959. If so, that should be specified, and great care taken as many CO2 measurements prior to 1958 are strongly distorted by local sources and sinks of CO2 (factories, roads, forests). In sum, this graph provides an excellent teaching opportunity. Specifically, it can be used to show how so called "skeptics" use incorrect values (3.25 instead of 3 for the IPCC central estimate of climate sensitivity; mislabel data (labeling a mathematical estimate as "Global Warming Models"); do not properly understand the data they are analyzing (ie, presenting it as a plot showing climate sensitivity, when the technique can show Transient Climate Response at best, and is likely to underestimate it); and do not take into account proper caveattes on the data (due to the unmentioned confounding factors). As such, it presents an excellent opportunity to show the difference between the pseudo-science of so-called skeptics and the genuine science as reported on by the IPCC. It also is an excellent opportunity to show that the vast majority of objections to climate science only masquerade as being science based, but are in fact political in nature, employing as they do pseudo-science rather than science to back up their claims.
  24. Sphaerica and Tom Curtis, Thank you for your replies. I have not heard of a response to this issue before. I will have to read it carefully and repeatedly to understand it all. If I have further questions I will ask. I appreciate your time.
  25. Continued from HERE Tom Curtis says: "RW1's bizarre claims assume that solar forcing results in no feedback response. That is, if the world's oceans are heated by 1 degree C by an increased GHG concentration, that will result in increased evaporation and an increase in absolute humidity (and hence a water vapour feedback), but that an increased temperature of the same proportion brought about by a brighter sun will not increase evaporation at all, nor melt any snow, or in any other way have feedbacks. RW1 can only attribute this view to climate scientists because, as always, he operates in complete disregard of what climate scientists actually say." What I'm saying is the ratio of surface radiative power to post albedo incident solar power, from which the so-called 'zero-feedback' response is ultimately derived, is already giving a measure of the lion's share of all the feedbacks operating in the system, including especially water vapor and clouds, as the two are by far the most dynamic components of the whole atmosphere. "Still more bizarre is RW1's claim that CO2 should result in less warming because of the energy needed to modify the internal energy structure of the atmosphere. What is bizarre here is that inside the troposphere, there is no significant difference in the change in temperature structure with time under GHG and solar warming. But solar warming heats the stratosphere, while increased GHG cools it - so as usual, RW1 gets the science completely backwards." GHG induced warming results in the warming of the atmosphere, does it not? Warming air expands and in doing so does work against its surroundings, which requires some of the internal energy to be expended, leaving less available to heat the atmosphere (and ultimately the surface).

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