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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 126 to 150 out of 389:

  1. RickG @123, scaddnp @125, being fair to RW1, power is just energy over time, and the Watt is a unit of power, not of energy. For convenience, when measuring the energy balance of the Earth, climatologists use the unit of watts/meter^2 rather than joules/second meter^2, which would be more formally correct when talking about energy. Talking about the "power that escapes the atmosphere" rather than the "energy that escapes the atmosphere" would be peculiar; but talking about the "power that is transmitted" or the "power that passes through" the atmosphere is not, so I don't see your point.
  2. Well, I am used to the more usual definition of power as rate of energy conversion. The GW usage just sounds so strange when used instead of energy flux. Mix it in with amplifier analogues and its a real recipe for confusion. There is a lot to said for accuracy ( though I know I am pot calling kettle black at times). On other hand, met anyone not acquainted with GW using power in this peculiar way when discussing radiative physics?
  3. So the 3.7 W/m^2 does not represent the reduction in the atmospheric window, nor does it represent the incremental absorption?
  4. Tom Curtis (RE: 125), "The combination of these two effects will reduce the total energy leaving the atmosphere by 3.7 w/m^2" Wonderful. Now please provide me the documentation for this. What you don't seem to understand is I already know this is what is being claimed - I don't need to you to tell me it's true.
  5. Here is a question: If the 3.7 W/m^2 does not represent the reduction in the atmospheric window, then what is the reduction in the atmospheric window from 2xCO2?
  6. RW1 - The reduction in the atmospheric window represents only a small portion of the 3.7 W/m^2, as Tom Curtis said. Sorry I don't have exact numbers, but (as I have a day job) I haven't put in a request for the HITRAN data. If you look at the actual spectra of top of atmosphere (TOA) emissions, you will see the GHG blocked bands: The baseline of around 225K (around 650 microns) in the first graph represents the lapse-rate cooled greenhouse gas emission at the altitude where the IR can actually reach space without being intercepted by more GHG's. The higher this goes, the cooler the gases, the lower the temperature for emission, the lower the bottom of that curve. And hence the lower the integrated power over the entire spectra. My question to you is: Why does it matter? What's the issue with the 'window' versus lowest temperature of fully intercepted bands? I'm genuinely curious, especially since you've been poking at that for some days now - why is the percentage involved in 'window' narrowing important relative to the total integrated power blocked by a doubling of CO2? Do you have an argument based upon 'window' size? The reason I ask is because I don't see why the distribution would be an issue - the total energy imbalance (change in emitted energy with doubled CO2) is what is important as a forcing, rather than exact spectral distribution (and I say that as someone who works with spectrometers all the time!).
  7. I tried. I'm going to get to bottom of this. I'll be back when I know and can show the proof.
  8. RW1 @129, if you already understand this then why are you asking a question which is almost nonsensical, and is certainly irrelevant, given that knowledge? Your ask it again @130. However, it is irrelevant for all except the most abstruse studies. What concerns us it the total change in Outgoing IR Radiation, not the change at particular wave numbers. It is also very difficult to calculate independently. For each wave number effected, you would need to calculate the energy flows by radiation and convection/latent heat from the surface to the top of the atmosphere, including both upwards and downwards energy transfers. Line By Line models do in fact calculate exactly that for every wave number (or small band of wave numbers depending on their resolution), so if you were to ask a scientist who regularly dealt with LBL models, they would no doubt be able to find the information you seek. But unless you can show a very good reason why it matters, I see no reason to pander to your request, anymore than I would pander to a geocentrist's request to show the gravitational impact of Mount Everest on the moon's orbit. Given the very accurate prediction of LBL models as shown here, and the detailed discussion of that accuracy by Science of Dooom (linked by scaddenp @102 above) and the many quoted direct claims that the change in OLWR from a doubling in CO2 is 3.7 w/m^2, you have no reasonable basis to doubt that figure. You need to come good with a very good reason as to why you doubt the 3.7 w/m^2 figure, and as to why you persist in your obtuse question.
  9. Tom, "if you already understand this then why are you asking a question which is almost nonsensical, and is certainly irrelevant, given that knowledge?" That is a very troubling question. It appears that if the 'answer' supplied can't be put into the exact format required, it's either alleged to be undocumented (when it actually was documented) or alleged to be unacceptable. Sadly, we've seen this drag on for hundreds of comments. How this rather elliptical debating process can be considered scientific eludes me.
    Response: [DB] Tamino has a new post up very "tangential" to those "elliptical" thinkers of whom you speak.
  10. "You need to come good with a very good reason as to why you doubt the 3.7 w/m^2 figure" Agreed - I'm working on it. I really don't doubt the figure - just what the figure supposedly represents. I'm not getting any 'proof' here of anything, so I'm left to figure it out on my own.
  11. RW1 - "I'm not getting any 'proof' here of anything, so I'm left to figure it out on my own." Actually, I will have to disagree with you. You've been pointed to the documentation, you have statements from several people who are quite familiar with line-by-line atmospheric calculations, and even George White sees a ~3.6 W/m^2 imbalance with his own runs of the HITRAN code. Unfortunately, as muoncounter pointed out, these efforts are met with disbelief, rejection, and (yes) denial - "I'm going to get to bottom of this. I'll be back when I know and can show the proof". I have the impression from this conversation that you will reject anything that does not conform to your preconceptions, and that is very sad. 3.7 or so W/m^2 is the difference in total planetary emissions upon doubling CO2, the amount of extra IR not leaving at a particular temperature, the change in outward directed energy. Please - we've offered this information honestly and clearly, as the best established data available. I would suggest you consider your own reasons for not believing it, and why you are so insistent that we are wrong.
  12. Guys, noticed RW asked at Realclimate: "’m wondering if someone can shed some light on this subject for me. I’ve searched around at length all over and cannot find a clear answer. The 3.7 W/m^2 estimated from simulations for the increase in ‘radiative forcing’ from a doubling of atmospheric CO2 – does the 3.7 W/m^2 represent a reduction in the atmospheric window or does it represent the half directed down due to isotropic re-radiation/redistribution (meaning a reduction in the atmospheric window of 7.4 W/m^2)???" Clearly absolutely nothing we have said has been understood at all. I doubt he will like Gavin's accurate response either.
  13. scaddenp @137, out of a morbid curiousity, what thread?
  14. Here it is: "RW says: 24 Feb 2011 at 8:50 PM Interesting thread. I have a question about some frequently referenced data: I’m wondering if someone can shed some light on this subject for me. I’ve searched around at length all over and cannot find a clear answer. The 3.7 W/m^2 estimated from simulations for the increase in ‘radiative forcing’ from a doubling of atmospheric CO2 – does the 3.7 W/m^2 represent a reduction in the atmospheric window or does it represent the half directed down due to isotropic re-radiation/redistribution (meaning a reduction in the atmospheric window of 7.4 W/m^2)???" [Response: It is the global mean change in outgoing LW flux at the tropopause (integrated over the whole spectrum) for a doubling of CO2. - gavin] [ -Edit: More ensuing discussion follows- ] The Yooper
  15. @138 How easy is it to get fooled?
  16. Another question: Do we all agree that not all of the absorbed infrared affects the surface? That a portion of it is directed up out to space and the remaining portion of it is directed down to the surface?
  17. RW1 - The 3.7 W/m^2 energy imbalance from doubling CO2 is kept in the Earth climate system, atmosphere and surface. This is the sum result of multiple absorption/emission events distributed throughout the atmosphere, as we've told you, and as (it appears) Gavin Schmidt has repeated. Each of those individually is isotropic, with nearly equal (due to horizon effects) probability of upwards or downwards. The sum radiation change upon doubling CO2 is that a global mean of 3.7 W/m^2 less energy leaves the top of the atmosphere. Shopping around for a different answer won't change that...
  18. KR, As stated before, you're just repeating and declaring conclusions I'm already aware of. This is not how science, logic and reasoning works.
  19. RW1 - You've repeatedly, and by multiple people, been told what the data is, and continue to argue for your (mis)perception of it. We've told you what the results are - denying the data is the unscientific approach here. Something to think about, RW1 - which is more likely? That everyone's interpretation of LBL analysis of CO2 forcing is somehow blatantly wrong? Or that George White (not published AFAIK, certainly not in climatology) is misinterpreting the results of the model he's run? I'm not asking for an answer from you, but just for you to consider the question. I'm out of this thread until real questions are discussed again.
  20. RW1 @143: Science works by verification and observation. In this instance, to verify the Line By Line models, or the Energy Balance models (which give essentially the same results) you would need to verify the physical laws involved, ie, the Beer-Lambert Law, Kirchoff's Law, Planck's Law, Wien's Law and Stefan Boltzmann's Law, not to mention the laws of convective heat transfer in the atmosphere and heat transfer from changes of state of H2O. Having done that, you then need to go through the models line by line to make sure they actually implement the relevant laws appropriately. You also need to have detailed records of the composition and temperature profile of the atmosphere, and confirm that they are correctly entered into the model. You also need to check the emissivity of the various compounds in the atmosphere and make sure they are correctly fed into the model. You have been referred to sources in this discussion where you can do each one of these things, either in little detail (Science of Doom), moderate detail (relevant textbooks) or great detail (relevant scientific papers). You have ignored all of that because, apparently, we cannot find a source that encapsulates all that knowledge into just one sentence. Having done all that, or accepted expert opinion that it was done correctly (which is the sensible approach in that none of the above is in dispute by any practicing scientist including well known skeptics such as Pielke and Spencer), you can then compare the results of the models with observation, as has been done here. In fact, line by line, if given approximately current information on atmospheric composition at each level, models have been shown to be accurate withing less than 1% - again something you have been shown in this thread. With only approximate information, the models are accurate to withing 5% or 0.2 w/m^2 for a doubling of CO2. Again this is not in dispute by any practicing scientist once a few transparent crack pots are excluded. Your problem is not that we are not confirming to how logic or science should work. Your problem is that we are, and for some strange reason, you don't like the answer.
  21. Oh. Jan 13, 2011 ... Guest Post by George White. Evolution of an Energy Budget ..... Trenberth's atmospheric window includes 40 W/m^2 coming from the surface and ...
  22. "Your problem is not that we are not confirming to how logic or science should work. Your problem is that we are, and for some strange reason, you don't like the answer." Whatever, Tom. I'm not getting the answers to the questions I'm asking. All I'm really getting is declarations.
  23. Oh. My. "co2isnotevil: February 10th, 2011 at 8:53 am "... looking up during the day you will see both primary and secondary IR directly originating from the Sun. This is not ‘back radiation’, but forward radiation from the Sun. Trenberth likes to call this ‘back radiation’ in order to give the false impression that GHG’s radiate this much." (from the joannenova thread still in progress). ____________ 'The question is,' said Alice, 'whether you can make words mean so many different things.' 'The question is,' said Humpty Dumpty, 'which is to be master ...'
  24. Oh, heck, RW/GW were answered previously e.g. and earlier. Sorry, I didn't realized I'd walked into the late stages of a thread-to-thread-to-thread Gish Gallop.
  25. "George White's arguments are rife with errors. (There was going to be a third and fourth post on his errors, but the page containing his essential argument is currently down.) One of the most egregious is the halving of the reduction in outgoing radiation due to IR gases. This is very easily verified for your self using the modtran model hosted by David Archer. This is an obsolete model available on the public domain, but it still shows a change in TOA OLR of -3.17 w/m^2 for a doubling of CO2 from the default settings. Note, that is the reduction in the Outgoing Longwave Radiation, it is not "the amount of IR radiation captured" or some other vague term designed to confuse. Based on this model, with 375 ppm CO2, approx 287.8 w/m^2, while with 750 ppm, approx 284.7 w/m^2 leaves the planet." How can 287 W/m^2 be leaving the planet? From Stefan-Boltzman, 287 W/m^2 = 266K (255K expected)? What about Trenberth's transparent window of 40 W/m^2? 287.8 W/m^2 + 40 W/m^2 = 327.8 W/m^2 = 275.5K (255K expected)????

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