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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 201 to 225 out of 386:

  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

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