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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 76 to 100 out of 386:

  1. thepoodlebites - If you look at the "Intermediate" tab of this discussion you will see half a dozen empirically observed estimates of climate sensitivity, along with another half dozen model based ones. Does this address your request for observational data?
  2. thepoodlebites - My "favorite paper"??? All of them are interesting. Some of the more directly relevant ones from your question (observations) are Hansen 1993 (energy changes since the last ice age), Tung 2007 (sensitivity from climate response to solar variations) and Bender 2010 (responses to the Mount Pinatubo eruption). All of the papers listed on the intermediate page are worth reading, though. I will not continue the climate sensitivity discussion on the "Is it the sun" thread - that's off topic.
  3. A question for the group - I hope this is a reasonable place for it, and even more that I'm phrasing this intelligibly. Given that we have raised the CO2 concentration quite high, it's now high enough that the oceans are acting like a sink despite their warming (above solubility pressures) - the oceans are absorbing 2ppm/year or so. If we maintain, as we are doing now, a CO2 concentration above that which would induce CO2 output by the oceans, does that remove one of the feedbacks (CO2 outgassing from said oceans) from the climate sensitivity calculations? In other words, does the forcing by CO2 emissions block the CO2 element of forcing feedback, and thus reduce climate sensitivity??
  4. RW1 - there is another way to look at whether the radiative change is correct or not without going into the mathematics deeply. Step 1/ assume scientists have the maths and physics right. Use the model to calculate TOA emissions. Not just the energy, but also the spectra. Compare with REAL measured spectra. Step 2/ Assuming that was right, you can see whether the calculation for incremental CO2 increase is also correct by doing the same procedure but doing it for different decades and seeing whether the change matches the change in CO2. Sound fair enough test? In fact you could do the calculation for downward IR at surface or for outward IR by satellite. For results, see the papers on this Now lets see George White produce some calculations from his approach that can match these empirical results.
  5. (Oops, the above is response to comments from RW1 at A swift kick in the ice
  6. scaddenp (RE: 79), I don't understand, sorry.
  7. If George was right, (ie should be 1.85W/m2), then the model result that calculated 3.7W/m2 would not agree with the actual measurements of IR. Similarly, if you compare spectrum measured in 1979 with that in 2004, if the incremental change in IR was wrong then the measurement wouldnt agree. This is experimental verification that 3.7W/m2 for doubling is correct. Furthermore, you check that the change in IR is due to CO2 by looking at the spectrum.
  8. I'm not following.
  9. scaddenp, RW1 - George White has stated that running the HITRAN models results in an imbalance of 3.6 W/m^2 (here, post #19). And then he, for some reason, halves that value. Which I cannot consider as other than a blatant mistake.
    Response: [DB] Fixed URL link.
  10. KR, You may recall the last time we went around this tree (the endless Lindzen and Choi thread), this came from the assumption that 50% of emitted IR photons go up and out - 50% down.
  11. Muoncounter @85, it comes from the incorrect assumption that line by line radiation models do not already apply that effect already, and then applying it again to the output of the line by line models.
  12. Yes, hence my attempt to show that science had it right by direct empirical means since I despaired that RW1 would understand the calculation.
  13. Tom Curtis (RE: 86), "it comes from the incorrect assumption that line by line radiation models do not already apply that effect already" Where is the documentation that the halving is already applied? That's all I'm asking for. I've looked around and cannot find it.
  14. I know I am sounding like a broken record but you start with Ramanathan and Coatley 1978.
  15. RW1, where is George White's documentation that it was not applied? Currently your "critical thinking" will not accept the results of several scientific papers, the two most seminal of which have been cited to you, it will not accept the IPCC report, it will not even accept the results of the public domain version of a radiation model designed by the USAF, and it will not accept the reports of a large number of people knowlegeable on the subject. But it will accept the say so of a single electrical engineer based on zero documentation to the contrary. This extreme contrast in willingness to believe shows it is not critical thinking at all. So, before we go any further, how about you show us the peer reviewed paper, or technical description of a line by line radiation model, or the code of such a model in which the effect is not applied already. Current evidence is that you will accept any belief contrary to AGW on zero evidence, but will not accept any belief supportive of AGW on even a mountain of evidence. Given that I am not going to waste my time presenting evidence to your that you will not consider anyway. (Afterall, I already have given that evidence to you in at least two different forms; both from very creditable academic sources.) So, either show me that you apply the same evidentiary standards you apply to Gearge White's ravings; or give principled reasons why you will not accept a straightforward truth that can be verified in any first year text on atmospheric physics, or on climate modelling?
  16. And also, in scientific arguments, nature is the arbiter. The codes can used to calculate what experiments should observe. If George was right, then the experiments should be giving results half what they in fact do so.
  17. saddenp @91, you mean that if the models where wrong, you would not get results like this: Note: the spectral lines have been deliberately offset so they can be seen clearly. Without the offset, it looks like this:
  18. Yes, precisely what I mean.
  19. It was and remains a simple question unanswered. If it's so obviously wrong as being claimed here, it should be easy to point to the documentation that the "halving" is already applied to the 3.7 W/m^2 forcing. I have search around too. I couldn't find anything.
  20. RW1 - When the HITRAN model (and others) indicate a 3.6/3.7 W/m^2 imbalance, they are indicating photons going outward. The line-by-line calculations include photons going up and down by absorption and re-emission, for every level of the atmosphere covered by the model. The imbalance is the end difference between incoming and outgoing, the leftover quantity. Not emitted in all directions from some level of the atmosphere, but just the value emitted to space. That's what you get when you model the absorption/re-emission over the entire atmosphere. What's going back down to lower levels of the atmosphere or to the surface is part and parcel of the model - the imbalance is only the portion going in one direction, whether that's positive or negative depending on conditions. I'm afraid that George White's misunderstanding of this (and subsequent "halving" of the imbalance) indicates his overall poor understanding of the models he's been running.
  21. KR, "The line-by-line calculations include photons going up and down by absorption and re-emission, for every level of the atmosphere covered by the model. The imbalance is the end difference between incoming and outgoing, the leftover quantity. Not emitted in all directions from some level of the atmosphere, but just the value emitted to space." OK, show me where this is documented.
  22. RW1 - Assuming that GW is using the HITRAN spectral database and something like JavaHAWKS for full atmospheric simulations, the spectral database includes absorption/emission spectra for a large number of IR interactive molecules. Full atmospheric emission modeling means looking at absorption, emission, and transmission across the full black body spectra of the Earth emission, over the depth of the atmosphere. Some IR gets radiated back to the surface, some gets radiated around and re-absorbed in the atmosphere, a certain percentage in the 'IR window' goes straight to space, etc. The output from JavaHAWKS is the amount of radiation that actually leaves the atmosphere. Now, I cannot speak for GW, but "imbalance" should be a difference between the outgoing radiation from JavaHAWKS and incoming from the sun (a reasonably known value). Not the amount isotropically radiated from some level of the atmosphere, but the amount finally leaving the atmosphere (one directional) at the end of the modeling. And that's because the model includes the isotropic (omnidirectional, spherical) radiation as part of the calculation, summing up the anisotropic portion as output. That's certainly what everyone else running these models gets; 3.6-3.7 W/m^2 anisotropic radiation going to space for a doubling of CO2. An imbalance (difference!) between incoming and outgoing, an amount going in one direction not balanced by an amount going the other. I hate to say it, but GW does not understand the model he's running...
  23. RW1 - To put it more clearly: If it's not an anisotropic emission, it won't show up. Isotropic emissions, absorptions, and re-emissions are part of the model, not part of the output spectra. Total power emitted from the atmosphere given the model conditions is the output - not a sub-portion of internal isotropic emissions that will then get bounced around.
  24. I'm willing to be shown incorrect on this issue (and I believe George is too), but you're talking around the crux of the issue. Words like "should" and "everyone else" isn't evidence to the contrary, and more importantly doesn't answer the fundamental question. In another thread, you said the total additional absorbed infrared from models/simulations from 2xCO2 was 7.4 W/m^2. And why haven't you said all this to George on his article and post on the issue at joannenova that you linked?
  25. KR, "To put it more clearly: If it's not an anisotropic emission, it won't show up. Isotropic emissions, absorptions, and re-emissions are part of the model, not part of the output spectra. Total power emitted from the atmosphere given the model conditions is the output - not a sub-portion of internal isotropic emissions that will then get bounced around." OK, where is this documented? Point me to the paragraphs or pages that state this is what the output spectra represent.

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