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

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

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

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

adapted fig 3a

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

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

It’s all a matter of degree

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

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

Basic rebuttal written by GPWayne

Update July 2015:

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

Last updated on 5 July 2015 by skeptickev. View Archives

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Further reading

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

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


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Comments 226 to 250 out of 386:

  1. Here is another showing which gases are most responsible for absorption at various wavelengths: Click
  2. Here is the composite absorption with the emitted energy spectrum (grey line), which gives 255K. You can see that where the wavelengths are completely saturated (i.e. the 15u band of CO2), the transmittance is zero. If the halving effect was included, the maximum for the saturated bands would be only 0.5 and not zero: Click
  3. RW1 immagine a light source and a absorbing medium. A transmission experiment will give you the fraction of light passing through the medium along the line connecting the source and the detector. On the contrary, the light eventually re-emitted by the medium has no preferential directions. You should not expect the transmittance to saturate at 0.5.
  4. Riccardo (RE: 229), "You should not expect the transmittance to saturate at 0.5" I don't. The point is if the half up/half down effect was included in the spectral data and Modtran simulation output, the maximum transmittance would be 0.5 since even if absorption is 100% half escapes to space anyway. That transmittance in the saturated lines is 1.0 means it represents the total absorption - not the downward emitted half.
  5. RW1 modtrans does take into account emission; infact, you can see light coming from the saturated bands. On the contrary, transmittance measurements or calculations don't.
  6. Riccardo (RE: 231), "RW1 modtrans does take into account emission; infact, you can see light coming from the saturated bands. On the contrary, transmittance measurements or calculations don't." OK, show me where or how it does this. Is it your contention that the reduction in transmittance from 2xCO2 is 7.4 W/m^2?
  7. RW1 any textbook will explain you this point. As for the contention, I did not quote any number, just explaining the meaning of transmittance with which apparently you're not familiar.
  8. Riccardo (RE: 233), Obviously you haven't followed the discussion in this thread. The definition of transmittance, in the context of this discussion, is the amount of surface emitted LW that passes straight through to space as if the atmosphere wasn't even there. The claim is this reduces by 7.4 W/m^2 when CO2 is doubled, because the referenced 3.7 W/m^2 from 2xCO2 supposedly already includes the effects of half up/half down.
  9. RW1 it's typical of many to show up transmittance spectra and draw conclusions from them. I was just trying to show how one should look at this kind of spectra. It is essential for a proper understanding of radiation in the atmosphere. Never mind, those are pretty standard concepts. You will easily find them whenever you think it's appropiate.
  10. "basically if there is some large negative feedback which makes the sensitivity too low, it would have prevented the planet from transitioning from ice ages to interglacial periods" Doesn't the above assume that the negative feedback is linearly related to T? Is that a safe assumption if so?
  11. MajorKoko - Feedbacks are amplifications (positive) or dampenings (negative) of a forcing change. So they're related to changes in T, not T itself. That said, there are definitely phase changes (melt of clathrates, lack of summer ice in the Arctic, etc.) where feedback levels can be expected to change. As to the original statement: "basically if there is some large negative feedback which makes the sensitivity too low, it would have prevented the planet from transitioning from ice ages to interglacial periods" The Milankovitch cycle forcing change between ice age and interglacial is estimated to be on the order of 3.4 W/m^2, for a direct forcing change of ~1C. Global temperature changes for those cycles, however, are on on the order of 5-6C or so. So a short term sensitivity of ~3C for a doubling of CO2 (3.7 W/m^2) with additional long term feedbacks (ice melt, vegetative changes, CO2 temperature/solubility changes from deep ocean, etc) matches the feedback amplification seen in the ice age cycle.
  12. In the Pielke Sr thread, dana1981 says: "I think I'm most disappointed that we never got an answer regarding the discontinuity between low climate sensitivity arguments and the paleoclimate record. I've never seen any low sensitivity proponent answer this question, and unfortunately it seems Dr. Pielke was unable to answer it as well." Also a long time ago I promised scaddenp I would address low sensitivity, and this is a start. The portion of Knutti and Hegerl (2008) that goes with "Advanced" figure 4 (click on Advanced tab above) (Various Estimates of Climate Sensitivity) is shown to the left.

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

  13. Just a quick comment - the question about glacial dust would certainly be something for carbon-cycle models to worry about but for a climate model, what matters is how much CO2 eventually ended up in the atmosphere. This is a known (from gas bubble) so model doesnt need to calculate it. Its tricky to see how uncertainties from glacial aerosols could lead to lower sensitivity given that the rise in CO2 is known. Albedo feedbacks would be different last glacial termination (they are so in the models), but can be reasonably estimated. (area covered by ice).
  14. Continuing further on this, I note that you have focused on the paleoclimate measures of sensitivity, though they are in broad agreement with the other measures of sensitivity. Schmidt at RC commented recently on this too. "It's certainly conceivable that climate sensitivity is a function of base climate and surely is at some level. How large that dependency is unclear. But you need to distinguish between estimates of sensitivity derived from comparing older climates to today, and estimates of variability within an overall different base climate. Comparing the LGM or Pliocene to today is the former, looking at the variations during an ice age would be the latter. There have been a couple of papers indicating that sensitivity at the LGM is different to today (Hargreaves - not sure of the year - for instance), but in each case the differences (while clear), are small (around 10 to 20%). - gavin" See here. You have commented previously that you thought climate sensitivity was low (hence no "C"AGW). What science did you examine that led you to that conclusion? At the moment, it looks you are trying to find science to back an a priori determination that sensitivity is low.
  15. I'm trying to understand the relationship between climate sensitivity and C02:temp feedback. Assuming that CS is 3C for the radiative forcing resulting from doubling atmospheric C02: 1)Over what time period is this realized? 2)Is this the limit of the temp:C02 feedback or is this just the first order effect? 3)Wouldn't the C02:temp feedback limit be dependent on the amount of C02 already in the atmosphere? 4)If the radiative forcing came from a non-C02 source, wouldn't the temperature rise be larger, as there'd be more 'room' for the feedback to occur?
  16. 240, Tristan, 1) No one knows for sure, because it's never been doubled this quickly before. The models give some insights, but this is hard to pin down. We're also pretty early in the process, so it's hard to even estimate it at the current rate of warming. We haven't hit any step-changes yet, and the system is sluggish. What we do know is that no matter how slowly it seems to happen, it is happening, and it is going to continue well beyond the point where we stop raising CO2 levels. 2) To my knowledge, this is the "Charney sensitivity" or "equilibrium sensitivity", meaning the final, end result sensitivity after everything has stabilized. Also note that while 3C is an easy working number, the assumed range is 2C to 4.5C, and it may even be lower (unlikely) or higher (also unlikely, but more possible than lower than 2). This is in contrast to the "transient sensitivity" we would see within 20 years of doubling CO2 levels, which would see all fast feedbacks come into play, but not some slower ones. An excellent paper to consider in studying this is Hansen and Sato (2011). They talk exactly about these issues in a fairly clear fashion, and compare current positions to what can be inferred from previous similar changes in climate. There are, really, I think (in my mind, not officially) three levels of feedbacks... very fast, slow, and very slow. Very fast includes humidity and cloud changes that happen quickly. Slow feedbacks involve things like albedo and CO2 feedbacks that require major ice melt and fast ecosystem changes. Then very slow feedbacks require even longer term things (the point where oceans warm enough to release rather than absorb atmospheric CO2, and major, large-scale ecosystem changes occur that in turn change albedo and release or absorb more CO2). But I think the hoped for answer is that 3C is all of these effects combined. [I will confess that someone else may be able to give you a more direct and perhaps different answer than this one... this is what I understand, but I could be wrong here. Hansen and Sato 2011 in particular talk about fast and slow feedbacks on other time scales.] The sad reality, though, is that we won't know if 3C is the accurate estimate of the final feedback result until 1,000 years pass. 3) That's why it's expressed in terms of a doubling of current concentrations, and not based on the incremental amount added. 4) Yes and no. There are logically slight differences in feedbacks depending on the source of a temperature increase, but overall feedbacks are driven by temperature change, regardless of the cause in temperature change. Refer to this chapter on efficacy (i.e. how one forcing differs from another) in the IPCC AR4 report. There would be more "room" for CO2 feedbacks, because the same amount of CO2 released would be proportionally larger to a lower starting level. But at the same time we'd have pumped less CO2 into the oceans to release there. More importantly, the CO2 feedback is only one of many. Other feedbacks (water vapor, albedo changes, etc.) are in aggregate probably more important. So that difference wouldn't amount to that much.
  17. Thanks Sphaerica, your response was just what the doctor ordered! xox
  18. You may be interested in Professor Shaviv's writings about climate sensitivity. He explains why he comes up with a lower number From: "Prof. Nir J. Shaviv, who is a member of the Racah Institute of Physics in the Hebrew University of Jerusalem. According to PhysicaPlus: "...his research interests cover a wide range of topics in astrophysics, most are related to the application of fluid dynamics, radiation transfer or high energy physics to a wide range of objects - from stars and compact objects to galaxies and the early universe. His studies on the possible relationships between cosmic rays intensity and the Earth's climate, and the Milky Way's Spiral Arms and Ice Age Epochs on Earth were widely echoed in the scientific literature, as well as in the general press." Chris Shaker
  19. cjshaker I'm a bit surprised to see this old and debunked Shaviv paper pop up again. Honestly, I do not find it that much interesting.
  20. A quote from the Climate-time-lag.html article: "How long does the climate take to return to equilibrium? The lag is a function of climate sensitivity. The more sensitive climate is, the longer the lag. Hansen 2005 estimates the climate lag time is between 25 to 50 years." While reading through Lacis et al regarding CO2 as a control knob, I noticed this diagram

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

  21. Depends what you call equilibrium Eric - the temperature units on the Lacis diagram are pretty large, and it clearly hasn't reached perfect equilibrium even after >50 years. Why you suggest it supports a 10 year equilibrium mystifies me. From the diagram the change has reduced to being relatively slight after ~25 years, but equatorial regions are still cooling after 50 years.
  22. Eric (skeptic) @245, the rapidity with which a system adjusts to a new equilibrium depends not just on thermal inertia, but also on the magnitude of the disequilibrium. With the enhanced greenhouse effect, the disequilibrium is small, being approximately 1 W/m^2. This is because of both the small initial perturbation and the fact that the full effects of positive feedbacks are not felt until the system approaches the equilibrium temperature. In contrast, in the model analyzed by Lacis et al, the initial perturbation is around 30 W/m^2. Consequently the system adjusts towards equilibrium much faster because of the much larger disequilibrium. Even so, as skywatcher @246 points out, the system has still not reached equilibrium after 50 years. Further, Lacis et al state that they use the Q-flux ocean model with a 250 meter mixed layer depth. Had they used a model with deep diffusion, time to equilibrium would have been significantly extended (by a few centuries, I suspect), but the early changes of the system would have been unaffected. It just would have taken longer to close the last 0.1 W/m^2 of disequilibrium.
  23. 245, Eric, 246, skywatcher, 247, Tom, I'd also point out that Lacis et al is dealing exclusively with fast feedbacks (like water vapor and clouds). As we are now seeing, things like the ice albedo feedback take a comparatively long time to develop, as would carbon feedbacks that result from methane release or major ecosystem changes. The point is, we are still, fortunately, talking about things in terms of a climate that could have a quick return to the old equilibrium if CO2 could somehow be drawn down. This will not necessarily be the case in the longer term, when those slower feedbacks begin to kick in, and so the reverse will consequently be just as slow (along with the fast feedbacks that go with the slow feedbacks instead of with the initial forcing). Beyond this, I am very, very concerned about how much CO2 the ocean has absorbed. In many past scenarios the ocean was the source of, not a damper on, added CO2. In this case the ocean is acting to hold down atmospheric CO2 levels by soaking up some of the excess. Even after we completely stop emitting CO2, where will it go? It can't go into the ocean, because it already is (and that is in balance). It has to go into biomatter, either on land or in the ocean, and in some way be sequestered, but the mechanics of it I would have to believe will take a very, very long time. And even after any part of it is drawn out of the atmosphere, the ocean will certainly respond by trying to maintain an equilibrium and so transfer it from the ocean to the atmosphere. If slow CO2 feedbacks involve things like the transition of huge swaths of the Amazon to savanna, or other ecosystems to desert, then this puts more CO2 into the atmosphere/ocean. But how does it then get back into biomatter? Temperatures must drop for rain forest or prairie to again take hold where savanna/desert has appeared, so that vegetation can then grow and put the carbon into other forms. But how does this happen until temperatures first drop? And how long will this reverse process take? The bottom line is that we're not getting anywhere near any of the fast-acting we-cut-atmospheric-CO2-back-to-285ppm scenarios any time soon, probably not for several hundreds of years, which will be more than long enough for us to see at least some if not many of the "slow feedbacks" take hold and therefore hard to reverse. [This with the understanding that "slow feedbacks" in past climate change events are going to be relatively fast in this case because we are pumping the CO2 into the atmosphere so abruptly and quickly as compared to increases due to most natural processes in the past.]
  24. Sphaerica, we're never getting back to 285 anytime soon because there is an exponential decay from whatever level we are at. Q1: is it necessary to return to 285 (it may not be possible anyway)? Is a higher level ok? Q2: What about storage in the deep ocean, would that help the recovery prospects?
  25. Eric (skeptic), the highest levels (per the Antarctica ice cores) achieved at any point in the past 800,000 years was 298.7 ppm. Humanity has seen that in the rear-view mirror long ago. To an uncertain future with great temerity we go.

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