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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 26 to 50 out of 83:

  1. re #25: I forget to give the url to Schwartz's reassessment/correction of his original work:
  3. please pay attention Quietman! We've already seen that paper (linked in John Cook's top post). My link is to the correction that Stephen Schwartz published in which he determined that the conclusions of the original version of the paper (your link), were incorrect and that his original climate sensitivity value was far too low.....
  4. chris I was asking if that was the original paper that you referred to. A little testy?
  5. CLIMATE SENSITIVITY "The sensitivity of the climate system to a forcing is commonly expressed in terms of the global mean temperature change that would be expected after a time sufficiently long for both the atmosphere and ocean to come to equilibrium with the change in climate forcing. If there were no climate feedbacks, the response of Earth's mean temperature to a forcing of 4 W/m2 (the forcing for a doubled atmospheric CO2) would be an increase of about 1.2 °C (about 2.2 °F). However, the total climate change is affected not only by the immediate direct forcing, but also by climate “feedbacks” that come into play in response to the forcing." "As just mentioned, a doubling of the concentration of carbon dioxide (from the pre-Industrial value of 280 parts per million) in the global atmosphere causes a forcing of 4 W/m2. The central value of the climate sensitivity to this change is a global average temperature increase of 3 °C (5.4 °F), but with a range from 1.5 °C to 4.5 °C (2.7 to 8.1 °F) (based on climate system models: see section 4). The central value of 3 °C is an amplification by a factor of 2.5 over the direct effect of 1.2 °C (2.2 °F). Well-documented climate changes during the history of Earth, especially the changes between the last major ice age (20,000 years ago) and the current warm period, imply that the climate sensitivity is near the 3 °C value. However, the true climate sensitivity remains uncertain, in part because it is difficult to model the effect of feedback. In particular, the magnitude and even the sign of the feedback can differ according to the composition, thickness, and altitude of the clouds, and some studies have suggested a lesser climate sensitivity." Climate Change Science: An Analysis of Some Key Questions, pp 6-7, Committee on the Science of Climate Change National Research Council "Climate models calculate outcomes after taking into account the great number of climate variables and the complex interactions inherent in the climate system. Their purpose is the creation of a synthetic reality that can be compared with the observed reality, subject to appropriate averaging of the measurements. Thus, such models can be evaluated through comparison with observations, provided that suitable observations exist. Furthermore, model solutions can be diagnosed to assess contributing causes of particular phenomena. Because climate is uncontrollable (albeit influenceable by humans), the models are the only available experimental laboratory for climate. They also are the appropriate high-end tool for forecasting hypothetical climates in the years and centuries ahead. However, climate models are imperfect. Their simulation skill is limited by uncertainties in their formulation, the limited size of their calculations, and the difficulty of interpreting their answers that exhibit almost as much complexity as in nature." Climate Change Science: An Analysis of Some Key Questions, p 15, Committee on the Science of Climate Change National Research Council Ref: The Real 'Inconvenient Truth' Some facts about greenhouse and global warming -, Updated August 2007
  6. JunkScience is a place where truth goes to die. I haven't looked at the link, yet. But aside from that, I don't see anything in comment 30 that disagrees with my points or chris's or Philippe's or the host of this website, RealClimate, IPCC, The Weather Channel, Scientific American, James Hansen, Shakira (well actually I don't know what she specifically says on the matter but she did perform for Live Earth and one or more inaugural balls), ... etc.
  7. "We should note that devoid of atmosphere Earth would actually be a less-cold -1 °C (272 K) because the first calculation strangely includes 31% reflection of solar radiation by clouds (which obviously could not occur without an atmosphere) while ignoring that clouds add significantly to the greenhouse effect. Granted it's kind of a bizarre to include clouds in one half the calculation and not the other but that is the way it's commonly done, so, for simplicity, just stick with ~33 °C." The reason for including the LW effects and not the SW effects of clouds is because what is being discussed in the effect of greenhouse agents via LW radiation. If we really consider what would actually happen with the removal of all greenhouse agents, including SW effects, then yes, the cooling won't be so great, but it would still be enough to cause dramatic cooling by ice/snow albedo feedback. Thus far, no techical errors, but the distinction between a real greenhouse and a radiative greenhouse, and that greenhouse agents do not 'form a blanket' is rather nit-picky and besides the point. Real greenhouses don't have fabric blankets either - they have glass (or some other generally SW transparent material). I don't cover my self with glass when I get into bed in the winter! What they all have in common is that they slow the flow of heat from hot to cold - by inhibiting convection, reducing thermal conductivity, and/or increasing LW opacity - so that a greater temperature difference (between inside and outside, between my skin and the air in my room, between Earth's average surface temperature and the temperature of a blackbody that would emit the same LW power to space) is required to sustain a rate of heat loss to balance a given heat supply (the sun, my metabolism).
  8. (Thus far, no techical errors) - I haven't varified the temperature calculated for no albedo and no greenhouse effect. Actually though, there is some error if they reduced the albedo to zero, because there is some surface albedo and some backscattering to space from clear air.
  9. Some less trivial errors popping up now, such as this: "Greenhouse gases do not emit energy in the same bandwidth in which they absorb energy and thus emissions from carbon dioxide are not absorbed by carbon dioxide." That might be true (? - something about quantum mechanics) if there were no absorption-line broadenning by doppler and pressure effects, ... BUT IT IS MOST CERTAINLY NOT TRUE in the case of atmospheric greenhouse gases and clouds, etc. Height of tropopause: "10-50Km or 6-30 miles above the surface" - NO WAY! WAY OFF! 10 km is a typical midlatitude value, but it never gets even halfway to 50 km (I think it's somewhere around 15 to 18 km in the tropics - around the 100 mb level - see Holton, chapter 12). 50 km is actually about the height of the stratopause. "Sidebar:" Even absolute errors that are larger than projected changes are tolerable because ... well, you know I'll be taller if I stand on my toes than flat on my feet; you essentially only need to know the dimensions of my feet to calculate the difference (perhaps some feedback from posture changes...). Another way of looking at it - suppose the relative error in change is about the same as the relative error in absolute values. 10 % of 288 K would be HUGE, yet a 10 % error in 3 K is not too bad. Of course there is not a particular basis for arguing that the errors are related precisely that way, but ... Aside from that, I can bet what's coming up - greenhouse effect short-circuited by convection. Okay, but models take that into account! Remember it's tropopause level radiative forcing that tends to be important in driving surface and tropospheric temperatures - which are convectively coupled; this does not mean they don't respond to anything. I'm not going to bother with the link anymore; I'm quite sure there's nothing new there.
  10. "Of course there is not a particular basis for arguing that the errors are related precisely that way," Actually they must not be that way; given that there is a correct value, the error range is quite sizable in projected changes. Too big, and yet not big enough.
  11. Okay, I skimmed the rest of it. There are some factual points that are true (and some that are true but irrelevant to anything), but the picture they paint is a worthless piece of trash. Forget the science, this is JUNK!
  12. "That might be true (? - something about quantum mechanics) if there were no absorption-line broadenning by doppler and pressure effects"... Actually, any macroscopic material in local thermodynamic equilibrium must have the same emissivity as absorptivity along any path length in any direction at any wavelength; otherwise, I've got some plans for a perpetual motion machine you might be interested in...
  13. Have a read: which has commentaries on Hug & Barrett's presentation on climate sensitivity. You can access the original paper direst from the web page.
  14. Hello! You could add that Schwartz updated and corrected (in some aspects) his analysis. He now claims that climate response time is 8.5 ± 2.5 years. According to this climate sensivity is 1.9 ± 1.0 K. Thus his estimate of climate sensitivity now is at the lower bound of the IPCC range.
    Response: Thanks for the link, I wasn't aware of Schwartz' response and have updated the article accordingly.
  15. So how about that Spencer & Braswell et al 2008? It has a few nice ideas about climate models and their feedbacks... and so far I haven't found anyone who has been able to debunk it. Has anyone found one?
  16. Sensitivity to cumulative carbon emissions is a new metric of sensitivity proposed in papers published in 2009 and discussed on the brand new blog Climate Physics Forums.
  17. Tom Dayton unintentionally reminded me that this is the right place to discuss the forthcoming Spencer paper on climate sensitivity. So this is in a sense a repost. For sure we need to wait to read the paper but one thing can already be said. From what Spencer himself says, he shows "monthly variations in the Earth’s net radiation [...] compared to similarly averaged tropospheric temperature". This is not wrong by itself but it should be clear that in this way he's looking just at the fast response component. From this work nothing can be said on the overall climate sensitivity which definitely includes components much slower than a month.
  18. The problem is that Spencer keeps repeating the same wrong conclusions in his blog ("These results suggest that the sensitivity of the real climate system is less than that exhibited by ANY of the IPCC climate models. ") so that they're still usefull for the skeptic community. I prefer not to comment on this attitude that he has in common with other (luckly few) skeptic scientists.
  19. Details about Riccardo's point about Spencer's papers are provided by chris in an excellent comment on another thread.
  20. Riccardo at 01:42 AM on 10 May, 2010 I think it is a bit dangerous for any of us to comment on a paper that has not yet been published. However, on my reading of the article published so far, it does not appear that Spencer is claiming that he can calculate the equilibrium climate sensitivity from his approach. He is comparing the observed response against the statistics FROM THE MODELS OVER THE SAME RESPONSE PERIODS, and thereby suggesting that the models are overestimating the temperature/flux reponse. From this, one can PERHAPS validly draw the conclusion that the models are overestimating the equilibrium climate sensitivity (expressed in temp/flux). It is therefore not valid (or at least not valid until we have seen the paper) to say that "nothing can be said on the overall climate sensitivity".
  21. PaulK, even assuming that the comparison with climate models is appropiate, my point still stands. One cannot judge the sensitivy just looking at the first part of it, let alone deduce the long term behaviour of the climate system nor of the models. For example, what if the models miss just a damping factor? In the short term they produce larger postive and negative variations, but they will average out. But, as pointed out in my last comment, my guess is that Spencerr will not explicitly say anything like this in the paper, it's something just for his blog.
  22. I am not sure this paper has been mentioned yet. See Lin 2010. Obtains 3.1K for sensitivity, constraining the estimates a little to between 2.8K and 3.7K
  23. here is a more readable formatting of Lin 2010. ;)
  24. I think on this subject somewhere at the top of this page the paper(s) from Spencer should also be clearly addressed - while Schwartz was probably what started this debate I believe right now the focus of the deniers is moving to Spencer ...
  25. From the main article:
    A 2008 study led by James Hansen found that climate sensitivity to "fast feedback processes" is 3°C, but when accounting for longer-term feedbacks (such as ice sheet disintegration, vegetation migration, and greenhouse gas release from soils, tundra or ocean), if atmospheric CO2 remains at the doubled level, the sensitivity increases to 6°C based on paleoclimatic (historical climate) data.
    Hansen et al estimate Earth System Sensitivity (ESS - that includes slow feedbacks) over Charney Sensitivity (CS - that only includes fast feedbacks) - or ESS/CS - at 2:
    Paleoclimate data permit evaluation of long-term sensitivity to specified GHG change. We assume only that, to first order, the area of ice is a function of global temperature. Plotting GHG forcing [7] from ice core data [18] against temperature shows that global climate sensitivity including the slow surface albedo feedback is 1.5°C per W/m 2 or 6°C for doubled CO2 (Fig. 2), twice as large as the Charney fastfeedback sensitivity. Note that we assume the area of ice and snow on the planet to be predominately dependent on global temperature, but some changes of regional ice sheet properties occur as part of the Earth orbital climate forcing (see Supplementary Material). pg. 220, James Hansen et al.(2008) Target Atmospheric CO2: Where Should Humanity Aim?,The Open Atmospheric Science Journal, 2, pp. 217-31
    A more recent estimate puts ESS/CS at about 1.4:
    As shown in Table 1, none of these assumptions greatly changes our estimate of ESS/CSacross all of the analyses presented in this article, the smallest value of ESS/CS we obtain is 1.3, and the largest is 1.5. Our combined modelling and data approach results in a smaller response (ESS/CS~ 1.4) than has recently been estimated using palaeo data from the Last Glacial Maximum, 21,000 years ago (ESS/CS ~ 2). This is probably due to the fact that transitions from glacial to interglacial conditions in the Quaternary involve large changes in the Laurentide and Eurasian ice sheets (see, for example, ref. 36), which result in a significant large-scale albedo feedback in these regions that is irrelevant for climates warmer that present. pg. 63, Daniel J. Lunt et al. (January 2010) Earth system sensitivity inferred from Pliocene modelling and data, Nature Geoscience, Vol. 3
    Either way the climate sensitivity that people have been talking about underestimates the warming that we can expect because by definition it omits the slow feedbacks -- which aren't necessarily that slow (e.g., the reduction in plant efficiency over the past decade, the saturation of some ocean CO2 sinks, Boreal forests in Canada, rising levels of methane emissions due to permafrost melt in Arctic tundra and Arctic shallow water continental shelves, e.g., near the coastline of Siberia.

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