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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...

Select a level... Basic Intermediate Advanced

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

Printable Version  |  Offline PDF Version  |  Link to this page

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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:

Comments

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Comments 276 to 300 out of 386:

  1. GHG induced warming results in the warming of the atmosphere, does it not? Warming air expands and in doing so does work against its surroundings, which requires some of the internal energy to be expended, leaving less available to heat the atmosphere (and ultimately the surface).
    RW1, an understanding of thermodynamics is not really something that you have under your belt, is it? I'm sure others will pick your statements to pieces, but do yourself a favour and in preparation learn about adiabatic processes. Seriously.
  2. RW1's original claim was that the forcing from Greenhouse Gases was three times that from solar. In defense of that claim he refers us to the ratio of incident solar radiation to upward long wave surface radiation to surface absorbed solar radiation, or 2.46 (396/161, see diagram below). With hesitance I say that RW1 should have used the ratio of LW surface radiation to total absorbed solar radiation, or 1.66 (396/239). I say "with hesitance" because his entire formulation is incorrect. For a start, 239 W/m^2 is not the solar "forcing". A "forcing" is the change in a value between two different, specified times. By convention, the reference time is 1750, notionally the pre-industrial era. Further, 396 W/m^2 is not the "greenhouse gas forcing" In fact, the difference between the upward LW surface radiation and the upward LW radiation at the top of the atmosphere (TOA) is the total greenhouse effect, but even that is not the total greenhouse forcing both because a forcing is a change between two times, and because it includes feedbacks as well as forcings. (For what it is worth, the ratio of the total greenhouse effect to total insolation is 157/239, or approximately 66%. What is more, the insolation contributes approximately 80% of mean global temperature, with the greenhouse effect contributing the majority of the extra warming and redistribution of heat, which equalizes temperatures contributing the rest.) Ignoring the terminological issues, which render RW1's claim almost incoherent, the simple fact is that the total greenhouse effect acts as a multiplier of energy from the sun. If we were in the dark of space, no amount of greenhouse gases would raise our temperature appreciably above the 2.5 K temperature of the cosmic background radiation. Therefore if insolation increased, then the total greenhouse effect also increases in proportion. For small increases in insolation, the ratio of effective insolation (incoming sunlight minus albedo) to upward LW surface radiation would remain constant. And therefore the increase in temperature from an increase in insolation of 1 W/m^2 would be approximately the same as the increase from a 1 W/m^2 forcing from CO2. Turning to RW1's second point, warming a gas does result in expansion, which does perform work. But a GHG forcing warms the troposphere but cools the stratosphere. In contrast an increased solar forcing warms both troposphere and stratosphere. Because the solar forcing is warming more gas (by a small percentage), if RW1's argument had any merit, it would indicate that solar forcing was weaker than GHG forcing. In fact, however, it is without merit. It does indicate that solar forcing must use more energy for a given increase in temperature, all else being equal. But energy leaving the system is not a function of how much energy is stored in the system, but of surface and atmospheric temperatures. Because of this, the increase in temperature is the only factor in determining if equilibrium has been restored, and the equilibrium temperature for equal solar and GHG forcings is approximately the same. I write this solely for the benefit of interested readers who may be confused by RW1's ramblings. He himself has a long demonstrated inability to learn or apply even the most basic of the relevant concepts, so I doubt he will gain any benefit from it. For the same reason I am unlikely to respond to any response he makes to my post. Again, he has a long history of simply regurgitating his initial confusion in slightly different words and imagining that thereby he is "debating". At the moment I do not have the time to waste pandering to his misconception.
  3. Tom Curtis, Let me clarify what I'm trying to say: Designating the +1.1C as the 'zero-feedback' starting point from the so-called 'Planck response' (i.e. the effective emissivity) is not valid because it arbitrarily separates the physical processes and feedbacks in the system that will act on additional forcings, like from GHGs, from those that currently act to maintain and control the system from the forcing of the Sun, for which there is no physical or logical basis. Put another way, one can't derive the 'zero-feedback' starting point from the absolute surface response to solar forcing, which itself is the net result of and maintained by all the physical processes and feedbacks in the system, and then claim there is some nebulous feedback acting on top of this that will amplify 'forcings' or imbalances even further, let alone 3-6x times greater. The 'brakes' - if you will, have already been put on all the feedbacks in the system from the many years and years of forcing from the Sun, including especially water vapor and clouds, as the two are the most dynamic components of the whole atmosphere. If you think they have not been put on (the brakes), why did the net surface energy flux from the forcing of the Sun 'stop' at only 390 W/m^2? Why didn't the feedbacks in the system, including especially water vapor and clouds, ultimately manifest themselves to an 'effective' emissivity of 0.22 (3.7/16.6 = 0.22), where a net surface energy flux of 1077 W/m^2 (about 100C!) has 837 W/m^2 'blocked' by the atmosphere and re-circulated back to the surface (240/1077 = 0.22)? In short, the absolute solar amplification factor of about 1.6 (390/240 = 1.625) is already giving a measure of incremental sensitivity to additional forcings or imbalances, only it represents an upper bound on sensitivity because net negative feedback on imbalances (a net response less than 1.6) is required for basic stability and maintenance of the current energy balance from the forcing of the Sun. If the logic is still not clear, here it is broken down into a series of separate questions: Do you agree that at the Earth's current global average temperature of 288K, the Earth emits about 390 W/m^2 from its surface (assuming an emissivity of 1 or very close to 1)? Do you agree that the globally averaged solar constant is about 342 W/m^2 and the average albedo is about 0.3, resulting in a net incident solar power of about 240 W/m^2? Do you agree that the 240 W/m^2 of incident post albedo solar power is forcing the climate system? Do you agree that the 240 W/m^2 forcing the system from the Sun results in an amplification at the surface of about 390 W/m^2 entering the surface from the atmosphere to sustain 288K? Do you agree that this accounts for all the physical processes and feedbacks in the system? If not, why haven't all the physical processes and feedbacks fully manifested themselves after billions of years of forcing from the Sun? Or even after the last few hundreds or thousands of years of forcing from the Sun? Do you agree that in order to amplify +3.7 W/m^2 of 'forcing' from 2xCO2 into +3C at the surface it requires +16.6 W/m^2 entering the surface from the atmosphere (288K = 390 W/m^2; 291K or +3C = 406.6 W/m^2 and 406.6 - 390 = 16.6 W/m^2)? Do you agree that watts of GHG 'forcing' and watts of solar forcing must obey the same physics in the system? That is a watt is a watt, independent of where it last originates from. Do you agree that a watt of post albedo solar forcing and watt of GHG 'forcing' can only do the same amount of work? Do you agree that 390/240 = 1.625? Do you agree that 16.6/3.7 = 4.5? Do you agree that 4.5 is 2.8x times greater than 1.625? If watts are watts, how can watts of GHG 'forcing' have a 3x greater ability to warm the surface than watts forcing the system from the Sun?
  4. Tom Curtis, Regarding my other point. Unlike additional solar forcing, with additional GHG 'forcing' there is no increased energy coming into the system, leaving only the existing internal energy available. Since GHG warming requires the troposphere to warm and the pressure is higher in the troposphere than it is in the stratosphere, I stand by my claim that if anything a watt of additional GHG 'forcing' would be a little less than a watt of solar in its ability to warm the atmosphere (and ulimately the surface).
  5. I meant to say: "...I stand by my claim that if anything a watt of additional GHG 'forcing' would be a little less than a watt of additional solar forcing in its ability to warm the atmosphere (and ulimately the surface).
  6. RW1:
    "Put another way, one can't derive the 'zero-feedback' starting point from the absolute surface response to solar forcing, ..."
    You are correct, but nobody does derive the zero feedback response to forcing from the absolute surface response to insolation. There is no point in further discussing your straw man.
    "In short, the absolute solar amplification factor of about 1.6 (390/240 = 1.625) is already giving a measure of incremental sensitivity to additional forcings or imbalances, only it represents an upper bound on sensitivity because net negative feedback on imbalances (a net response less than 1.6) is required for basic stability and maintenance of the current energy balance from the forcing of the Sun."
    1) It does not represent an upper bound. To assume that it does you must assume that the albedo of a sunless Earth would be identical to the albedo of the Earth as it currently exists. That assumption is, however, simply absurd. A sunless Earth would have an albedo of 0.7 (0.3 for treeless land areas, and 0.9 for frozen oceans) or higher. Consequently an approximate measure of the "solar amplification factor", if it is intended to reflect all feedbacks, is 2.05. What is more, your argument assumes that the "solar amplification factor" is constant over the range of temperatures that might be experienced, which is known to be false. It also assumes it is constant with regard to continental configurations (also known to be false). 2) Your whole presentation is nonsense. Let's define some terms: EI = Effective Insolation = Top of atmosphere insolation * (1-albedo); SR = Upward Long wave surface radiation OLR = Outgoing Longwave Radiation TGHE = total greenhouse effect = SR-OLR Given these definitions, we can define the solar amplification factor (SAF): SAF = (EI+TGHE)/EI Thus defined we see that your insistence that the Solar Amplification Factor remains constant under the greenhouse effect is just the insistence that (EI+TGHE)/EI = k, where k is a constant. That can only be true where TGHE = EI(k-1). So, your insistence that the ratio be constant is simply an assertion by fiat that the atmospheric greenhouse effect is completely independent of the concentrations of all greenhouse gases in the atmosphere. Put simply, your theory can only be correct if a pure nitrogen atmosphere has the same greenhouse effect as a pure CO2 atmosphere. And you want to assert this claim as a definition from which we are supposed to start reasoning. Well, you may be able to con some people, but I recognize the difference between science and utter nonsense, and it is the later that you are peddling.
  7. RW1 @279, increasing the GHG concentration will reduce the OLR radiation until equilibrium is reached. You assert that because it does not increase the incoming solar radiation, it cannot cause in increase in energy stored in the Earth's surface and atmosphere. This is logically equivalent to asserting that if you have a basin of water, being filled by a tap, and drained through a drain, that you cannot increase the water level by reducing the water flow out of the drain because doing so does nto increase the water flow from the tap. No more need be said.
  8. Tom Curtis, "You are correct, but nobody does derive the zero feedback response to forcing from the absolute surface response to insolation. There is no point in further discussing your straw man." 390/240 = 1.625; 3.7 W/m^2 x 1.625 = 6.0 W/m^2 = 1.1C from S-B. "What is more, your argument assumes that the "solar amplification factor" is constant over the range of temperatures that might be experienced, which is known to be false." Actually, no. The amplification factor is not constant and is indeed non-linear; however, each incremental watt causes proportionally less and less warming in the system, which is the opposite of what would be consistent with the incremental response being greater than the current absolute response (i.e. greater than about 1.6) "So, your insistence that the ratio be constant is simply an assertion by fiat that the atmospheric greenhouse effect is completely independent of the concentrations of all greenhouse gases in the atmosphere." I never said or implied anything of the sort. The ratio is not constant. If, from GHG 'forcing', the surface temperature were to rise by 1.1C the absolute surface response to solar forcing would increase. The new ratio would be 1.65 (396/240 = 1.65).
  9. Tom Curtis, "You assert that because it does not increase the incoming solar radiation, it cannot cause in increase in energy stored in the Earth's surface and atmosphere." I don't assert this at all. What I'm saying is additional GHG 'forcing' does not increase the total energy input into the system as additional post albedo solar forcing would. This does not mean that GHG 'forcing' cannot increase the total energy stored in the Earth's surface and atmosphere, as of course it can. My original point was that, if anything, a watt of GHG 'forcing' would be a little less than solar because some of the existing internal energy would have to be expended for the expansion of warming air against its surroundings.
  10. Tom, RW1 refuses to look at this as anything other than a linear system equivalent to electronic circuits. As long as he is in that trap you cannot help him out of it. He has tied himself in knots with his personal model of the system, and there's no way out of it, because he won't abandon his (grossly flawed) model, nor will he expand it to properly reflect the system being modeled. He'll argue in a thousand circles before he recognizes that he is wildly wrong.
  11. RW1,
    Do you agree that 390/240 = 1.625?
    Yes.
    Do you agree that 16.6/3.7 = 4.5?
    Yes, but utterly irrelvant.
    Do you agree that 4.5 is 2.8x times greater than 1.625?
    Yes, but utterly irrelevant.
    If watts are watts, how can watts of GHG 'forcing' have a 3x greater ability to warm the surface than watts forcing the system from the Sun?
    Because the ratio is a meaningless number and the question as phrased is a nonsensical question. Put another way... if the sun increased its output by 3.7 W/m2, the earth would warm by roughly the same amount. An additional 3.7 W/m2 from any source would result in 16.6 W/m2 at the surface. The reason has to do with this thing called feedbacks, and the fact that the system does not consist solely of a big flaming ball (the sun) and a little floating ball (the earth). It's more complicated than that, and you would be far, far better served studying the other issues than arguing, for the fifty millionth time, this same, old, tired point. Have you not yet figured out that no one on the entire planet agrees with you, or cares about your particular insight in this? Do you think you are Galileo? Or perhaps merely confused and lost? Take your pick, but either way, you're wasting everyone's time with the same, old, complete nonsense. It's time for you to get over yourself and go learn something.
  12. Hi! It is still very hard to grasp that incoming direct TSI can be modified by "earth climate" to such a high level that it in fact marginalizes the source's direct influence. A problem many sceptics seem to have, too. Coming from the discussion of the alternative TSI reconstruction from Shapiro e.a. http://www.skepticalscience.com/shapiro-solar-2011.html#comments , which is counter-argued in that linked article because his observations would imply a very low climate sensitivity when compared to the reconstructed temperature curves from Ljungqvist. I read here with special interest about Hansen 2008 and his long term comparisions from earth history. Hansen 2008 does not bring forth any TSI data, but from his footnotes you get the impression there is almost no significant shift in TSI levels throughout earth history. However, he also states "The possibility remains of solar variability on longer time scales.", which he debunks by pointing out the TSI development of the last decades (last page); which I find not entirly coherent. Is there any data on TSI levels throughout earth history? (For example, for the last 450k years from Figure 2 of this article? I never really understood what part of the 6 Degrees difference from the ice cores is attributable to GHG and what part to TSI. Sure is only that TSI was the driver of the shifts from warm to cold and vice versa. So, what level of TSI difference started and ended the shifts we observe from the ice cores?)
  13. Falkenherz wrote: "So, what level of TSI difference started and ended the shifts we observe from the ice cores?" Rather than explain to you again why the glacial / interglacial cycle is not caused by changes in TSI I'll just point you to the previous time I explained it.
  14. "it is still very hard to grasp that incoming direct TSI can be modified by "earth climate" to such a high level that it in fact marginalizes the source's direct influence" How about considering how different the climate of the earth would be (ice ball) without any GHG then? If you dont think that theory is believable then consider that you deduce surface temperature of any rotating planet anywhere given TSI, albedo, aerosol and ... atmospheric composition (ie GHG).
  15. Falkenherz, if you are referring tp the galcial/interglacial cycles, the evidence points to changes not in TSI but in its distribution over the surface. That itself is an argument for high sensitivity to radiative forcings in general.
  16. CBDunkerson, thanks for correcting me again. But my question is still open: So, are there really no significant changes in TSI throughout the last 450k years? Philippe, I take it I then have to talk about insolation instead of TSI? So, let me rephrase: What was the difference in insolation or whatever W/m2, in order to trigger the shifts during the last 450k years?
  17. Falkenherz, Tamino has a good explanation with the maths, Wobbles part1 and part2, on the WB machine: http://web.archive.org/web/20080501124634/tamino.wordpress.com/2007/11/19/wobbles-part-1/ http://web.archive.org/web/20080419120634/http://tamino.wordpress.com/2007/12/02/wobbles-part-2/ Wiki has the skinny on Milankovitch, I'm surprised you seem to be not yet familiar with that: http://en.wikipedia.org/wiki/Milankovitch_cycles Berger and Loutre have published quite a bit on the subject, check them out. The litterature is out there.
  18. Falkenherz, it depends on what you mean by 'significant'. Current TSI (sometimes still called 'the Solar constant' even though we now know it isn't actually constant) is about 1361 W/m^2. The Maunder Minimum ~1700 was less than 1 W/m^2 lower. Thus, the most profound swing in TSI of the past several thousand years was a change of less than 0.1%. The difference from peak to valley of the ~11 year cycles is also about 0.1%, but obviously maintained over a shorter period. Over longer time scales TSI is increasing by about 0.1% per ~140,000 years as the Sun grows older and hotter. Yet, these 'tiny' changes in TSI have noticeable effects on the Earth's climate due to feedback sensitivity. The fact that current greenhouse gas forcings are already larger than any solar variation of the past few hundred thousand years should thus be of some concern.
  19. Philippe, I ignored Milankovitch cycles (MC) because they are uncontested and I assumed they would have a certain known impact on global temperature, thereby initiating ice ages. Reading through the links you provided, this assumtion is wrong. If I understood correctly, nobody really seems to know the physics of the trigger for global temperature changes, only that MC must be a trigger, and it is assumed that glacial changes on the landmass-rich northern hemisphere play a key role. In other words, there is no initial rise of global temperature as the initial trigger, but rather some severe local imbalances. This just in short, because there is that other article specifically on MC. My research here is about climate sensitiviy, and specifically why consenus seems to be that it is high. So right now I am puzzled why people assume a high climate sensitivity if we don't know the physical trigger process. After all, if I understood correctly, it seems like local insolation can peak at 600 W/m2, which would probably be a very strong trigger with only a low sensitivity required. I am unsure where to continue discussion. Maybe I best move on to the MC article. (I start feeling like a hyperlink nomad and comment-parasite. Do you guys maybe have a forum?) CBDunkerson, thanks for confirming no real strong TSI changes connected to the ice age cycles.
  20. @ Falkenherz
    "I ignored Milankovitch cycles (MC) because they are uncontested"
    Perhaps not by you (at this moment) nor by most scientists (some do) but there are those who deny this, daily.
    "nobody really seems to know the physics of the trigger for global temperature changes"
    You project here. Try reading this post (including the comments threads, which should be mandatory).
    "My research here is about climate sensitiviy, and specifically why consenus seems to be that it is high."
    Um, "consensus" is that climate sensitivity is bewteen 1.9 (or so) and 4.2 (or so) with a central estimate of 3.0 being strongest. That you characterize that as "high" speaks volumes.
    " if we don't know the physical trigger process"
    More projection, again. Suggestion: more research & reading (by you), less trying to shoehorn reality into the worldview you have chalked out for it.
  21. "we don't know the physical trigger process" How on earth do you deduce that? The physical trigger is change in insolation distribution in the northern hemisphere, which in a low CO2 atmosphere sets up a web feedbacks on albedo and GHG. Untangling this web quantitatively has been a slow process. Have a look at fig 6 of Hansen and Sato 2011 and tell me again that this isnt understood.
  22. Daniel, I do question about reality, and if reality is reality, I am confident that my questions will be answered. scaddenp, Hansen and Sato 2011 are a very difficult to read for me. If I understand their chapter 5 on the Holocene correctly, they use climate forcings calculated from GHG and sea level changes (=ice sheet approximisations) and apply climate sensitivity "consensus" values and thusly produce temperature curves which match the ice core data. Seeing that there is yet another article which again specializes within more details with regards to my questions on the ice core data, I will move on to the discussion of Shakun e.a., link provided by Daniel#295.
  23. Guys, can someone tell me if the Knutti & Hegerl graph is free to be reproduced in Wikipedia? Is it already in the Commons? The Nature Geoscience page says "all rights reserved"...
  24. Dear Moderator, I am stuck. Apparently, the comment function on the article "Shakun e.a." seems to be bugged. It took several days before my questions actually appeared. Now, sometimes I can see my questions I posted there, sometimes not. I suspect that some answers to my questions might not have gotten through or are stuck. Could you pls check? (http://www.skepticalscience.com/news.php?p=4&t=151&&n=1391)
  25. I'm sure Ari will be loading this one into the next weekly installment, but Trenberth and Fasullo have apparently constrained sensitivity even further -- or at least made a major advancement.

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