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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 101 to 125 out of 386:

  1. RW1 - To use a database like HITRAN, you set up your parameters (in whatever spectral software you are using, such as JavaHAWKS) for two different conditions, run it twice, and look at the differences between the outputs. The output of interest is the summed energy radiated from the atmosphere given a particular surface temperature and atmospheric mix. The difference between them (~3.6 W/m^2 for doubling CO2 with HITRAN data, 3.7 for more up to date models) is the difference in total radiated energy - outgoing energy. Not isotropic radiation from a particular level of the atmosphere, but the difference in total emissions. Changes in atmosphere modify the emissivity of the Earth, as per the Stefan–Boltzmann law; the amount of thermal radiation emitted at any particular temperature. And that leads to imbalances with incoming sunlight that result in climate changes as energy accumulates or leaves. It's as simple as that - what is the sum difference between radiated powers after an atmospheric change. That 3.6/3.7 Watts is the integrated difference in total radiation going out to space at a particular temperature - which is the very definition of "radiative imbalance".
  2. RW1 - you are asking for documentation of what is implicit in the equations. Lets see if I can attempt it. At heart of equations, you consider a small slice of atmosphere. It has radiation from in from below, (from surface and lower layers) and from above (from upper layers in atmosphere). The equations capture absorption, transmission, emission (in ALL directions - which of course is the inputs to layers above and below) for a given gas composition, P,T. The integral of all the layers is what then allows you to calculate what comes out of the top. All the interaction is captured. You know it correct because the model results agree with empirical measurement. Science of doom explains the textbook.
  3. scaddenp, I'm not finding the information and/or documentation I'm looking for to verify the claims made by you and KR. You are saying the 3.7 W/m^2 increase is not the reduction in the atmospheric window?
  4. Sigh. The Science of Doom takes you through text book. Is that documentation enough? The problem seems to be that you are looking for a statement that doesnt exist because it would make no sense. The way real physics is done is bears little relationship to way you are trying to approach it. We are trying desperately to show why that is. As far as I can see you either: a/ study the physics b/ see that since model matches measurement so model must be right. I am guess that are ignoring the textbook, SoD, papers, because they dont relate to George White's "logic" and you search in vain for an analogous treatment. However, this is the right way to do it. I'm beyond my power to help you further.
  5. My goodness, the answer is yes or no. You are claiming the 3.7 W/m^2 does NOT represent the reduction in the atmospheric window, right? (This is what I'm assuming you're saying). I do not find this information in the stuff you've referenced, and I've continued to search online to no avail. What we are talking about here represents a fairly simple thing. I sent an email to one of the links from the source you referenced to inquire: SpectralCalc.com Hopefully they will respond.
  6. Rahmstorf 2008, linked in the Advanced version of this post, gives 3.7 W/m^2 as an undisputed figure for CO2 forcing. Without any feedbacks, a doubling of CO2 (which amounts to a forcing of 3.7 W/m2) would result in 1°C global warming, which is easy to calculate and is undisputed. ... consensus holds that a doubling of CO2 causes a radiative forcing of 3.7 W/m2, which in equilibrium would cause 3°C±1.5°C of global warming.
  7. muon - the issue is does "reduction in the atmospheric window" mean the same thing to RW1 as I think it means. You can say yes, but I suspect that RW1 then has corollary from that shows a very different understanding.
  8. scaddenp - Yeah, I thought that was a peculiar phrase, which seems to add an unnecessary layer of complication. All I did was point to the link, as it seemed (in #105) that he couldn't find it. How he chooses to interpret this particular 3.7 W/m^2 is up to him, although both KR and you have made it very clear.
  9. I believe the "atmospheric window" issue is tied (again) to George White - he believes the window of IR going straight to space is >90 W/m^2, whereas Trenberth estimates 40 W/m^2, and asserts that all greenhouse gas effects operate by narrowing that window. He seems to neglect lapse rate and GHG concentration effects raising the altitude (and dropping the temperature) of emission, and in addition argues that the 90 W/m^2 represents a limiting band on GHG effects. RW1 - The models operate by calculating upwards and downwards emissions from all levels of the atmosphere, and the 3.7 W/m^2 represents all the effects: band broadening due to higher GHG concentrations, band deepening due to higher effective altitudes of emission to space, higher reemission to the ground, etc. So the answer to your question is partially, although not readily picked out of the other effects.
  10. The problem of getting your "physics" from George White instead of from a textbook. Is George untroubled by lack of match with empirical data?
  11. KR, My question isn't related to what the number for the window is. scaddenp, No, the problem is no one is answering my question.
  12. @ #111 "No, the problem is no one is answering my question. Or perhaps its just not the answer you want to hear.
  13. RW1 - "You are saying the 3.7 W/m^2 increase is not the reduction in the atmospheric window? " Hmm, but we have this rather odd expression about "reduction in the atmosphere window". What does this mean? KR identifies it with a GW idea. Can you phrase the question in a way that we can understand, and preferably makes physical sense?
  14. scaddenp, I'll break it down into a series of separate small questions: 1. Do you agree that some of the emitted surface infrared power passes through the atmosphere unabsorbed by GHGs or clouds? 2. Do agree that the remainder is absorbed by the atmosphere? 3. Does the 3.7 W/m^2 of 'radiative forcing' represent a reduction in the atmospheric window of 3.7 W/m^2? 4. Does the 3.7 W/m^2 of 'radiative forcing' represent an increase of 3.7 W/m^2 in the amount of infrared absorbed by the atmosphere. My understanding is your answer to 1 & 2 is YES and your answer to 3 & 4 is NO. Is this correct?
  15. RW1 - The answers to your four questions are "Yes, ~40 W/m^2", "Yes, although a fair amount of energy also goes into the atmosphere via convection and latent heat (~20%)", "Only partially", and "Almost, it's the amount prevented from leaving via various effects - more absorption and higher/colder emissions". Sorry, but these are obviously important questions for you, and I would be doing a disservice by giving un-nuanced answers.
  16. KR, The actual number for the atmospheric window is irrelevant to the particular question at hand here. Whether it's 40 W/m^2 or 90 W/m^2 - it doesn't matter, nor do I care. The estimated 3.7 W/m^2 from 2xCO2 either represents a reduction in the atmospheric window or not. The fact you seem to be side stepping this fundamental question is quite revealing. It's a ridiculously simple and straightforward question with a simple yes or no answer. I can see no one here is interested in getting to bottom of this, so it appears like I'll have to do some more searching around and figure out for myself.
  17. RW1 - "The estimated 3.7 W/m^2 from 2xCO2 either represents a reduction in the atmospheric window or not." Wrong. It's partially a reduction in the "window", and partially a reduction, a drop in the intensity, in the GHG bands - the ones already inhibited by the presence of greenhouse gases. Not yes or now, but "in part". As GHG concentration rises, the effective emission altitude goes higher and higher in the troposphere, and hence (due to the lapse rate) comes out of colder and colder GHG's. They emit less than warmer lower GHG's - the additional altitude means that the repeated reduction in IR transmission as part gets emitted up (to higher levels) and parts down attenuate the IR levels. That and widening bands, the reduction of the window, combine to provide the 3.7 W/m^2 effect from doubling CO2. That is why I gave a nuanced answer, one that actually answered your question without conveying incorrect information. It's not A or B - it's both.
  18. KR, Let's take this one question at a time. What does the atmospheric window represent? Please define it for me.
  19. KR (RE: 117), Then the answer is no. What's so hard to understand here? I'm trying to find specifically where the disagreement lies. This is pretty basic stuff.
  20. RW1 - actually that's what I dont understand. What do you mean by "atmosphere window"? A clearer understanding of that might illuminate this.
  21. By 'atmospheric window', I'm referring to the amount of the emitted surface power that passes through the atmosphere completely unabsorbed by GHGs or clouds.
  22. RW1 - The answer is not "no", it is "in part". I've (repeatedly) clearly answered your question - narrowing of the atmospheric window is part of the 3.7 W/m^2, and deepening of the intercepted bands due to higher effective emission altitude is also part of the 3.7 W/m^2. It's not an either/or question!
  23. RW1: By 'atmospheric window', I'm referring to the amount of the emitted surface power that passes through the atmosphere completely unabsorbed by GHGs or clouds. How do you distinguish what is surface emitted from other emitted sources. And why do you use the word "power"? What is power?
  24. RW1: 1) When physicists refer to "the atmospheric window" they refer to a portion of the spectrum in which radiation is not absorbed, so radiation can pass through that "window" without appreciable loss or distortion. The atmosphere has several windows - one at the frequencies of visible light, another in the IR spectrum, and still others in the radio spectrum. 2) One of the atmospheric windows in the IR spectrum is in that range of frequencies where the majority of the surfaces IR radiation is emited. As a result, about 40 w/m^2 of IR radiation escapes to space without being absorbed by any atmospheric components (except clouds, if present). 3) Increasing CO2, O3 or H2O content into the atmosphere, or introducing novel GHG can narrow this window slightly, but the effect is very small. 4) Outside of the atmospheric window, IR radiation from the Earth's surface is entirely absorbed by GHGs; but 5) Those GHGs then emit radiation at the same frequency at an intensity that depends on their temperature. The IR radiation emitted towards space by GHGs is then absorbed by higher GHGs, which in turn emit radiation at an intensity depending on their temperature, which is in turn absorbed and so on until the atmosphere is thin enough for the upward emitted radiation to escape to space. 6) Because the radiation outside the atmospheric window that escapes to space is emitted high in the atmosphere, it is emitted by gases that are much cooler than the surface. Therefore, that radiation has a much lower intensity, ie, transmits much less energy than the radiation emitted from the surface at the same frequency. The difference between the energy that is radiated to space outside of the atmospheric window and the energy originally radiated from the surface at those same frequencies is the fundamental basis of the green house effect. 7) If you increase the concentration of a GHG, then the altitude at which radiation from that GHG will effectively escape to space will increase. Because the altitude has increased, the temperature of the radiating gas is lower, so the total energy radiated is also lower. 8) If you double the CO2 concentration, the atmospheric window will narrow slightly as the absorption band of CO2 widens. This does not mean no IR radiation will escape in the frequencies where the absorption band widens - it just means that the IR radiation in those frequencies will come from a higher, ergo cooler, ergo less energetic altitude, reducing the total IR energy escaping in that frequency band by about a third. 9) At the same time, IR in the frequencies of the previously existing absorption band will come from slightly higher in the atmosphere, and therefore carry less energy (because the emitting CO2 is colder). 10) The combination of these two effects will reduce the total energy leaving the atmosphere by 3.7 w/m^2 That is the full and complete answer to your questions (given space limits). It has been given to you ad nauseum above but you refuse to hear the answer because it is not framed according to the frankly fallacious model of the Green House effect used by George White. However, we cannot ignore the physics and give you answers that only make sense if framed in terms of George White's fallacious physics. If you try frame your question in terms of the actual physics, however, you will find you have already been answered repeatedly.
  25. " And why do you use the word "power"? What is power?". It's the surest sign that you are dealing with someone who has got their education from George White. This incorrect usage has been pointed out to RW1 before.

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