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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 176 to 200 out of 201:

  1. RW1 - Why is it that you cannot accept that it's two effects adding up to the total IR reduction?
  2. "Why is it that you cannot accept that it's two effects adding up to the total IR reduction?" Simple. I haven't seen the evidence showing/prooving it, nor do I understand what a "deepening in the GHG emission bands" actually means.
  3. I can't find the manual online. I emailed Ontar to see if they can provide it.
  4. Tom, What use would the 'average transmittance' number be if it were NOT how I interpreted it? Maybe you're correct, but then the information is totally useless as far as I can tell.
  5. Tom, "Which is more likely, that a program developed by the air force for research and which has been used in various incarnations since 1988 with good correspondence to observational results has an error that produces up to 20% errors in its output? Or that you are simply mistaken in your interpretation of average?" Not the program itself, but the web interface integration of the program. Where do you see the 2.764 W/m^2 in the line by line output, either directly or indirectly? I don't see it in there.
  6. Tom, Nor do I see the 252.801 W/m^2 or 255.565 W/m^2.
  7. Tom, Is it just a coincidence that using the average transmittance numbers and dividing the surface difference by 2 yielded nearly exactly 240 W/m^2 leaving (for 255K)? Maybe, but it certainly warrants further investigation.
  8. RW1, I cannot keep up with your stream of consciousness posting, and nor will I try to. Sit down, think it out, and work our your objections and problems after a little thought. As regards the "average transmittance", I have seen a comparison of the Modtran model at David Acher's site with the data from a satellite over Barrow Island, and the match is pretty good. Close enogh that you can't tell the difference by eyeball, although if you overlaid them I'm sure some differences would jump out. The "error" you have found is too large for that to be plausible. If you have a further problem with it, work the problem out line by line as I suggest. If you are correct, there will be no difference in the reult. If I am correct, you initial calculation will be shown to be in error. As regards the window, the figure above @ 171 shows the radiance for 325 ppm CO2 overlaid on the radiance for 750 ppm CO2 on the right. As you can see, the main difference is on the wings of the trough, where a slight step pattern is deeper with 750 ppm than with 325 ppm. That is not a change in the atmospheric window, because IR radiation at those frequencies were already absorbed, possibly completely absorbed as far as radiation from the surface is concerned. If you look even closer, (closer than the resolution will allow, unfortunately), you will also see that the center of the trough is slightly deeper. You will also see the walls of the trough at the top are slightly wider (which is a reduction of the window). You will also see some of the secondary troughs generated by CO2 are deeper. There is one just to the right of the main CO2 trough where a single spike shows up in the center. Here for comparison is a modtran graph for 10,000 ppm CO2 with not H2O or O3: You will notice that that small spike noted above has become a deep trough that overlaps with the first trough, with a resulting large reduction in the atmospheric window. But that widening took place step by step, and in each step, it was always the smallest effect.
  9. Tom, "As regards the window, the figure above @ 171 shows the radiance for 325 ppm CO2 overlaid on the radiance for 750 ppm CO2 on the right. As you can see, the main difference is on the wings of the trough, where a slight step pattern is deeper with 750 ppm than with 325 ppm. That is not a change in the atmospheric window, because IR radiation at those frequencies were already absorbed, possibly completely absorbed as far as radiation from the surface is concerned. If you look even closer, (closer than the resolution will allow, unfortunately), you will also see that the center of the trough is slightly deeper. You will also see the walls of the trough at the top are slightly wider (which is a reduction of the window). You will also see some of the secondary troughs generated by CO2 are deeper. There is one just to the right of the main CO2 trough where a single spike shows up in the center." I know all of this. The total atmospheric window is simply the quantity of the whole spectrum of surface emitted infrared that passes through the atmosphere completely unabsorbed and goes straight out to space. Visually, there is a slight widening of the CO2 absorbing bands at 750ppm, which narrows the more overtly visual part of the 'window', but it's a specific quantity - not just a visual reduction. The outputs of these programs are detailed numbers, specifically the transmittance - not just what can be seen overtly in visuals of a graph. The decrease in transmittance directly tells us how much more outgoing surface power, across the entire emitted spectrum, is absorbed by the atmosphere.
  10. Tom, "The decrease in transmittance directly tells us how much more outgoing surface power, across the entire emitted spectrum, is absorbed by the atmosphere." If 'transmittance' does NOT tell us this, then what does it tell us?
  11. RW1 @184, evidently you do not know all this. You may think you do, but that is another question entirely. Consider the "atmospheric window". An "atmospheric window" is any part of the spectrum where transmittance is sufficiently high that you can place a telescope on a mountain, and observe the stars effectively at that frequency. Alternatively, it is a part of the spectrum where the transmittance is sufficiently high that you can use it as a channel for communication to space. If you want to see the atmospheric window in the IR spectrum, you should look at the back radiation at the surface: Clearly, if you looked up at 680 cm^-1 wave number, all you would see is the thermal radiation of the atmosphere. You would not even be able to see the sun in that portion of the spectrum, from the surface. In contrast, the intervals between 810 and 950 probably have sufficiently high transmittance to be useful for telescopes (and sidewinders). That is a atmospheric window. There is another, smaller one on the other side of the O3 trough. With that in mind, closing the window means reducing the IR radiation from the surface that escapes to space, particularly in those parts of the spectrum; and it is a minor effect. Deepening of the CO2 emission band means that in that part of the spectrum outside of the atmospheric window, the amount of IR from the atmosphere itself is reduced because it comes from a higher altitude and hence has a colder temperature. These effects are not strictly independent. For example, at the right edge of the CO2 trough, there are transmittances that rise from around 0.2 to 0.8 over a 60 wave number interval. Over this interval, both trough deepening,and narrowing of the atmospheric window occur with rising CO2. On the equivalent left side, however, transmittances peak around 0.2 because of the overlapping effects of H2O. The trough deepening, however, contributes almost as much to the reduced OLR as does the equivalent on the other side. Speaking of which, on the graph shown, total area under the line equals the total power (watt's per square meter) radiated to space, so the difference in area is the difference in radiated power. As you can see, the most significant part of this comes from the deepening of the trough on the wings, and that is approximately equal on both sides, even where transmittances are very low. Therefore it is clearly not a narrowing of the atmospheric window.
  12. RW1 @185, sorry, I don't recognize the quote. Could you please cite the source and link to it if on the web. If not on the web, could you please embed the quote in a wider context.
  13. Tom, RW1 185 quotes himself. No other source for it. http://www.google.com/search?q=%2Bdecrease+%2Btransmittance+"outgoing+surface+power"+"emitted+spectrum"+"absorbed+by+the+atmosphere"
  14. > total atmospheric window is simply the quantity No, it's not a quantity. It's a term defined in various ways in papers published in science journals. It's never defined as a quantity.
  15. Tom, When I use the term "atmospheric window" I mean the total transmittance - the specific amount of emitted surface power that passes through the atmosphere unabsorbed and goes straight out to space. If that is not the technical definition, then I stand corrected, but that's what I mean when I use the term.
  16. Are you claiming that the 3.7 W/m^2 of additional absorption from 2xCO2 does NOT represent a 3.7 W/m^2 reduction in transmittance? What I don't think you understand is that unless the specific wavelengths are saturated, some of the outgoing surface power still passes through them unabsorbed, and this amount is included in the transmittance. Increasing the concentration of CO2, for example, will reducing the amount that passes through at wavelengths NOT already saturated (i.e. widening the band or deepening the trough). The effect of more CO2 at saturated wavelengths will just reduce the height from the surface where 100% absorption occurs.
  17. RW1 @190 &191, given that at many frequencies, the atmosphere has an optical thickness greater than 1 (ie, transmittance is 0 for less than the full thickness of the atmosphere) than much of the IR absorbed by that region of the atmosphere that actually radiates to space does not come from the surface, but only from lower regions of the atmosphere. So an increase of absorption by 3.7 w/m^2 may have absolutely no effect on transmittance, or the atmospheric window (as you have defined it). Further, as much of the heat in the atmosphere is carried there by evaporation or transpiration, there is not even a necessary correlation between surface radiation and the thermal radiation of the lower levels of the atmosphere. Certainly that correlation is broken over antarctica in the winter, and may be broken at other places periodically as well. That is why Line By Line models use temperature profiles in developing their predictions, either a simple lapse rate (as in Modtran) or measured (or modelled) values in more sophisticated programs. Worse for your interpretation, a decrease in transmittance will automatically mean that a higher proportion of radiation from lower in the atmosphere is absorbed higher in the atmosphere, even with opticat thicknesss less than 1, but greater than 0. Because the higher gas is cooler (in the troposphere) it will radiate less energy, thus reducing the total IR radiation leaving the planet. That means a change in transmittance has more effect than simply reducing surface radiation to space. The only way to properly calculate its effect is, as the LBL models do, calulate its effect on each layer of a large number of layers of the atmosphere (in modtran's case, 33). The LBL models take account of radiation flows in both directions. That is, for each layer, they determine its emission at each individual wavenumber (or wavenumber couplet for modtran), based on its temperature. They then apply that radiation as both upward and downwelling radiation. For each layer, they also take the total incoming radiation (upward and downward), multiply by the transmittance for that layer, and apply the result as upwelling or down welling radiation from that layer as appropriate. Here is a diagram illustrating the process from Science of Doom: Although this only indicates transmittance in one direction, be assured it is calculated in both. In the thread from which this comes SoD is developing a simple radiation model, and you can see in the code that he makes the calculations first for upwelling, and then for downwelling radiation. (By the way, this illustration also appears in SoD's thread on theory and experiment in atmospheric radiation, from which I got the diagrams which showed the close correlation between the model predictions and observations. That thread has already been linked here. So your claim that all you have received is statements, not evidence, is nonsense.) Because the transfers in radiation are calculated for each wavenumber, and for each level independently, there is no single calculation that corresponds to what you are seeking, ie, a level in which all incoming radiation is from the surface, and all upwelling radiation goes to space. But that does not mean that both the upwelling and downwelling emittance from each level is ignored, or that the absorption at any level is ignored which is what is required for George White's adjustment to make any sense. Of course, in the LBL models, the total upwelling radiation of the highest level (emitted and transmitted) is just the Outgoing Long-wave Radiation. And the difference between that for 375 ppm and for 750 ppm is the increase of the greenhouse effect for doubling of CO2. So, if you want to verify Modtran, and all the other LBL models programed by different teams around the world, and all the energy balance models also programed by different teams around the world, which all come up with essentially the same result; which just happens to match observations almost exactly, you either need to accept the observational match as confirming the models, or you need to go through the models line by line. There is no other short cut.
  18. "given that at many frequencies, the atmosphere has an optical thickness greater than 1 (ie, transmittance is 0 for less than the full thickness of the atmosphere) than much of the IR absorbed by that region of the atmosphere that actually radiates to space does not come from the surface, but only from lower regions of the atmosphere. So an increase of absorption by 3.7 w/m^2 may have absolutely no effect on transmittance, or the atmospheric window (as you have defined it)." And where does the radiation from the lower regions of the atmosphere come from? So this is what you're claiming? That the 3.7 W/m^2 does NOT represent a reduction in total transmittance, as I have defined it? I just want to be clear. "Further, as much of the heat in the atmosphere is carried there by evaporation or transpiration, there is not even a necessary correlation between surface radiation and the thermal radiation of the lower levels of the atmosphere." Define what you mean by "correlation". I understand that a good amount of the heat in the atmosphere is carried there by evaporation and transpiration, but those amounts are in addition to emitted surface power and are non-radiative, which means they have to be returned to the surface in equal and opposite amounts, because all the infrared energy leaving at the top of the atmosphere is radiative. It's true that some of the kinetic energy moved into the atmosphere from the surface by evaporation and transpiration can radiate some energy into the atmosphere, but again it has to be offset by the surface radiation in equal and opposite amounts. If some of the surface originating kinetic energy is radiated into the atmosphere and that energy is ultimately radiated out to space, the amount of kinetic energy returned to the surface will be less, having a cooling effect on the surface, effectively reducing the emitted surface power by the opposite amount.
  19. RW1 @193: "And where does the radiation from the lower regions of the atmosphere come from?" Any substance with an emissivity greater will radiate energy with a total energy proportional to its emissivity times the fourth power of its temperature. That is where the radiation comes from, from the gases in the lower atmosphere which radiate in the IR spectrum and have non-zero temperatures (primarily water vapour and CO2). The heat that warms that gas comes evapo/transpiration from the surface, radiation from the surface, and atmospheric absorption of incoming solar radiation, although at any given layer, a large part of it will come from thermal radiation from adjacent layers, or convective heat transfer from adjacent layers. "So this is what you're claiming? That the 3.7 W/m^2 does NOT represent a reduction in total transmittance, as I have defined it? I just want to be clear." If your definition of total transmittance is "... the specific amount of emitted surface power that passes through the atmosphere unabsorbed and goes straight out to space", then no it is not. A small part is reduction of transmittance, but a more significant part is the reduction of thermal radiation from lower levels of the atmosphere to space, as per the diagram @171. "Define what you mean by correlation ..." The normal statistical sense. What I am pointing out is that because not all energy transfers are radiative, situations can arise in which the atmosphere returns more energy to the surface than it receives from the surface. This will only happen when there is a temperature inversion, as sometimes happens with low lying clouds. In Antarctica in the winter it can happen on a continental scale because Antarctica is receiving no insolation, and there is still an energy transfer from the Antarctic Ocean to the Antarctic interior carries by the atmosphere. However, when you say "If some of the surface originating kinetic energy is radiated into the atmosphere and that energy is ultimately radiated out to space, the amount of kinetic energy returned to the surface will be less, having a cooling effect on the surface, effectively reducing the emitted surface power by the opposite amount", you appear to be making an error. Specifically, when energy is transferred to the atmosphere, it makes no distinction in the source of that energy when it radiates. So, the sum total of the energy it receives is radiated away, and half of that energy must be downwelling, and half upwelling. And if the sum of Insolation plus back radiation is less than the sum of Surface radiation plus energy transfer by evapo/transpiration and (a small) energy transfer by by collisions between gas molecules and the surface, then the surface will indeed cool. You also may be not making a mistake, and I have simply misunderstood you. It is true that the presence of evapo/transpiration and convection, by making energy transfer more efficient, cool the surface compared to the temperature it would be if all energy transfers in the atmosphere were radiative (about 70 degrees C). So in that respect, the fact that evapo/transpiration carries energy into the atmosphere, a portion of which does eventually escape to space does mean the surface is cooler than it otherwise would have been. Having said that, I do not see the relevance to the basic point at issue - is it George White, or all the world's radiative transfer modelers who are correct in their interpretation of the output of radiative transfer models?
  20. "Worse for your interpretation, a decrease in transmittance will automatically mean that a higher proportion of radiation from lower in the atmosphere is absorbed higher in the atmosphere, even with opticat thicknesss less than 1, but greater than 0. Because the higher gas is cooler (in the troposphere) it will radiate less energy, thus reducing the total IR radiation leaving the planet. That means a change in transmittance has more effect than simply reducing surface radiation to space." How do you figure? If anything, it seems a decrease in transmittance will shorten the height from the surface where the atmospheric absorption occurs. "The only way to properly calculate its effect is, as the LBL models do, calulate its effect on each layer of a large number of layers of the atmosphere (in modtran's case, 33). The LBL models take account of radiation flows in both directions. That is, for each layer, they determine its emission at each individual wavenumber (or wavenumber couplet for modtran), based on its temperature. They then apply that radiation as both upward and downwelling radiation. For each layer, they also take the total incoming radiation (upward and downward), multiply by the transmittance for that layer, and apply the result as upwelling or down welling radiation from that layer as appropriate. Because the transfers in radiation are calculated for each wavenumber, and for each level independently, there is no single calculation that corresponds to what you are seeking, ie, a level in which all incoming radiation is from the surface, and all upwelling radiation goes to space. But that does not mean that both the upwelling and downwelling emittance from each level is ignored, or that the absorption at any level is ignored which is what is required for George White's adjustment to make any sense." Have you verified with GW that this is what he's claiming? Because that's not my interpretation of it. The model simulations he's using are multi-layered through the atmosphere - he's simply showing the aggregate effect through all the layers. Is it just another coincidence that he's getting an incremental absorption or reduction in transmittance of 3.7 W/m^2 for 2xCO@ from his HITRAN based simulations?
  21. "What I am pointing out is that because not all energy transfers are radiative, situations can arise in which the atmosphere returns more energy to the surface than it receives from the surface. This will only happen when there is a temperature inversion, as sometimes happens with low lying clouds. In Antarctica in the winter it can happen on a continental scale because Antarctica is receiving no insolation, and there is still an energy transfer from the Antarctic Ocean to the Antarctic interior carries by the atmosphere. However, when you say "If some of the surface originating kinetic energy is radiated into the atmosphere and that energy is ultimately radiated out to space, the amount of kinetic energy returned to the surface will be less, having a cooling effect on the surface, effectively reducing the emitted surface power by the opposite amount", you appear to be making an error. Specifically, when energy is transferred to the atmosphere, it makes no distinction in the source of that energy when it radiates. So, the sum total of the energy it receives is radiated away, and half of that energy must be downwelling, and half upwelling. And if the sum of Insolation plus back radiation is less than the sum of Surface radiation plus energy transfer by evapo/transpiration and (a small) energy transfer by by collisions between gas molecules and the surface, then the surface will indeed cool. You also may be not making a mistake, and I have simply misunderstood you. It is true that the presence of evapo/transpiration and convection, by making energy transfer more efficient, cool the surface compared to the temperature it would be if all energy transfers in the atmosphere were radiative (about 70 degrees C). So in that respect, the fact that evapo/transpiration carries energy into the atmosphere, a portion of which does eventually escape to space does mean the surface is cooler than it otherwise would have been." Tom, All I'm saying is that globally, energy has to be conserved. Any kinetic energy moved from the surface into the atmosphere, some of which ultimately leaves radiatively at the top of the atmosphere, has to reduce the amount of emitted surface power by an equal opposite amount due to less being returned to the surface in kinetic form, which has the effect of reducing the surface temperature; thus reducing surface emitted radiation. I know about the Antarctic temperature inversion. It's highly localized.
  22. "Any substance with an emissivity greater will radiate energy with a total energy proportional to its emissivity times the fourth power of its temperature. That is where the radiation comes from, from the gases in the lower atmosphere which radiate in the IR spectrum and have non-zero temperatures (primarily water vapour and CO2). The heat that warms that gas comes evapo/transpiration from the surface, radiation from the surface, and atmospheric absorption of incoming solar radiation, although at any given layer, a large part of it will come from thermal radiation from adjacent layers, or convective heat transfer from adjacent layers." Agreed, but ultimately what matters here is the net combined effect of all these things relative to surface emitted radiation. Aferall, that's what we're talking about here is it not? That's what determines global average temperatures, right? Heat flows - how much from the surface is coming back from the atmosphere and how much is passing through. This is determining the heat flux or power flux at the surface, which ultimately is determining the temperature.
  23. "Having said that, I do not see the relevance to the basic point at issue - is it George White, or all the world's radiative transfer modelers who are correct in their interpretation of the output of radiative transfer models?" Yes this is crux, but if GW is so obviously wrong as you claim, where is the smoking gun? And why haven't you presented it to him? I mean if it's so egregiously wrong, it should be easy to point directly to the specific evidence that disproves it, right? I admit I have not yet verified if what he's claiming is correct or not, but you have neither verified what the IPCC is claiming the 3.7 W/m^2 represents from the model simulations. I've looked all through the IPCC 2007 report, I don't find this information - they seem to be really ambiguous about where exactly the 3.7 W/m^2 is derived from. I've also looked all over the internet and cannot find verification either way. Regardless, I'm determined to get to bottom of this - even it means I have to get the MODTRAN software and run the simulations myself.
  24. Enjoy! http://www.modtran.org/ http://download.cnet.com/Modo/3000-2054_4-77505.html
  25. "Having said that, I do not see the relevance to the basic point at issue - is it George White, or all the world's radiative transfer modelers who are correct in their interpretation of the output of radiative transfer models?" There is yet another possibility too. They assumed or convinced themselves that there was a remote possibility that the full 3.7 W/m^2 of incremental absorption could somehow make it back to the surface through multiple absorptions and re-emissions. I've seen this claim argued before, though ultimately never convincingly. Maybe they used this as a rationalization to count it all as a "just in case" precaution. I don't know. Without knowing the detailed specifics of the outputs of these model simulations there's no way to know.

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