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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 76 to 100 out of 386:

  1. thepoodlebites - If you look at the "Intermediate" tab of this discussion you will see half a dozen empirically observed estimates of climate sensitivity, along with another half dozen model based ones. Does this address your request for observational data?
  2. thepoodlebites - My "favorite paper"??? All of them are interesting. Some of the more directly relevant ones from your question (observations) are Hansen 1993 (energy changes since the last ice age), Tung 2007 (sensitivity from climate response to solar variations) and Bender 2010 (responses to the Mount Pinatubo eruption). All of the papers listed on the intermediate page are worth reading, though. I will not continue the climate sensitivity discussion on the "Is it the sun" thread - that's off topic.
  3. A question for the group - I hope this is a reasonable place for it, and even more that I'm phrasing this intelligibly. Given that we have raised the CO2 concentration quite high, it's now high enough that the oceans are acting like a sink despite their warming (above solubility pressures) - the oceans are absorbing 2ppm/year or so. If we maintain, as we are doing now, a CO2 concentration above that which would induce CO2 output by the oceans, does that remove one of the feedbacks (CO2 outgassing from said oceans) from the climate sensitivity calculations? In other words, does the forcing by CO2 emissions block the CO2 element of forcing feedback, and thus reduce climate sensitivity??
  4. RW1 - there is another way to look at whether the radiative change is correct or not without going into the mathematics deeply. Step 1/ assume scientists have the maths and physics right. Use the model to calculate TOA emissions. Not just the energy, but also the spectra. Compare with REAL measured spectra. Step 2/ Assuming that was right, you can see whether the calculation for incremental CO2 increase is also correct by doing the same procedure but doing it for different decades and seeing whether the change matches the change in CO2. Sound fair enough test? In fact you could do the calculation for downward IR at surface or for outward IR by satellite. For results, see the papers on this Now lets see George White produce some calculations from his approach that can match these empirical results.
  5. (Oops, the above is response to comments from RW1 at A swift kick in the ice
  6. scaddenp (RE: 79), I don't understand, sorry.
  7. If George was right, (ie should be 1.85W/m2), then the model result that calculated 3.7W/m2 would not agree with the actual measurements of IR. Similarly, if you compare spectrum measured in 1979 with that in 2004, if the incremental change in IR was wrong then the measurement wouldnt agree. This is experimental verification that 3.7W/m2 for doubling is correct. Furthermore, you check that the change in IR is due to CO2 by looking at the spectrum.
  8. I'm not following.
  9. scaddenp, RW1 - George White has stated that running the HITRAN models results in an imbalance of 3.6 W/m^2 (here, post #19). And then he, for some reason, halves that value. Which I cannot consider as other than a blatant mistake.
    Response: [DB] Fixed URL link.
  10. KR, You may recall the last time we went around this tree (the endless Lindzen and Choi thread), this came from the assumption that 50% of emitted IR photons go up and out - 50% down.
  11. Muoncounter @85, it comes from the incorrect assumption that line by line radiation models do not already apply that effect already, and then applying it again to the output of the line by line models.
  12. Yes, hence my attempt to show that science had it right by direct empirical means since I despaired that RW1 would understand the calculation.
  13. Tom Curtis (RE: 86), "it comes from the incorrect assumption that line by line radiation models do not already apply that effect already" Where is the documentation that the halving is already applied? That's all I'm asking for. I've looked around and cannot find it.
  14. I know I am sounding like a broken record but you start with Ramanathan and Coatley 1978.
  15. RW1, where is George White's documentation that it was not applied? Currently your "critical thinking" will not accept the results of several scientific papers, the two most seminal of which have been cited to you, it will not accept the IPCC report, it will not even accept the results of the public domain version of a radiation model designed by the USAF, and it will not accept the reports of a large number of people knowlegeable on the subject. But it will accept the say so of a single electrical engineer based on zero documentation to the contrary. This extreme contrast in willingness to believe shows it is not critical thinking at all. So, before we go any further, how about you show us the peer reviewed paper, or technical description of a line by line radiation model, or the code of such a model in which the effect is not applied already. Current evidence is that you will accept any belief contrary to AGW on zero evidence, but will not accept any belief supportive of AGW on even a mountain of evidence. Given that I am not going to waste my time presenting evidence to your that you will not consider anyway. (Afterall, I already have given that evidence to you in at least two different forms; both from very creditable academic sources.) So, either show me that you apply the same evidentiary standards you apply to Gearge White's ravings; or give principled reasons why you will not accept a straightforward truth that can be verified in any first year text on atmospheric physics, or on climate modelling?
  16. And also, in scientific arguments, nature is the arbiter. The codes can used to calculate what experiments should observe. If George was right, then the experiments should be giving results half what they in fact do so.
  17. saddenp @91, you mean that if the models where wrong, you would not get results like this: Note: the spectral lines have been deliberately offset so they can be seen clearly. Without the offset, it looks like this:
  18. Yes, precisely what I mean.
  19. It was and remains a simple question unanswered. If it's so obviously wrong as being claimed here, it should be easy to point to the documentation that the "halving" is already applied to the 3.7 W/m^2 forcing. I have search around too. I couldn't find anything.
  20. RW1 - When the HITRAN model (and others) indicate a 3.6/3.7 W/m^2 imbalance, they are indicating photons going outward. The line-by-line calculations include photons going up and down by absorption and re-emission, for every level of the atmosphere covered by the model. The imbalance is the end difference between incoming and outgoing, the leftover quantity. Not emitted in all directions from some level of the atmosphere, but just the value emitted to space. That's what you get when you model the absorption/re-emission over the entire atmosphere. What's going back down to lower levels of the atmosphere or to the surface is part and parcel of the model - the imbalance is only the portion going in one direction, whether that's positive or negative depending on conditions. I'm afraid that George White's misunderstanding of this (and subsequent "halving" of the imbalance) indicates his overall poor understanding of the models he's been running.
  21. KR, "The line-by-line calculations include photons going up and down by absorption and re-emission, for every level of the atmosphere covered by the model. The imbalance is the end difference between incoming and outgoing, the leftover quantity. Not emitted in all directions from some level of the atmosphere, but just the value emitted to space." OK, show me where this is documented.
  22. RW1 - Assuming that GW is using the HITRAN spectral database and something like JavaHAWKS for full atmospheric simulations, the spectral database includes absorption/emission spectra for a large number of IR interactive molecules. Full atmospheric emission modeling means looking at absorption, emission, and transmission across the full black body spectra of the Earth emission, over the depth of the atmosphere. Some IR gets radiated back to the surface, some gets radiated around and re-absorbed in the atmosphere, a certain percentage in the 'IR window' goes straight to space, etc. The output from JavaHAWKS is the amount of radiation that actually leaves the atmosphere. Now, I cannot speak for GW, but "imbalance" should be a difference between the outgoing radiation from JavaHAWKS and incoming from the sun (a reasonably known value). Not the amount isotropically radiated from some level of the atmosphere, but the amount finally leaving the atmosphere (one directional) at the end of the modeling. And that's because the model includes the isotropic (omnidirectional, spherical) radiation as part of the calculation, summing up the anisotropic portion as output. That's certainly what everyone else running these models gets; 3.6-3.7 W/m^2 anisotropic radiation going to space for a doubling of CO2. An imbalance (difference!) between incoming and outgoing, an amount going in one direction not balanced by an amount going the other. I hate to say it, but GW does not understand the model he's running...
  23. RW1 - To put it more clearly: If it's not an anisotropic emission, it won't show up. Isotropic emissions, absorptions, and re-emissions are part of the model, not part of the output spectra. Total power emitted from the atmosphere given the model conditions is the output - not a sub-portion of internal isotropic emissions that will then get bounced around.
  24. I'm willing to be shown incorrect on this issue (and I believe George is too), but you're talking around the crux of the issue. Words like "should" and "everyone else" isn't evidence to the contrary, and more importantly doesn't answer the fundamental question. In another thread, you said the total additional absorbed infrared from models/simulations from 2xCO2 was 7.4 W/m^2. And why haven't you said all this to George on his article and post on the issue at joannenova that you linked?
  25. KR, "To put it more clearly: If it's not an anisotropic emission, it won't show up. Isotropic emissions, absorptions, and re-emissions are part of the model, not part of the output spectra. Total power emitted from the atmosphere given the model conditions is the output - not a sub-portion of internal isotropic emissions that will then get bounced around." OK, where is this documented? Point me to the paragraphs or pages that state this is what the output spectra represent.

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