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What would a CO2-free atmosphere look like?

Posted on 11 March 2011 by Chris Colose

The ability for CO2 to warm the surface of a planet through the absorption of infrared radiation is well known.  What is much less appreciated, however, is just how effective of a gas it is in maintaining the greenhouse framework that helps to characterize the modern climate.

The question of how the climate would change in a completely CO2-free atmosphere was brought up recently in a testimony to the subcommittee of the House Science and Technology Committee.  An answer was provided by MIT scientist Dr. Richard Lindzen, who suggested that such a hypothetical removal of all the CO2 in the air would translate into a global cooling of about 2.5 degrees, presumably in Celsius (see here, about 47 minutes into the video).  Dr. Cicerone, who was also on the panel, expressed disagreement although didn't really provide a better answer of his own.

It may be that Lindzen was just giving an estimate off the top of his head in a hearing, but the question is nonetheless interesting to explore further because it provides perspective on how to decompose the greenhouse effect into its individual components and the underlying implications of having a mixture of both condensing (i.e., those that reach saturation values and precipitate from the air) and non-condensing greenhouse gases.  On Earth, water (which is radiatively active as a gas or a cloud) is the only condensable substance in the atmosphere, while the "long-lived" greenhouse gases (mostly CO2, CH4, O3, or N2O) do not precipitate from the air under modern temperature or pressure regimes. 

One might naively suggest that removing all the CO2 in the atmosphere would produce a similar temperature change as doubling CO2, just in the other direction.  Even without considering the complex nature of feedbacks, this conclusion would be wrong however.  The radiative forcing generated by CO2 change goes as the logarithm of the concentration, and thus removing the whole CO2 inventory would generate a much larger impact than had you doubled it.  In fact, when you get to very small amounts of CO2 (on the order of a few parts per million), the radiative forcing is much faster than logarithmic since you’re opening up new opacity in the central 15 micron band where Earth is strongly emitting (it is for this reason methane changes are commonly cited as being “more powerful than CO2,” which is only true molecule-for-molecule in the modern atmosphere, as methane is starting off at much lower background concentrations.  Methane has no intrinsic properties that make this the case, and when comparing with CO2 side-by-side is even worse of a greenhouse gas).

Even this qualitative reasoning might have led Lindzen to a better ballpark answer than just 2-3 C, which would be even too small if feedbacks were completely neutral.  It’s still qualitative however, and getting a better handle on the question requires radiative transfer modeling and simulations that can adequately handle a greenhouse-free atmosphere without blowing up.  From here, the first question one can ask is how much of the total greenhouse effect is provided by CO2?  This is dependent on the background atmosphere itself, since overlapping absorption with other molecules will give a different number than if that molecule were all by itself, even at the same concentration. When you do these calculations with overlapping absorption, it was found by Schmidt et al (2010) that CO2 contributes to about 20% of the modern greenhouse effect.  The greenhouse effect can be defined energetically as:

where Ts and Te are the surface (288 K) and emission temperature (255 K), respectively. σ is the Stefan-Boltzmann constant.  It follows that CO2 alone provides nearly 30 W/m2 of radiative forcing, much larger than the ~4 W/m2 when you double it in the modern climate.  Assuming the same climate sensitivity, Lindzen’s estimate of a 2.5°C drop for a -30 W/m2 forcing would imply that currently doubling CO2 would warm the planet by only a third of a degree at equilibrium, which is well outside the bounds of IPCC estimates and even very low by most skeptical standards.

At this point, we need to incorporate feedbacks into the problem in order to get a better feel for how nature really operates.  It is often mentioned as a somewhat routine talking point that “water vapor is the most important greenhouse gas.”  This is true in the sense that it makes up the bulk of the infrared opacity in our atmosphere (~50%, and clouds another 25%), but because it condenses at Earth-like temperatures is very short-lived in the air as it goes through the evaporation, condensation, and precipitation cycle.  Because of the temperature-dependency on the pressure at which water saturates, this makes water vapor a feedback.  Another interpretation would then be that the non-condensing greenhouse gases (chiefly CO2, only about 5% of the greenhouse effect is provided by ozone, methane, N2O, etc) are the “most important,” since they provide enough warming so as to produce the skeleton by which the water vapor greenhouse effect can be strong enough.

There have been a number of studies which examine the evolution of the climate system with no CO2 in the atmosphere.  Such experiments are described for example in Pierrehumbert et al (2007) , or by Voigt and Marotzke (2009)From these papers, one can trigger a full snowball Earth with a sufficient reduction in atmospheric CO2.  A substantial reduction in water vapor (shown below, from Lacis et al (2010) as well as increase in the surface albedo are important feedbacks here, showing that removing the non-condensing greenhouse gases (mostly CO2) in the atmosphere can collapse nearly the entire terrestrial greenhouse effect.  What’s more, since the albedo increases substantially, the total greenhouse effect can be thought of as providing even more than 33 K of warming relative to Earth’s blackbody emission temperature. In the Lacis et al experiments, removing the CO2 from the atmosphere generates a cooling of around 30 C, an order of magnitude difference from Lindzen's answer.

 

 

Figure 1: Time evolution of global surface temperature, TOA net flux, column water vapor, planetary albedo, sea ice cover, and cloud cover, after zeroing out all of the noncondensing GHG’s.  From Lacis et al (2010)

 

Why might we care about such a hypothetical situation anyway? Aside from Lindzen, there are a number of interesting applications to low solar or low CO2 cases.  A concern with “large” climate changes (i.e., on the scale of snowball Earths or runaway greenhouses) is that there’s bifurcation (loosely, tipping points) in the system.  This is to say that one can conceivably draw down CO2 from the atmosphere and trigger a snowball, although it would take extremely high values of CO2 (much higher than the original amount) to get back out of this glaciated state.  In case skeptics bring it up, this is at least one way to have an ice-covered planet with very high CO2 levels.  Indeed, precisely how to escape a full-blown snowball is one of the grand unresolved questions in climate science.

Once you're out of a snowball however, you're left with a very hot climate until weathering can draw down CO2 to moderate values. Shown below (from Pierrehumbert et al., 2011, accepted) is a bifurcation sketch of the temperature as a function of the CO2 content in the atmosphere, with a lower solar insolation than today (specifically, Neoproterozoic insolation).  One can do a similar type of diagram against the incoming shortwave radiation, but in any case the presence of an albedo feedback makes multiple temperature solutions possible, even for the same incoming stellar radiation and greenhouse effect.  For example, if the Earth were magically ice-covered today, this would be a completely stable situation and there would be no tendency to escape that state unless the greenhouse effect was substantially enhanced or the sun got brighter.  This is a big problem in planetary habitability studies, especially if a planet succumbs to a snowball fate early in its history when the sun is faint. Once you begin to melt ice however, the temperature jumps rapidly to a very hot solution.

This is germane to a recent paper by Rosing et al (2010) which purported to show that the "faint sun" paradox can be resolved through a lower albedo rather than through a substantially enhanced greenhouse effect.  This paper is interesting, and an example of good scientific skepticism, in the sense that the authors are proposing a new idea for a subject still open for research.  The idea is problematic however, since even for Neoproterozoic insolation, you kick over into a snowball at about 3x present CO2, and the situation is even worse for early Earth insolation, which is some 20-25% lower than today.  There is no mechanism to adjust the albedo in such a way as to offset temperature changes, whereas the long-term silicate weathering thermostat acts as a negative feedback between CO2 concentations and temperature over geologic time.

 


Figure 2: Bifurcation diagram for a zero-dimensional energy balance model. Calculations performed with L = 1,285Wm−2. For CO2 inventories up to approximately 1,000 Pa, the inventory can be converted to a mixing ratio in parts per million by volume (ppmv) by multiplying by 6.6. The CO2 concentrations on the upper horizontal axis are stated as fractions (e.g., 0.0065 corresponds to6,500 ppmv). The vertical dashed lines marked “Left” and “Right” indicate the left and right boundaries of the hysteresis loop for the case with ice albedo equal to 55%.  Adopted From Pierrehumbert et al., 2011, accepted

 

Lindzen has argued for a relatively insensitive climate system in the past, in which case it would be difficult  to explain the magnitude of large climate changes in the past, ranging from snowballs, the PETM, glacial-interglacial cycles, etc.  However, arguing that the climate would cool only be 2.5 degrees when you remove all the CO2 in the atmosphere is really just a made up number and ignored several articles on the subject that show otherwise.  Just the opposite, evidence shows that CO2 provides the building block for the terrestrial greenhouse effect, both because it absorbs strongly near the peak emission for Earth, and because it allows Earth to be warm enough to sustain a powerful water vapor greenhouse effect. 

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Comments 51 to 100 out of 102:

  1. 18 Chris Colose Thanks for the reply. Do you mind clarifying something. "but non-linearity in sensitivity is rather small over the ranges of climate of interest to us right now. Certainly you don't want to compare snowball Earths to say, the PETM directly, but I haven't seen anything suggesting it's a big deal for evaluating modern global warming." Here you're suggesting that you don't think the magnitude of the problem is so great when comparing modern conditions to say the LGM? Anyway your references lead me to this review (which includes Crucifix as an author). I thought it was useful in presenting the strengths and weaknesses of various CS estimates as well as being easy to follow. Would this be a balanced assessment of the science? Chris ignore this if you think it's straying too far away from the CO2 free atmosphere subject.
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  2. HumanityRules, No, I don't think climate sensitivity itself changing is a fundamental issue, and is more of a higher order effect,at least over a reasonable range of climates (like the Pliocene, LGM, modern day, etc). Your paper looks like a good review. Knutti and Hegerl (2008) is also a great read on the issue. It's very tricky stuff tying to get a good sensitivity range estimate, and 2 to 4.5 C per CO2 doubling as the IPCC, 2007 cites is still a fairly large range to work with, but it's the most credible starting point from which to proceed. Peru, Sorry I didn't respond. I have been traveling over the last 30 hours. I would email a Venusian dynamicist for some authoritative insight on the cloud structure of Venus and how it is maintained (there's still a lot of uncertanties on this topic though, something experts will always stress if you ask them), but there's a lot of differences from Earth conditions. The clouds on Venus are often grouped into different layers, an upper layer (~60–70 km), the middle (50–57 km) cloud layer and a lower (48–50 km) layer. In the upper levels, sulfuric acid is created by photochemistry, and in fact most solar absorption occurs in the high atmosphere making it already different from Earth-like conditions. A large number of sulfur compounds, and some water vapor that is still left over, provide sources for the global cloud cover, and there is condensation by H2SO4 vapor lower down on sulfuric acid particles. This makes the low level clouds more prone to spatial and temporal variations due to convection. There's some literature on these hourly-scale fluctuations, and Bullock and Grinspoon (2001) is a good source for geologic timescale variations in these clouds and how changes in SO2 sources or water vapor amounts may impact things. But it's a lot more complex than just high temperatures, and one can find some similarities but also many differences to Earth's cloud dynamics. There's no straight-forward reason that global cloud cover should increase if you make Earth hotter, and even if it does, how that impacts the surface temperature depends on the altitude and optical properties of the cloud. For clouds formed by condensation in updrafts, you're not likely to get cloud cover close to 100% of the globe. On Earth the microphysics and evaporative/condensation processes are largely controlled by the strength of vertical motions, so to get clouds you need to figure out where the air is being lifted (but by conservation of mass, it needs to be sinking somewhere else). If you take a course in synoptic-scale meteorology, you'll probably spend most of a semester learning different diagnostic tools to figure out, very simply, if the air is rising or sinking. The last thing to note in my already lengthy post, is that evaporation does not scale with Clausius-Clapeyron as the globe warms, since there are surface energy budget constraints that regulate the amount of evaporation that can occur. In fact, in very highly opaque atmospheres, you can increase the temperature by turning CO2 up but if your input solar energy term is not sufficient, you won't be able to increase evaporation. You can still increase the vapor content though, but again, what it means for cloud distribution is anyone's guess.
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  3. This entire line of argument relies on model results that will become less reliable and relevent the further they get from the climate's component values against which they were built. The parametisation values will quickly move away from reality as new equilibriums are established in the real world for the case where CO2 was magically instantaneously removed from the atmosphere. Put another way, our models today have a snowballs chance in hell of being even vaguely accurate when moddeling climate heading towards a snowball earth.
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  4. Chris Colose: But what a 50ºC or 70ºC hot, super-sauna Earth with enormous amounts of water vapor in the atmosphere (as in the Archaean Eon and in the snowball aftermath) would mean for the atmospheric dynamics? What kind of clouds cover would dominate the Earth, high level (cirrus), low level (cumulus and stratocumulus) or monstruous convective (cumulonimbus)? How much of the planet would be cloud-covered? Could it be near 100%? How big would be the Tropical Storms? Could they reach the warm Poles?
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  5. Peru, Once again you ask good questions, but neither I (nor the scientific community) has full proof answers to them. It is not unreasonable to suggest that cloud water increases in a warming climate, allowing high clouds to become more optically thick, as well as clouds at all levels to have higher water vapor content. It is possible that both the cloud albedo and cloud greenhouse effect increase substantially, only for the net effect to not change much. As I noted in my last comment though, if you keep the temperature fixed, but reduce the solar radiation (and increase the greenhouse effect), you also reduce precipitation. Precipitation efficiency also matters, and there's some papers showing that this goes up for warm-rain regimes in the tropics, but the mechanisms that control cloud fraction is not well known, nor what it means for the radiative budget. There's also some work showing that biological stress in a warming climate can impact clouds by changing biologically sourced cloud condensation nuclei (e.g., here)
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  6. TTTM - I guess that depends on what you mean by "vaguely accurate". The gross parameters of energy balance dont depend on any parameterizations, so I would say GCM have pretty good chance of say estimating global planetary temperature within a degree or so. We can do it for other planets in solar system. Which parameterizations do you think are so critical as to make this impossible?
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  7. "The gross parameters of energy balance dont depend on any parameterizations" In what sense? Of course they do in any sense we care about unless you're trying to obfuscate by indirectly noting the fact the sun hasn't changed. "Which parameterizations do you think are so critical as to make this impossible?" In general...all of them. Clouds. Precipitation. Any parameters concerning the movement of energy from the equator to the poles. You name it, they'll be significantly wrong on the way to any "end point" and therefore may contain surprises that have considerable impact on that "end point" Your way of thinking assumes snowball earth is inevitable and therefore the path to that point is pretty much irrelevent. At the end of the day, its not a scientific result having used models to determine the earth's climate response to CO2 removal, its just an interesting play with the models given the assumptions upon which they've been built.
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  8. Okay, I jumped too quick on that one. However, how robust is the development towards snowball. Given increasing albedo, decreasing water vapour, that heat balance is going south for all values of parameterizations that seem reasonable. The interesting question is whether its complete snowball or there is enough tropical heat generating water vapour to be partially ice-free. Ask another, if had snowball, (100% ice cover), no water vapour (and thus no cloud), is there enough heat to create significant water vapour? This is at least an easily scenario to model. If this leads to a melt, then CO2 free atmosphere can be confidently stated as leading to only partial snowball.
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  9. No one denies that there are many factors which are very difficult to simulate in simple models, and it's far more constructive to discuss these issues and their relevance, rather than just labeling it as an issue. For the snowball problem, inherent uncertainties in estimating parameters for a number of factors are problematic(especially the lapse rate, which is very shallow on the winter hemisphere of a snowball due to the inhibition of convection, which in turn eliminates the prospect of a very strong greenhouse effect, other factors include surface albedo, clouds, and boundary layer dynamics). Different models and the sensitivity to different parameters are discussed in a number of papers on snowball Earth already, as well as Ray P et al's coming review paper. Isolating the impact of paramters on initiation/deglaciation is one purpose of the Snowball model intercomparison project (SNOWMIP). GCM's are also prone to physical uncertainties, though assumptions one needs to make will at least lie closer to the fundamental underlying physics That some of the most powerful computers in the world can get into a snowball however, and rather easily, suggests that no robust barriers to the initiation of such a state exist.
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  10. "especially the lapse rate, which is very shallow on the winter hemisphere of a snowball due to the inhibition of convection, which in turn eliminates the prospect of a very strong greenhouse effect" I find that looking at the limit of an effect is very useful in understanding the direction an effect will ultimately take. Using the entirely reasonable albedo argument, a snowball earth may well be stable. ...if it could be "forced to exist". However, one cant ignore the path to get there and whether its actually reasonable. Changes in cloud cover combined with increased equator-pole oceanic transfer of heat due to a larger heat differential could compensate for any increasing ice based albedo and decreasing cloud cover is quite a reasonable expectation from a cooling, drying atmosphere. So its not an unreasonable expectation that land masses could tend to ice up locally, particularly the larger ones inland and at altitude, but the oceans remain largely liquid and many smaller land masses largely ice free. AFAIK this doesn't contradict the little evidence of snowball earth that exists either but its a quite different result to a solid snowball.
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  11. "An answer was provided by MIT scientist Dr. Richard Lindzen, who suggested that such a hypothetical removal of all the CO2 in the air would translate into a global cooling of about 2.5 degrees, presumably in Celsius" Yeah, that's about right. As you know effective temperature of a body is the temperature of a black body that would emit the same total amount of thermal radiation. Effective temperature of Earth as seen from outer space is about 255 K. This value does not depend on CO2, neither on any other GHG for that matter. It is entirely determined by the solar constant and the short wave albedo of the planet. We can also define effective temperature of Earth for a restricted frequency range like the main CO2 band between 18 and 22.5 THz (which includes the wings) the same way. Currently it is about 240 K. As for realistic temperatures (let's say between 220 & 290 K) a black body emits about 16% of its radiation in this frequency band, it means effective temperature of Earth outside the main CO2 absorption band should be some 257.5 K to still produce a 240 W/m2 overall OLR. Now, if we removed the carbon dioxide from the atmosphere, effective temperature inside and outside this band would both be 255 K, which is a decrease of 2.5°C. Of course it is a very crude approximation. The actual value can be anywhere between 2°C and 3°C. Not to mention the fact if we removed all the carbon dioxide, we'd kill plant life, which would change short wave albedo by eliminating evapotranspiration entirely. Fortunately there'd be no one to notice it, because with no food people are inclined to starve to death.
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  12. BP... I don't think even Lindzen has any research that backs that up. It's little more than a bit of wishful thinking pulled from a tattered top hat that would justify his sloppy research.
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  13. BP @61, "Of course it is a very crude approximation. The actual value can be anywhere between 2°C and 3°C." Quick, submit a rebuttal to Lacis et al. in Science, all based on a short bog post and a "very crude approximation"-- post normal blog science lives, and what is more we do not have to worry about the consequences of doubling CO2 b/c its effects on global SATs are minimal! Hang on though, first you might want to read their paper....I see no consideration for feedbacks in your "analysis" BP, other than the plants dying because they have no "food". From the main post, b/c you seem to have not read it: "There have been a number of studies which examine the evolution of the climate system with no CO2 in the atmosphere. Such experiments are described for example in Pierrehumbert et al (2007) , or by Voigt and Marotzke (2009). From these papers, one can trigger a full snowball Earth with a sufficient reduction in atmospheric CO2. A substantial reduction in water vapor (shown below, from Lacis et al (2010) as well as increase in the surface albedo are important feedbacks here, showing that removing the non-condensing greenhouse gases (mostly CO2) in the atmosphere can collapse nearly the entire terrestrial greenhouse effect. What’s more, since the albedo increases substantially, the total greenhouse effect can be thought of as providing even more than 33 K of warming relative to Earth’s blackbody emission temperature."
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  14. #62 Rob Honeycutt at 03:08 AM on 17 March, 2011 I don't think even Lindzen has any research that backs that up I am not talking about Lindzen's research, but actual data and simple back of the envelope calculations. It does not show changes in surface temperature directly, it's about the temperature of terrestrial photosphere. However, if environmental lapse rate does not change dramatically, it should translate to a surface temperature anomaly of the same order of magnitude.
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  15. BP... My point is that you're taking the same position as Lindzen and doing pretty much exactly what Lindzen did in front of the Congressional Subcommittee. Pulling a napkin out of your hat and saying, "Eh, I think would be about 2.5 degrees cooler" without hard research to back up the statement. Conversely, those who have done the hard research (Lacis, as mentioned above by Albatross) come up with completely different numbers. For me, napkin calculations just don't sway my opinion as much as published research. Now, if you could get that napkin worked up into a published paper then you (and Lindzen) might have something worthwhile to discuss.
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  16. #63 Albatross at 03:20 AM on 17 March, 2011 one can trigger a full snowball Earth with a sufficient reduction in atmospheric CO2 Perhaps one could. And a snowball Earth has much higher short wave albedo indeed. However, it has no bearing on our present situation. Area of ice covered surface is small relative to the entire surface in this interglacial, on top of that where there's ice there's no insolation for half a year. Therefore even if all the ice melted, the effect on global albedo would be negligible. Especially if low level clouds which tend to develop over ice free cold regions are taken into account as well. It just shows how fast climate sensitivity decreases with increasing temperatures. This is why Holocene temperature variability is an order of magnitude smaller than during glacial periods, don't you think?
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  17. BP... Don't you think it's just a little bit disingenuous to make a statement about Holocene variability compared to glacial periods by tossing up GISP2 data? You'd be more clear and accurate to say "Holocene variability above the Arctic circle..." The actual variability within the last glacial is not that much greater than the Holocene, and it's pretty well accepted (to my understanding) that Vostok is probably more indicative of global temperature than is GISP2.
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  18. BP, First off, a "CO2 frequency-band effective temperature" is highly wavelength dependent, moving from even colder than 240 K at the center to even warmer than 240 K toward the wings, and with spectral overlap with the other GHG's. You wouldn't even be able to define how the temperature would change using your methodology for a doubling of CO2, since the "bite" in the spectrum by more CO2 doesn't really get deeper at the center. Secondly, CO2 absorbs in a much more important place in the spectrum than in other conceivable spectral domains, say, toward the far tail of the Planck function. The "no-feedback" temperature change is approximately RF/(4*sigma*T^3), where RF is the radiative forcing and the denominator is the derivative of the Stefan-Boltzmann equation for a blackbody. For T=255 K, the denominator is about 0.26 K/(W/m2), which allows you to approximate the traditional ~1 C temperature change for a CO2 doubling. In contrast, by removing all the CO2, the RF is even more so a real value is closer to 7 C. As Albatross noted, your estimates don't include feedbacks at all, either surface albedo or the water vapor feedback.
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  19. BP @61, assuming it was sensible to calculate a no feed back change to temperature from the removal of CO2, your method of doing so is so sloppy as to be worthless. Using Modtran, setting it to a US standard atmosphere and then adjusting the temperature down to approximately equal the effective temperature (-0.9 degrees), I then removed the CO2. The result was to increase OLR by 27 watts, increasing the effective temperature by approximately 10 degrees. The version of Modtran I used is obsolete, so this figure is not exact; but it is a far better estimate than your estimate by sloppy calculation based on generous (to your argument) estimates.
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  20. #69 Tom Curtis at 05:39 AM on 17 March, 2011 The result was to increase OLR by 27 watts Impossible. You did something terribly wrong. The flux emitted by a black body between 18 THz and 22.5 THz (wavenumber 600 cm-1 & 750 cm-1) at 255 K is 39 W/m2. If current abundance of CO2 would decrease it by 27 W/m2, effective temperature of the photospehere in this frequency range would be 195 K (-102°C), which is way below any temperature ever observed in terrestrial atmosphere.
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  21. OK, 195 K is -78°C. Still, it is much lower than global effective temperature in the 18-22.5 THz range.
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  22. BP @70, it turns out I did make an error. Proceeding more accurately, the Modtran model is here. The settings are: CO2 exp 1 - 375 ppm, exp 2 0 ppm CH4 1.7 ppm; Trop Ozone 28 ppb; Strat Ozone Scale 1; Ground Temperatures Offest -0.5 Hold Water Vapour "Pressure"; Water Vapour scale 1; Locality 1976 US standard atmosphere Standard Cirrus Model Sensor altitude 70 km; lookdown Except for locality, offset temperature, cloud type and CO2 concentration, these are the default settings. Ground temperature is 287.7 degrees K. With 375 ppm, the OLR (Iout)is 239.582, a close approximation to the 239.8 w/m^2 black body radiation for a temperature of 255 K. With 0 ppm CO2, the OLR is 262.818 The difference is 23.236 w/m^2. The black body radiation of 262.8 degrees represents an effective temperature of 260.9 degrees K, or a change of 6 degrees over the case with CO2. Clearly you and Lindzen are still underestimating even the no feedback case for the removal of CO2.
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    Moderator Response: [DB] Ok, not "utter". :)
  23. The whole idea of arbitrarily removing all the CO2 from the atmosphere and contemplating the resulting temperature is contrived and is no more useful than contemplating the temperature while ignoring (unknown) feedbacks. Ignoring feedbacks doesn't make the answer wrong, it is still correct and just as useless in any practical sense. Its far less OMG though, isn't it.
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    Moderator Response: (DB) Your opinion is noted; however the excellent post by Chris still stands. The fact that others have found this thread intensely interesting & informative still stands, as well.
  24. TTT @73, "Ignoring feedbacks doesn't make the answer wrong..." Interesting contorting, but Lindzen's answer remains wrong, and it is troubling that you are willing to bend over backwards to dismiss that fact and defend yet another of Lindzen's monumental faux pas. The biosphere is a fully coupled and interactive system, you cannot ignore feedbacks. Well, you could but your answers/science would be wrong as demonstrated here. Your argument is ironic, given that Lindzen and Spencer insist that climate scientists are ignoring their hypothesized strong negative feedback. Yet here you seem to be arguing that "ignoring feedbacks does not make the answer wrong". No. Yet more inconsistencies and contradictions by 'skeptics'.
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  25. Yeah, rofl Albatross with your insistance that your fantasy figure is better then my fantasy figure. " The biosphere is a fully coupled and interactive system, you cannot ignore feedbacks." The biosphere is full of carbon and hence carbon dioxide.
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  26. TTT, Instead of ROFL, you might want to try and engage in some actual science. And to be quite honest, your post @75 makes no sense whatsoever.
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  27. TTT @73 and 74, the issue of the Earth's probable temperature with very low, or no CO2 is a critical issue with regard to climate sensitivity. If it is a fact that with no CO2, water vapour levels would fall sufficiently that the mean global temperature was -19 degrees C, or less, then the notion that climate sensitivity is 0.5 degrees per doubling of CO2, or even 1.5 degrees per doubling, are absurd. That might be a merely academic point. Where it not for one fact, there would be no way to independently test the consequences of very low CO2. But, as it happens, during at least two periods in the past the Earth had very low CO2. Consequently model experiments on very low CO2 can be tested against geological data from those events. What is becoming evident is that low climate sensitivities are inconsistent with the Earth entering a snow ball (or slush ball) state. And they are even more inconsistent with the Earth leaving such a state, in that without a strong greenhouse effect, the strong negative forcing of global ice cover cannot be overcome. This leaves aside the rhetorical issue, ie, the reason Lindzen concocted his figure. Lindzen asserts a low value for the temperature change with no CO2 to create in his audience a false impression that climate sensitivity is low. Giving a more accurate figure would have been counterproductive to his rhetoric. So perhaps your "fantasy figure" is better than Albatross's "fantasy figure" for you, but only if it is your intent to deceive.
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  28. "Instead of ROFL, you might want to try and engage in some actual science. And to be quite honest, your post @75 makes no sense whatsoever. " I'll spell it out for you then. You say you cant ignore feedbacks because they DO exist and hence ignoring them would not make sense when deriving an "answer" but along precisely the same lines of reasoning you cant ignore CO2 either because CO2 DOES exist. But of course you can ignore any effect you want and both answers are perfectly valid within the context of the assumptions made on their calculation. Just because you dont like it, doesn't make it wrong. The ROFL part is because you're insisting the answer with feedbacks is more valid than the answer without when in fact neither answer is valid in any practically useful sense.
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  29. @Tom "So perhaps your "fantasy figure" is better than Albatross's "fantasy figure" for you, but only if it is your intent to deceive." I'm not the one who feels strongly about a certain "answer" to the question. I doubt sensitivity is constant and therefore whether its 1K, 3K or 4.5K per doubling right now isn't as important as some say it is. I do feel strongly about criticisms leveled at people because they quote an answer calculated in a different way.
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  30. Tom Curtis @77, You said it better than I could. TTT, seems to be missing the entire point of this exercise. And TTT, does not understand that it is not about what I or you "like" or not, it is a question of what the physics state/dictate. The physics state that Lindzen is wrong, but I understand that being a "skeptic" means that one has to concede nothing and rarely, if ever, admit fault. Here we have yet another example of that, this time courtesy of TTT.
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  31. Hypothetically, one could say that without hands to wave, hypothetical "skeptics" would be incapable of communication. Hypothetically. One could also posit that without CO2, communication might well be impossible. Without a seance, anyway. This conversation only hypothetically happened anyway. Electrons being virtual. The Yooper
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  32. The reason I created this post was not just to address Lindzen, but to create some thinking about broader issues in planetary climate. In a PRACTICAL sense, these questions might not be important right now on Earth, but they are important for example to astrophysicists exploring the limits of habitability. This is important as many new planets continue to be discovered (e.g. Gliese 581 g), since the extrasolar planet database is now at ~500 new worlds, a few of which could potentially support liquid water depending largely on their atmospheres. These questions are also important for considering some of the big questions in Earth's past, such as getting into and out of a snowball, how to offset a faint young sun, etc. While having zero CO2 sustained in the atmosphere might be very hard to do, it is a type of extreme case with much of the same physics operating as other extreme cases, like a 30% fainter sun. In the snowball deglaciation problem, we consider the opposite spectrum in which a substantial fraction of the atmsophere is CO2. This is also important for considering the evolution of Mars or Venus. While climate science on the blogs does not generally talk about these things, and there's not a heavy denial thinktank attacking them (because how Venus evolved doesn't effect fossil fuel industries or the fear of governmental control), there's still many scientists out there who do think about them. That said, defending Lindzen's answer as just as valuable "as any other answer" is complete hogwash. It doesn't matter if you double CO2, make an atmosphere of 90 bars of CO2, or think about a 50 ppm only atmosphere. The point is to understand the underlying physics, to get our understanding as close to reality as possible, and this does not involve making things up. It's perfectly fine to consider exotic cases in modeling, but you want to be self-consistent in obeying physical constraints (e.g., you can't remove some 10 C of the greenhouse effect and hope to keep the water vapor and ice the same). Even with compeltely neutral feedbacks, Lindzen is still off by a factor of 2-3. It is also legitimate to criticize the efforts being done so far constructively, but saying that it's all fantasy just because we are dealing with something you may not be familiar with, might be "too far out" from modern Earth, or whatever other lopsided rationale you build up displays great ignorance into the field of planetary climate.
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  33. "That said, defending Lindzen's answer as just as valuable "as any other answer" is complete hogwash." I see. And so the amount of warming CO2 creates in the atmosphere is not useful to know even though its essential to understand that before moving on to understanding the feedbacks? Its ALL hand waving Dan and Chris.
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  34. TimTheToolMan - The amount of warming CO2 creates is a very important question. It's unfortunate that Lindzen's answer makes absolutely no sense given known physics, data, and constraints - and given the observations and the physics, defending such an incorrect answer is indeed "complete hogwash".
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  35. "defending such an incorrect answer is indeed "complete hogwash"." Somewhere along the line, I am suddenly defending Lindzen's answer. This is typical of AGW arguments. If they cant argue their point successfully then they turn it into a point they CAN argue against successfully. MY point about AGWers criticising Lindzen is that their criticism is largely of Lindzen answering without feedbacks and this criticism is invalid. THE answer (whether Lindzen's or anyone else's) of CO2 warming in the atmosphere irrespective of feedbacks is a perfectly valid answer in its context.
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  36. TTTM, I have no idea why you are still arguing this. First off, a “no-feedback” sensitivity is useful in that it provides a baseline from which to compare the impact of internal feedbacks on the system (in fact, defining this reference system is a central aspect of feedback analysis). What this ‘reference system’ is depends largely on the application of interest. When we traditionally talk about positive and negative feedbacks, they are ‘positive’ or ‘negative’ relative to a blackbody reference system that restores radiative equilibrium via the Planck function when something like CO2 or solar is perturbed (largely because this is well understood). For someone interested in soil moisture and vegetation changes in global warming, it might be more useful to let other variables freely change, define this as the reference system, and define the feedback as just those processes of interest. If you change the reference system, you also change the feedback. This is an important point and not just semantical. But Lindzen was asked a straightforward question: would Earth be hotter or colder with no CO2? To his credit, he managed to get the sign right! But he threw in his own quantitative estimate, and the answer he sold implied both a reference system and a feedback. When you’re talking to policy makers, they are interested in the real world, and the real world is a reference system plus feedbacks. He was not asked “how would the temperature change if everything in the world stayed the same and we removed CO2,” in which case he would have still been wrong, just less so. If he wanted to give a no-feedback answer, than he should have made that caveat in the testimony, in which case I would still have made this post for educational and thought-experiment purposes anyway. But you are simply arguing that Lindzen is allowed to make up whatever number he wants, whether it be 2.5 C or 250 C colder. That’s not how science works, sorry.
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  37. TTT @79: First, Lindzen did not just arrive at the figure by a different calculation. He got it wrong. Even if we suppose he was determining the value by using the law value for climate sensitivity (0.5 degrees per doubling) that he accepts, despite the fact that it is contrary to all the evidence, then he still significantly understates the relevant cooling. Second, the idea that climate sensitivity is inconstant in a range between 1 and 4.5 K per doubling is just nonsense. The net greenhouse effect on Earth including feedbacks is at least 33 degrees C. If climate sensitivity varied in the range you indicate, then that net greenhouse effect should vary in the range from 11 degrees C to 49.5 degrees C. In other words, the mean global surface temperature should vary from -8 degrees C to 30.5 degrees C without any changes in forcing. Given the observed range of natural variation in the Holocene, even if we assumed (contary to fact) that they were unforced, the range of variation of climate sensitivity would be limited to plus or minus 0.05 degrees C. Of course, climate sensitivity probably does vary depending on the lay out of the continents, and as a function of MGT. But that is hardly heartening to deniers, because studies of such variation show we are currently in a trough of low sensitivity, and that increasing temperatures will probably result in increasing sensitivities.
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  38. I see. Now Tom gets to dictate what I've "indicated" with a rubbish interpretation on what varying sensitivity means and I have no say in the matter. Real class SkS.
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    Moderator Response: [DB] Your reply to Tom was deleted both due to the lack of substance and the devolution of the comment into childish behavior. When you start to personalize things you cross the line. Feel free to repost a reply to Tom, along with a substantive argument based on the peer-reviewed science. Show everyone why you are right. Merely disagreeing, and being disagreeable while doing it, adds no value to this dialogue. There are other venues for that type of behavior.
  39. TTT @88, from your deleted comment, it is clear that you did not intend to say that climate sensitivity is arbitrarily variable in time; but rather that its potential variability is governed by some other factor(s). What other factor(s) is that, and what is the range of sensitivities you believe to be operable? And (given the topic of this thread) how does it make a difference in determining the after feedback mean global temperature in the situation of zero CO2 in the atmosphere?
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  40. TimTheToolMan @90, the only study I have seen on the effect of changes in forcing on climate shows only slight changes (12% or less) over a range of values, but with much higher and smaller forcings shows a much higher (up to 33% higher) value. Taking the modern best estimate of climate sensitivity of 2.8 degrees per doubling of CO2, that means the minimum value is around 2.5 degrees per doubling, and the maximum around 3.7. If we consider the minimum value only, that would indicate a fall in temperature after feedbacks of 14.6 degrees C for the total removal of CO2 from the atmosphere. That is likely way to small a reduction, as shown in the post above. However, it is a best case for Lindzen and shows him to have an error of 83% at minimum. An alternative approach would be to compare climate sensitivities determined from geological data for warm and hot periods. As it happens, climate sensitivities determined for the LGM and for the Pliocene (cold and warm periods respectively) are very similar; as indeed are sensitivities determined using GeocarbSulf by Berner for the whole phanerozoic. Regarding the "not at all subtle references", they are too subtle for me. All I know is that TTT are the initials of you chosen name.
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  41. Well after all that I'm still unsure what you're saying. Are you saying that sensitivity has varied between 2.5C and 3.7C per doubling or something else? I dont know how much variation there is in sensitivity (nor do I think anyone knows or even CAN know with the data we have) but the following thought experiment might help in understanding where I'm coming from. Right back on topic and using my favorite method of looking at an extreme, what would the sensitivity be on a hypothetical hard snowball earth? For the first many doublings, it would be very low. Maybe even approximately zero. Only once CO2 levels have got very high would they have an effect and then it will jump hugely. What does this mean to me? Well it means that sensitivity depends on the climate's "state" and the CO2 level itself. Some states will be more sensitive than others.
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  42. TTTM @92 (First, do you object to TTTM as you do to TTT?)
    Right back on topic and using my favorite method of looking at an extreme, what would the sensitivity be on a hypothetical hard snowball earth? For the first many doublings, it would be very low. Maybe even approximately zero. Only once CO2 levels have got very high would they have an effect and then it will jump hugely. What does this mean to me? Well it means that sensitivity depends on the climate's "state" and the CO2 level itself. Some states will be more sensitive than others."
    There is probably something in this. As it happens, in figure 2 above we have a means of testing this intuition. In figure 2, the slope of the (solid) lines represent climate sensitivity. By zooming in on the graph a bit, and laying a ruler along side, it can be determined that the climate sensitivity in the snowball earth case is 1.5 degrees C per doubling of CO2 (and higher on the right of the graph). In the non-snowball earth case, the climate sensitivity is about 2.5 degrees C per doubling of CO2 (and again, higher as it gets warmer). So, let's consider what this means for Lindzen's testimony. First, it means that climate sensitivity in the non-snowball earth case is high enough to initiate a snowball earth in the event of removal of all CO2 from the atmosphere. So, in the no CO2 scenario, the global meant temperature would be closer to 240 degrees K then the approximately 284 degrees K he testified to. Second, it means even the lowest climate sensitivity is about three times the value he uses. And you have to have a snowball Earth to have a climate sensitivity that low. Third, the climate sensitivity is very stable over a large range of temperatures and CO2 concentrations. The major instability is an albedo forced bifurcation which therefore cannot be included as a climate sensitivity per se. So, Lindzen's testimony continues to look very shabby.
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  43. " In figure 2, the slope of the (solid) lines represent climate sensitivity." And we've come full circle. I would put no faith in those figures because they're little more than guesses dressed up as science. We simply dont have any knowledge of how the climate will respond when its a long way from what we know about and have measured. Parameterisations are definitely going to be wrong in the models. What is this fascination with attacking Lindzen?
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  44. Is figure 2 really taken from Pierrehumbert's accepted paper? That graph makes no sense to me. Our global mean temperature is around 288K and according to the graph that puts us correctly in the partially ice-covered state (only just) at about 130 ppm CO2 and thats not right. Looking the other way, 390ppm CO2 puts us on the curve at 292K (too hot) and into the ice-free state (the polar bears and penguins might have something to say about that)
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  45. And then there's the issue of where the CO2 is coming from. The best case (lowest ice albedo @ 55%) requires 66,000 ppm CO2 to escape snowball earth. We're at 390ppm right now and we've burned about half our oil and quite a lot of coal too. Sure, there's plenty of coal left, but enough to get to 66,000ppm CO2? How about the worst case (highest ice albedo @ 65%) which is closer to 660,000ppm CO2?! Maybe the numbers add up and I'd be interested to see his paper to see what he says about that.
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  46. The CO2 came from unabated volcanic activity and tectonic forces over millions of years. Snowball Earth by Paul F. Hoffman and Daniel P. Schrag, Scientific American, January 2000
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  47. I would agree that it makes little sense to look at things like Fig. 2 with a magnifying glass, but the general picture I have presented here is robust to different models and parameterizations, and also within the geologic record (particularly cap carbonates as a key indicator that the Neoproterozoic glaciations and deglaciations involve a jump between extremely different states). Different cloud parameterizations and feedbacks, simulation of circulation, etc may yield different results, and while it is useful to know whether it takes 2000 or 4000 ppm of CO2 in the air to trigger glaciation under Neoproterozoic insolation, it plays little bearing on thinking through these problems on a forum like this. This is how science advances in general, by building a useful scaffold upon which more technical details can be later hung. I feel like this is self-evident to most people here, and TTTM is just trying to argue; certainly, the results of something like Fig. 2 emerge in simple zero-dimensional energy balance models as well as in full flown GCM’s, and are not hand-waving or other such “useless” exercises as he’d have people believe. There is no fascination with attacking Lindzen either, he was just wrong, and admitting that rather than trying to defend it through “arguments from complexity” would be one more step toward a good discussion. Concerning the rather weak climate sensitivity in a snowball Earth, there are several published mechanisms for why this is the case. The first is the very weak water vapor feedback, which is relatively unimportant in regimes where the tropics are frozen. Without the help from water vapor, CO2 has limited power to warm up a Snowball Earth to the point of deglaciation. This is much less true for condensed water, which is much more efficient of an infrared absorber than water vapor. Second, there is a rather weak vertical temperature gradient in the summer hemisphere of a snowball, and even weaker in the winter hemisphere in which convection is suppressed (much like over Antarctica in the winter). The greenhouse effect can only operate insofar as you have colder air aloft to work with, so a weak lapse rate inhibits the ability for CO2 to do much. In regimes where you could condense CO2 as a cloud, you could generate a scattering greenhouse effect (rather than the traditional absorption and re-radiation we are used to) that works largely independent of the lapse rate (though this also serves as a limit to CO2 accumulation). This could play a role in Polar Regions in the winter time during the Neoproterozoic at high CO2 levels, early earth snowballs, or on early Mars for example.
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    Moderator Response: [DB] Fixed Fig 2 reference per request.
  48. @Bibliovermis "The CO2 came from unabated volcanic activity and tectonic forces over millions of years." But 66,000ppm (best case) is a lot of CO2. Thats what I mean by wondering whether the numbers add up. Is there enough time to do it? And where is it now? Do those numbers add up too? "I would agree that it makes little sense to look at things like Fig. 2 with a magnifying glass, but the general picture I have presented here is robust to different models and parameterizations" The thing is that people are now using those numbers in reasoning. Assuming the graph is from data output from those models, its pretty clear that the model didn't get it right and it didn't get it right in the area it was most likely to "get it right". What confidence does that give us about the areas where we have no knowledge? "while it is useful to know whether it takes 2000 or 4000 ppm of CO2 in the air to trigger glaciation under Neoproterozoic insolation, it plays little bearing on thinking through these problems on a forum like this." Its not 2000 or 4000 though is it? Its 66,000 or 660,000 and those numbers are much higher than I've seen our historic atmospheric concentrations of CO2 quoted to have been. Perhaps I've simply missed higher ones but the point is that this is a "sanity check" item. And because its so high (if it really is considered very high), then the question of where it came from and where it went to becomes necessary to consider. "are not hand-waving or other such “useless” exercises as he’d have people believe. " That hypothesis (that these results have some semblance of reality) has yet to be shown. Your answer is itself "handwaving". "Second, there is a rather weak vertical temperature gradient in the summer hemisphere of a snowball..." ...fundamentally assumes snowball earth is a a hard snowball which is quite an assumption.
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  49. OK, At least I've spotted the cause of the fact that the graph bears no resemblance of today's situation... The value of L used is a fair bit lower than today's value. I'm going to have to find where that value comes from and how confident we are in it. Certainly the only value we've ever measured is much higher.
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  50. Tim... Have you watched Dr Alley's lecture yet?
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