Dear Gene--Your comment suggesting that the greenhouse effect can saturate and so adding CO2 will have no greater effect was one of the two major criticisms from when Arrhenius made the proposal in 1897 or so that a rising CO2 concentration could change the climate. This criticism was refuted long ago, yet keeps coming up, so I'll attempt to refute it again and let's let it rest in peace. If you, as you say, want to find a way to reduce the greenhouse effect of the gases, the way to do that is to have lower GHG concentrations.
The understanding (aside from the meticulous hand-done calculations of Arrhenius) that overcame the criticism regarding the bands saturating came in the early 1960s with the first computer models of atmospheric radiation, which made clear that one really has to think about the atmosphere as a series of layers. It is easiest in my view to think of these layers as having a thickness of a given infrared opacity at a particular wavelength (in reality this is done by the atmosphere at each wavelength--but consider just one wavelength near the center of the CO2 absorption band). So, aside from some nonlinearities, each layer might be thought of as having an IR opacity of some amount (and calculations would be more an more accurate the thinner the layer) and so would have some amount of carbon dioxide--and each layer would also have some average temperature, varying in the vertical (at the lapse rate in a convectively active region like the troposphere). With the arrangement of layers, one can calculate the radiative flux at any given level. To get the downward radiation, layers above radiate down based on their temperature and their opacity (they radiate as they absorb) and the flux has to pass through the layers between the given level and the level of the layer radiating accounting for each layer's opacity. If there is still some opacity, one counts outer space as radiating down at zero (or 2.8) Kelvin. To get the upward flux, one looks to layers below, doing the same thing, and when one goes through the layers below, one has the rest of the radiation coming upward from the surface, which act as a black body radiator (or slightly less given the surface type). And, of course, one has to do this at all wavelengths and consider all absorbers (the infrared radiation subroutines of global models tend to do the calculation in a number of spectral bands and check their representativeness against more detailed spectral calculations). When one does this for the world as a whole (which involves a lot of approximations, etc. but okay conceptually), and we calculate the upward flux at the top of the atmosphere (and across all gases, etc.), one finds that, viewed from space, the Earth radiates as would a blackbody at 255 K (and this gives off the amount of energy that matches the net absorbed solar radiation--so incoming less 30% reflected). The average global surface temperature is about 288K and with the typical lapse rate of 6.5 K/km (the rate of decrease in temperature with height), the flux to space is coming from about 6 km up [(288-255)/6.5]. At the surface, one can also calculate the upward and downward radiation. Upward is based on average surface temperature of 288 K, and downward radiation is about 83% of that [to see a diagram, go to http://faculty.gg.uwyo.edu/neil/teaching/Geomorph/lect_images/EarthsEnergyBa lance.png as an example]. Do a bit of math and this suggests that the average temperature for the downward radiating temperature is about 13-14 K lower than the surface temperature, so the average height of the downward radiating temperature for all gases on average is about 2 km. So, now let's increase the CO2 concentration (or any other greenhouse gas, and including water vapor as temperature rises). Basically, layers with the same opacity increment as before will be thinner. And so, looking up from the surface, with thinner layers, the radiation from each layer that contributed to the downward flux will be coming from a layer that is at a warmer temperature, so the total flux will be greater (that is, the natural greenhouse effect will be enhanced). That will lead to an accumulation of heat at the surface that will make the layer warmer and so more radiation will be emitted upward, absorbed at a lower average layer and the system will warm. >From space, looking down into the atmosphere before its temperature changes, each layer will have a higher opacity and be thinner, so the radiation making it to space will be coming from layers that are, on average, higher and therefore colder. So, less radiation will be going to space. The planet will therefore be out of balance, absorbing more solar than the energy being radiated away. This will lead to warming until the atmosphere gets warmed enough so that the temperature of the layers (now higher up in the atmosphere) that are radiating to space are warmed up enough to restore the energy balance. The assertion that a wavelength band through the whole atmosphere is saturated and so adding more of the gas will have no more effect is thus just plain wrong. What matters is both the amount of the gas and the temperature of the emitting layers. Were the atmosphere isothermal, this all would not matter, but it is not and cannot be given that the pressure decreases with height. Once one has more radiation being trapped, then one has to get to the temperature change, and this involves more processes that I won't cover--just to note that they combine to make it so that the relationship between temperature change and CO2 concentration is logarithmic, but this should not be interpreted to mean that adding more CO2 is not leading to changes in the fluxes of radiation due to saturation of the bands. Mike MacCracken On 4/2/09 7:38 AM, "[email protected]" <[email protected]> wrote: > > All the discussion here is making sense. Clearly the initial goal is to > first reduce solar radiation energy and/or greenhouse layer radiation energy > incident on the surface in the Arctic region and to reduce the melting of > ice and to evaluate potential negative consequences. It is not to cool the > planet initially, just the surface in the Arctic. Indeed, if not for the > Artic and the melting the rest of the planet is currently doing fine. > However, when sunspots return at some future time and the rest of the planet > is warming, what is learned in the Arctic may be applicable. > > The use of the term runaway is a bit misleading. The radiation back from the > greenhouse layer is graybody radiation and it has a limit when it becomes a > black body and then adding more greenhouse gas has no impact and produces no > additional heating of the surface. The greenhouse layer heating ultimately > saturates when it becomes a black body, independent of additional increases > in the concentration of the greenhouse gases, hence no runaway. > > Anybody have any good ideas for safely cooling the greenhouse layer gases so > as to reduce the greenhouse layer graybody radiation flux to the Earth's > surface? > > -gene > > -----Original Message----- > From: [email protected] > [mailto:[email protected]] On Behalf Of Sam Carana > Sent: Thursday, April 02, 2009 12:01 AM > To: Geoengineering > Subject: [geo] Re: the limits of geoengineering? > > > Sure, François, but let's acknowledge that doing nothing is also taking a > decision. Doing nothing, with the argument that this was better because > something might not go as planned, that's not really an option. People are > doing things without fully knowing what will happen, i.e. we're emitting > greenhouse gases at such a scale and to such an extent that feedback effects > are taking place that could result in a runaway greenhouse effect even if we > did magically decide to stop emission altogether. > > If we're to start somewhere (and I advocate that we do), then we might as > well start close to the arctic, and if we'll be starting now (as I > advocate), we can start gradually, thus getting a better understanding as we > go (small-scale). Of course, any such attempts will be regarded as a trial, > and of course, if something unexpectedly did go wrong, there will need to be > plans for adjustments or even to abandon further efforts. That speaks for > itself. But we should get things started now in order to be able to monitor > things. If there are any reasons to believe that something could go wrong, > please post a message with details. But the longer we wait with this, the > greater the chance that immediate and more dramatic large-scale actions will > need to be taken without much insight in what will eventuate. > > Cheers! > Sam Carana > > > > On Thu, Apr 2, 2009 at 3:32 AM, f.m.maugis <[email protected]> wrote: >> Sam, >> >> I do not agree completely with you. The question of temperature >> increase or decrease is not so simple. It seems to me that the >> important action is to try to decrease the temperature somewhere on >> the planet. Then, we see what happen. It is very possible that >> decreasing 2 or 3 degrees in Africa can affect Siberia with 7 degrees. >> For the moment nobody knows. Trials have to be done. >> >> Sincerely >> >> François MAUGIS >> http://assee.free.fr >> ====================================================================== >> ====== >> ============= >> -----Message d'origine----- >> De : [email protected] >> [mailto:[email protected]] De la part de Sam Carana >> Envoyé : mercredi 1 avril 2009 15:49 À : Geoengineering Objet : [geo] >> Re: the limits of geoengineering? >> >> >> >> Andrew, >> >> Looking at global historic data of warming, one might at first glance >> conclude that only a relatively small amount of global warming did >> cause the arctic melting we're witnessing now. So, the conclusion >> would then be that we similarly needed to reflect just a small amount >> of sunshine back into space to have the reverse effect. >> >> However, what complicates things is that global warming is amplified >> in specific areas, notably around the North Pole. East Siberia was at >> times 7°C warmer than normal during the summer of 2007 and the East >> Siberian permafrost region alone contains 500 billion tonnes of >> carbon. So, we would need to achieve much more cooling there to avoid >> further melting. In other words, it's going to be hard to come up with >> exact amounts of water vapor or aerosols that needed to be released to >> achieve the same amount of cooling worldwide. >> >> And of course, we don't need to achieve the same amount of cooling >> everywhere. What is most urgent is the arctic melting of sea-ice and >> permafrost. Therefore, I suggest to make a start by getting ships as >> proposed by Stephen Salter to cruise as close to the arctic as possible. >> >> Such a proposal should be discussed and considered at the Major >> Economies Forum on Energy and Climate, set in Washington this month, >> followed by a summit in Italy in July. Obama has invited the leaders >> of 16 major economies, as well as the secretary general of the United >> Nations, to participate in this forum. >> http://voices.washingtonpost.com/44/2009/03/28/obama_sets_internationa >> l_clim >> a.html >> >> This Forum seems to focus only on energy. We should present the forum >> with a similar Open Letter that we recently presented to Dr Pachauri, >> head of the IPCC, in which we suggested that a comprehensive approach >> to global warming should consist of three parts: >> >> Part A: Emissions reduction >> Part B: Carbon stock management >> Part C: Heat transfer and radiation management >> >> I hope to convince further members of this group to become signatories >> to such an Open Letter to this Forum. >> >> Cheers! >> Sam Carana >> >> >> >> On Wed, Apr 1, 2009 at 8:31 PM, Andrew Lockley >> <[email protected]> >> wrote: >>> I'm not so concerned about the absolute physical limits of the >>> process (as in max temperature or forcing reached). What concerns me >>> is the ability to avoid dangerous climate change by using geoeng to >>> arrest or reverse dangerous effects. >>> I'd be really interested to see calculations that show whether geoeng >>> can or cannot 're-freeze' the Arctic. If it cannot, then maybe John >>> Nissen is right and we have a very short time to act. If it can >>> re-freeze it, then the risks of 'full scale' SRM geoeng probably >>> outweigh the benefits by a substantial margin at this stage, and for >>> some time to come. (Much further research is needed into geoeng and >>> also ice dynamics and carbon cycle feedbacks). >>> I think that this is one of the most fundamental things we have to >>> demonstrate when considering the case for geoeng. Is there anyone on >>> this list who'd be willing to bring forward a paper or informal >>> research on the interaction between ice-albedo feedback and SRM geoeng. >>> A >>> >>> 2009/4/1 Eugene I. Gordon <[email protected]> >>>> >>>> As you probably know greenhouse gas is a positive feedback on global >>>> surface temperature but saturates when it becomes a black body. As >>>> the concentration of greenhouse gas goes up the layer become blacker >>>> and blacker (in the IR that is) As it becomes blacker the amount of >>>> radiation sent back to the Earth's surface saturates. IT IS NOT A >>>> REFLECTOR NOR IS THE LAYER GOING TO GET HOTTER AND RADIATE MORE >>>> ENERGY BACK AS THE EARTH WARMS UP. So there is a concentration of >>>> CO2, methane, water vapor etc. above which the greenhouse layer >>>> sends back a maximum amount of energy independent of the >>>> concentration. The positive feedback saturates. Does anyone know >>>> what that level is and what fraction of the radiation from the >>>> surface is currently returned? In any case there is no tipping point >>>> and there may be a limit to how high the surface temperature can go >>>> (of course there is a limit, it is about 24 C). Geoengineering other >>>> than CO2 removal controls the forcing function (the sun intensity at the > Earth's surface). >> There is a lot of loose talk about tipping points that needs to be >> eliminated. >>>> ________________________________ >>>> From: [email protected] >>>> [mailto:[email protected]] On Behalf Of Andrew Lockley >>>> Sent: Tuesday, March 31, 2009 6:49 PM >>>> To: geoengineering >>>> Subject: [geo] the limits of geoengineering? >>>> >>>> There's been quite a few suggestions on this list that we need to >>>> use geoengineering whilst we still can. >>>> I'm not convinced by this argument, and I'd like to investigate how >>>> long the 'window of opportunity' for geoeng is. >>>> Lenton and Vaughan found that Aerosols could counter a doubling of >>>> natural CO2, and space sunshade (if possible) could go much further >>>> This seems to suggest that we have quite a while to act. Even in >>>> the even of the start of a methane pulse which gives a doubling of >>>> co2 equivalent, we can still ramp up geoengineering intervention >>>> quickly if needed. I suggest that we'd have at least 5yrs or so >>>> warning, even of a really sudden methane pulse from permafrost. >>>> However, the big risk, as I see it, is that ice-albedo feedback will >>>> be too strong in the arctic to overcome with geoeng. I know that >>>> re-freezing will occur in winter, but the thickness of the ice may >>>> not be sufficient to last all year, even with geoeng. If this >>>> happens, we may be unable to stop the methane pulse from permafrost >> occuring (if it indeed does). >>>> I don't have the skills needed to do the complicated sums, nor do I >>>> have access to clever computers that can do them for me. Could >>>> someone therefore suggest how long we've got before the 'window of >> geoengineering opportunity' >>>> closes - if it indeed does? >>>> A >>> >>> >>>> >>> >> >>> >> >> >> > > > > > --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "geoengineering" group. To post to this group, send email to [email protected] To unsubscribe from this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/geoengineering?hl=en -~----------~----~----~----~------~----~------~--~---
