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 -~----------~----~----~----~------~----~------~--~---
