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& Quoting David & Alison Webster <dwebster@glinx.com>:> > Hi Patrick & All, Nov 26, 2006 > Thanks for these URLs. The top one led me to > http://www.aip.org/history/climate/cycles.htm#M_52_ > which contains a good account (or so it seems to me) of climatology > over the years. > > One passage in this account has me baffled and perhaps someone can > explain. The passage being "The changes in the atmosphere could also > answer the old persuasive objection to Milankovitch's theory -- if > the timing of ice ages was set by variations in the sunlight falling > on a given hemisphere, why didn't the Southern Hemisphere get warmer > as the Northern Hemisphere cooled, and vice-versa? The answer was > that changes in atmospheric CO2 and methane physically linked the two > hemispheres, warming or cooling the planet as a whole.(52*) " > > What configuration of tilt and orbital distortion could lead to one > hemisphere receiving more insolation in e.g. summer than the other > hemisphere would receive 6 months later ? Tilt would have to be > symmetrical and even if the sun was at times not exactly at the > intersection of the major and minor axis of the orbital ellipse, > surely this assymetry would not flip in the space of 6 months. > Yours truly, Dave Webster, Kentville > Dave: Puzzling and interesting. While it's fresh in the mind I think the answer may be that you are focussing on the primary effect, the change in insolation with earth's obliquity. As you say, this must be symmetrical, just phase shifted 6 months in the two hemispheres. However, the more important secondary effect, the amplification of this, is not symmetrical. According to the AIP sources, part of the reason Milankovitch's (M) ideas were discounted for several decades was that the primary effects predicted are so feeble -- only a ~1 percent change for the extremes of obliquity -- while the global cooling during periods of glaciation was much more severe than this would predict. (Another reason was that the M cycles didn't match the simplicity of the 4 periods of glaciation for which there was evidence back then: this evidence was simply incomplete -- there turned out to be many more than 4, which do match M cycles better). So you need secondary amplification mechanism(s), found for example in changes in the earth's albedo. The N hemisphere cools a bit in summer as the obliquity of earth's axis increases a little (axial tilt relative to the plane of its orbit, one of the M cycles). It then snows in winter a bit more and longer and the snow comes a bit earlier in the fall in the N. hemisphere, so summer snow cover increases on land. This increases the reflectivity of part of the N. surface, so it cools a bit more than it otherwise would do. This slight extra cooling persists through the next year, so the the snow cover increases some more and so does the cooling until finally all of the N. land surface is covered by snow, the albedo cooling effect is now massive and then secondly, the N. seas finally freeze over too, locking up water as snow fails to melt and turns to glacial ice. The same weak primary driving force, lowered insolation, acts 6 months out of phase in the S. hemisphere, but now has little effect because there is not much land down there to capture the extra snow, only a thin bit down in Patagonia (in contrast to Canada + N. Europe + Russia-Siberis in the north). So the snow falls and melts on the sea which cools a bit but doesn't change its reflectivity, so there's no amplification in the south. This leads to the apparent paradox mentioned in the clip, that the Antarctic and Greenland ice core evidence points to symmetric world-wide cooling during the glacial periods, not to an effect just concentrated in the N. hemisphere. This requires an explanation linking climates in the two hemispheres. The explanation offered is that the N and S hemispheres' climates are linked via the "greenhouse gases", water vapour, methane and CO2. I don't know how this works since I don't have the supercomputer model, but guessing simplistically, much of the vegetation dies in the north and the swamps freeze over, so the CO2 and Me levels emitted into the atmosphere drop, and their greenhouse contributions fall, amplifying the northern albedo cooling some more. Because atmospheric mixing is relatively fast (relative to ocean mixing), this effect proceeds with little delay in the S and well as the N, linking the two climates via the changes in greenhouse gas concentrations (not sure if this also applies to water vapour though). The huge difference in land masses in the N and S critical to this explanation is mentioned briefly in reference 16 of your clip, or of one of the AIP links that I pursued -- I forgot to note which. Steve
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