Stars increase in energy output as they age. As a hueristic during the Phanerozoic the figure of about 1% per 100 million years is given for the sun. During the Ordovician the sun would have been in the region of 4-4.5% less energetic
are commonly available and have been for a long time. If you find it odd that CO2 was at 3000ppm during the Ordovician, and yet the planet was glaciated, don't you think you ought to try to find out why that might have been the case?
Anyone thinking seriously about this issue should understand such a basic concept as increasing solar output over time, in fact you wouldn't even need to leave PO.com to know about it.
As I wrote a few months back: " Only 100 million years ago 1000 ppm was required for the Earth not to be in a cool period, and 500 ppm was probably the threshold for full glaciations, and during the oft-cited Ordovician, CO2 needed to above 3000 ppm just to avoid an ice age".
Ok, lets look at the holes in this argument. If the suns output is gradually increasing through time and the "balancing theory" you propose is working then there should be a continuous drop of CO2 during periods of relatively stable temperatures. You see this in a gross sense in the early Palaeozoic with CO2 dropping overall from 6000 ppm to around 3000 ppm . The devil, however, is in the details, In the case I mention of the Ordovician during early Ordovician the earth was certainly in a greenhouse, temperatures were around 25 C and CO2 was near 4000 ppm, at the end of the Ordovician temperatures drop to somewhere around where they are today and CO2 ....is still at 4000 ppm (didn't do a very good job of staving off a glaciation did it?). The question is then if we have this balance between increasing solar output and CO2 why did temperature suddenly drop.......and of course you'll say "well everyone knows CO2 and the sun aren't the only thing that controls climate" and that is my point exactly. To suggest there is a CO2 "tipping point" you have to have an airtight model, which we do not have. Further to this idea of a balance between solar output and CO2 lets look at the late Carboniferous to early Permian....CO2 and temperature were pretty close to where they are now....but you are saying solar output has steadily increased since then....that would be 300 hundred million years of increase, yet the temperature and CO2 is the same now. With increasing solar output should not temperatures be much higher now? And to that end the IPCC tells us that the contribution from solar (TSI) is miniscule at current. So if its been steadily increasing through the Palaeozoic given your theory it would have had negligible influence on climate back in the Palaeozoic? If that were the case it seems this argument about CO2 balancing increasing solar output is rather far reaching.
That being said it is apparent you are referring to the work of Dana Royer who has published a number of papers on his view that there is a tight link between CO2 and temperature in the Phanerozoic. Shaviv and others subsequently argued his calculations were incorrect and that the sensitivity of temperature and CO2 seems to be quite small.
http://www.phys.huji.ac.il/~shaviv/ClimateDebate/RoyerReply.pdfAs well Royer recently has noted that there are problems with the link between CO2 and temperature:
However, some intervals in Earth's past fail to show any consistent relationship. One conspicuous example is the Miocene (23.0–5.3 Myr ago), an Epoch where multiple advances of the Antarctic ice sheet are juxtaposed with a period of global warmth ≈15 Myr ago. Most CO2 records during this period are low [<300 ppm by volume (ppmv)]and do not covary with temperature ). These records imply that other radiative forcings such as changes in paleogeography or meridional heat transport were disproportionately more important than CO2 at this time.
When you know the result you want it isn't too difficult to come up with a model. Do you actually know what the balance between CO2 and temperature was in the Ordovician or for that matter now? Of course not simply because it is a multivariate problem where the actual forcings, feedbacks are not understood for the variables known and there is always the variables which are unknown. If the models couldn't have predicted the disconnect in CO2 and temperature over the past decade or so how can you use them to predict what the balance was back in the Ordovican?
And by the way....noone actually knows what caused the glaciation at the end Ordovican, there are theories the best being gradual drift of amalgamated continents over the southern pole, transgression followed by isolation and gradual freezing.
