CO2 and T, again

Its evident that some people are still confused by the T/CO2 relationship, so I'll have another go in fairly simple terms.

Let me start with the "official" position, if you like. There are two cases: the current one, where we're pumping out CO2; and the ice age one, where CO2 varies naturally. There is no reason to expect them to have the same timescales.

In the first case, we know that CO2 is going up quickly - on geological scales, its going up vertically (see inline from wiki). This is then expected (basic physics, simple and GCM models) to lead to T increases more-or-less immeadiately - with a decaying lag of a few decades for thermal inertia. Note that we have already, in about 100 years, caused CO2 to increase by about as much as the glacial-interglacial difference that takes 5 kyr-ish.

In the second case, the situation is not so clear. At glacial terminations, there is a lag between T and CO2 of about 800 years. The std explanation for this is that T increases, causing CO2 to increase a bit later, which feeds back into a T increase, thence CO2, etc etc, until we come out of the glacial period. Where exactly the CO2 comes from isn't clear. What triggers the initial T rise appears to be orbital variations (*not* solar variations) becuae the timing seems to fit.

BTW, for "how do we know that the CO2 increase is anthro..." there are any number of reasons why this is obvious. See RC for a nice polite explanation. A rather shorter one is at myths #5 - to summarise: we know we've emitted CO2, we know Co2 has gone up, you'd have to believe some magical process (a) removess the human CO2 and (b) emits its own to just compensate. Oh - and if you think that volcanoes pump out more CO2 than us, you should ask why there are no volcanic spikes in the record; and why it increases smoothly.

Now onto the "unoffical" position: which is "(a) CO2 lags T by 800 years in the ice cores (b) therefore CO2 lags T by 800 y always (c) therefore the current T rise is not caused by CO2". (a) is true-ish, see above. (b) is a non-sequitur (and indeed its obviously wrong, unless you think there was a truely vast T spike 800 years ago - and before the MWP-ers jump in, it would have to be far bigger than said by even the biggest MWP boosters). CO2 doesn't even always lag T for all the ice core record. But the main error in (b) is that different mechanisms are at work. Presently, we *know* we're increaasing the CO2. In the ice cores, there is no particular reason to expect CO2 to lead. That leave (c) unsupported.

Notice, though, that the unofficial/solar position *doesn't* actually explain the interglacial T/CO2 cycle at all. There is no evidence for solar variation on the appropriate timescales. This particular little point gets conveniently forgotten in the enthusiasm for something-anything-other-than-CO2.

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For my sins, I decided to listen to Murray "I have a theory" Salby talking about his ideas about why the recent rise in CO2 isn't human-caused (note that isn't his most recent UK tour; that's back in April). By all means read my notes below if you're interested in the various ways that he is wrong…
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I've been reading ClimateAudit (hey, only out of balance) and a poster their was musing that plants draw CO2 down to a 270ppm minimum (it isn't) got me wondering why CO2 doesn't fall much below 200pmm in the ice core record? Indeed, while we talk about why it goes up, do we know why it doesn't go right down? Is 200 ppm some kind of plant determined limit given the way the planet is geologically configured atm?

[Hmm, dunno is the answer -W]

By Peter Hearnden (not verified) on 11 Mar 2007 #permalink

Will do Eli, RT(W?)FR and all that ;)

By Peter Hearnden (not verified) on 11 Mar 2007 #permalink

Carbon dioxide follows temperature and not the other way around because there is a period of time where gases within a glacier are able to mix with the surrounding atmosphere, it is not for many years (800?) that the gas bubbles are completely removed from the system by ice entrapment. Therefore, when ice cores are dated the gas within them is actually older.

[Gas ice age differences certainly complicate the problem. But its not the whole problem, and these results are with that difference removed as best we know how -W]