Posted by: reasic | 15th Mar, 2007

800-year CO2 Lag Explained

I recently received what I thought was a very good question on the issue of the 800-year lag between the rise in CO2 and temperature during the warming period between ice ages. Seeing that this issue is used by many skeptics as proof that CO2 does not affect our climate, I thought I’d put a little more effort into this specific question and post my response, instead of just commenting. Here is the question, as posed by “Jeff”:

Thanks for taking the time to refute the show’s points! I still have a problem with the “CO2 lags temperature” rebuttal from the RealClimate website, though. Simply, it seems to be incredibly lame, and, in my opinion, just plain wrong. This is his argument (written in 2004, well before the release of the TV show):

“From studying all the available data (not just ice cores), the probable sequence of events at a [ice age] termination goes something like this. Some (currently unknown) process causes Antartica and the surrounding ocean to warm. This process also causes CO2 to start rising, about 800 years later. Then CO2 further warms the whole planet, because of its heat-trapping properties. This leads to further CO2 release.”

(There’s more, but the above sentences are the meat of the argument).

Here’s the problem. I don’t think the author has any understanding of feedback theory, because he’s describing a classic positive-feedback system, which is *not* what the historical record seems to shows. Further, if we use his argument, the world’s temperature should have continued to spiral up and up and up, which it did not do. In other words, following his last sentence: “This leads to further CO2 release” should have been added “which leads to more warming, which leads to further CO2 release, which leads to more warming, which leads to…”

You see the problem?

Instead, if CO2 indeed lags temperature, one could just as easily make the point, and make it more accurately than the above argument (given the historical geological record) that, in fact, CO2 aids in temperature *regulation*. Like this:

1. Temperature goes up, which causes release of CO2.
2. CO2 goes up, which causes temperature to go down.
3. Temperature continues to go down, CO2 goes down.
4. With lower CO2, temperature begins to rise again.
(return to ‘1' and repeat)

So I have to wonder - what in the world is going on? Why hasn’t such a glaring inconsistency been explained? This really bothers me, because, if CO2 indeed lags temperature, it would seem to blow the whole theory of CO2-induced climate change out of the water. Is there any other hard data (not models!) which support this theory?

That first question I tried to answer was probably the most confusing, and as a result, was most criticized. So, I’ll try to clear it up here.

First, we must establish what is known about the relationship between CO2 and temperature. I saw a comment recently by Fermiparadox, that I thought summed it up rather nicely:

In a nutshell: The sun radiates mostly in the visible spectrum, for which CO2 is transparent. The same amount of energy has to be reradiated back to space by the earth. Ideally, the sun and the earth emit a black body spectrum. Because the temperature is lower, the earth does this mostly in the infrared spectrum. But for IR, CO2 is not transparent, so it absorbs the radiation coming from below. It also reradiates it, but in all directions, and half of this back to the surface. This mechanism is well known, and has been for a long time.

This has been proven through research, so there is no doubt that greenhouse gases warm the atmosphere.

Second, the scientist that discovered the time lag between the rise of CO2 and temperature during the warming period between ice ages actually concluded in his paper that this lag “is still in full agreement with the idea that CO2 plays, through its greenhouse effect, a key role in amplifying the initial orbital forcing”:

First, the 800-year time lag is short in comparison with the total duration of the temperature and CO2 increases (~5000 years). Second, the CO2 increase clearly precedes the Northern Hemisphere deglaciation.

The idea that this lag somehow debunks the theory of anthropogenic global warming did not originate from the scientists who discovered the lag. It has come from right-wing pundits and skeptics.

Finally:

…if we use his argument, the world’s temperature should have continued to spiral up and up and up, which it did not do. In other words, following his last sentence: “This leads to further CO2 release” should have been added “which leads to more warming, which leads to further CO2 release, which leads to more warming, which leads to…”

You see the problem?

Yes, I understand your concern. I’m no expert, but I’ll try my best to explain this warming process as I understand it. First, some unknown event triggers the warming (and there are several theories on what this event is). Then, the warming triggers a mixing of the deep ocean, which releases CO2 (a process known as “outgassing”). There is a finite amount of CO2 in the deep ocean, so CO2 cannot be released into the atmosphere indefinitely. Also, it is believed that the 800-year period is also the mixing time for the deep ocean, further solidifying this theory. So, there is a limit to the amount of natural CO2 that can be released, making a never-ending cycle of temperature increase unlikely.

Your question was very well-worded and civil, and I thank you for that. I know this is a very complex issue, so I hope this answer has helped to clear up any confusion on the matter.

UPDATE: This post has been refuted by a skeptic named Lubos Motl. See my response here.

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