Temperature drives CO2?

For years now, we have been seeing graphs of CO2 and temperature records, with an observation that the pattern of CO2 lags temperature change. The claim is then that temperature drives CO2, not the other way around. Often this is expanded to the proposition that the rapid recent increase in air CO2 has nothing to do with the huge amount that we have emitted.

In the past, indeed CO2 has responded to sea temperature changes. That is no surprise. And CO2 has not been driving temperature. The reason is that nothing has been driving CO2. Unless something pushes up (or down) CO2, it can't force anything. And for millenia, the total amount of carbon cycling through various forms on the Earth's surface has been fairly constant.

None of this has anything to do with our present situation. We are looking at the response to burning 370 Gigatons of carbon (so far). There's no doubt what is driving that CO2 production. And it's a new introduction to the surface total. It is about 2/3 of what the atmosphere held 150 years ago, so it's big. We are about to find out how CO2 drives temperature.

Below the break, I'll look at some recent expressions of the Temp drives CO2 meme.

Ice ages

This is the traditional form. You see a plot like this:

with the observation that CO2 peaks and rises lag temperature rises by a few hundred years. That article starts:

"There are still people who insist that changes in CO2 can explain the pattern of glacial and interglacial periods."

but doesn't say who they are. Know anyone? It's not the IPCC! In the AR3, in 2001 with the evidence very recent, they weren't sure, but had no trouble with the idea that CO2 lags temperature:

" From a detailed study of the last three glacial terminations in the Vostok ice core, Fischer et al. (1999) conclude that CO2 increases started 600 ± 400 years after the Antarctic warming. However, considering the large uncertainty in the ages of the CO2 and ice (1,000 years or more if we consider the ice accumulation rate uncertainty), Petit et al. (1999) felt it premature to ascertain the sign of the phase relationship between CO2 and Antarctic temperature at the initiation of the terminations."

Fluctuations in recent CO2

For a while now, variants of graphs matching short-term changes in CO2 with temperature have been circulated, popularized particularly by Murry Salby in various lectures (though not written documents). An apostle is a commenter called Bart, or Bartemis. In a typical version, the annual difference of CO2 is said to be predicted by the difference between average temperature anomaly and an "equilibrium value", which is a fudge parameter. Plots like this are shown:

It shows Southern Hemisphere temperature compared with the annual difference of the CO2 series, with an offset to get them to match. It shows a good correspondence of the El Nino peaks, and some ability to trach the other short term ups and downs.

Why the SH, you might ask? Bart says it's not a cherry pick, but is because the SH has most ocean. And yes, the global temp is not so good:

The peaks still line up, but the trends drift apart. NH is worse. The reason is that the SH had a particular ability to match the fairly modest rate of temp rise with the longer termCO2 difference trend, while keeping the scaling right. Other plots don't. It's hard to see that as an ocean effect.

It illustrates the main issue. Diffencing the CO2 curve emphasises the short term fluctuations in what is otherwise a rather boring CO2 history. But it is the long term AGW rise that is of interest, and differencing suppresses that. The trend would have been there as a constant, except that the two curves are arbitrarily displaced to match, so even that has gone. The wiggle matching in no way explains the main effect - the 30% or so increase in CO2 since we've been emitting.

This is clearly shown in the equivalent AR3 plot of differenced CO2:

Now the differences are not displaced, and are clearly positive. It's that steady positive value that is to be explained. And the AR3 shows the explanation - the emissions, which are much steadier, but rise with the air increases, and always exceed them. The question is not how the CO2 got into the atmosphere, but where some of it went.

Predicting CO2

That wiggle matching has been going on for a while, but Bart also has a plot which does try to do the right thing - to integrate his differential form and predict CO2 rise from temperature (his plots here). He calls it "very high fidelity":


But is it? I've reproduced the last plot, the differences in red, here, along with a quadratic regression in black:

Here the CO2 curve is just 24-month smoothed, and the temperatures were modified as integrated 0.184*(GISSLO-0.424). These are slightly different to Bart's. I used his numbers first, but the curve was rather different; with these parameters it looks the same. There may have been some rounding. In any case, the point is that the quadratic regression fits much better than the GISS derived version.

But the quadratic version could have been expressed as integrated 8.832E-5*(t-429) where t is time in months from the start (Feb 1959). It's of the same form as the GISSLO fit, but using t instead of GISSLO. So simple date is a better predictor than temperature!

Well, not quite. In fairness, I should optimise the coefficients by regressing against GISS as I did against time. That gives the green curve in the above plot. It's very close to the time regression. So a better conclusion is that GISS is no better that time. On the longer term, there is absolutely no basis for saying that temperature drives CO2.

Conclusion

Short term temperature fluctuations, like ENSO, cause short term CO2 fluctuations of a few ppmv. And very large longer term fluctuations, like Ice Age terminations, cause larger fluctuations of up to 100 ppmv. All dwarfed by the current increase, which is clearly driven by our emissions.

Differencing the recent CO2 history shows up short term fluctuations, which show this matching. But this says nothing about the effect of our steady emissions, which differencing and translating removes. When the differenced version is integrated as a predictor, it offers no benefit.

IPCC derivative