CO2 Acceleration

The question arose recently, is it really true that atmospheric CO2 concentration is not just increasing but actually accelerating? Let’s take a look at some data.

Mauna Loa recently released the estimated CO2 concentration for December 2008, bring the year to a close. The initial value was a repeat of the value for November, which would be a very unusually low value (CO2 generally increases from November to December as part of its annual cycle). But this appears to be just a simple mistake because the value has been revised, the new figure fitting expectation well and conforming to the existing annual cycle pattern. Here’s the current data from Mauna Loa:

CO2 concentration shows both a long-term increase (indicated by the smoothed curve) and an annual pattern. We can remove the annual pattern (and the average value) in order to define CO2 anomaly:

I’ve plotted a straight-line fit to the anomaly data, which indicates visually that the CO2 concentration follows a curved path rather than straight, one which curves upward. So let’s remove, from the original data, both the average annual cycle and a best-fit straight line, which will give us CO2 residuals:

On this plot I’ve superimposed a parabola (a 2nd-order polynomial). The parabola indicates that the CO2 concentration does indeed curve upward, and the amount of curvature indicates that the growth rate of CO2 concentration is increasing at about 0.024 +/- 0.005 ppm/yr/yr. That increase is most definitely statistically significant (in fact it explains nearly 91% of the variance of these residuals). That does not mean that CO2 concentration is following a parabola, or that the growth rate is increasing with perfect regularity; it isn’t. But it does confirm that the growth rate is not constant, it’s been increasing.

We can also take the original CO2 anomaly data and determine the growth rate separately for each decade. We get this:

Clearly the decadal growth rates are on the rise, although just as clearly the increase in growth rate has not been steady. Nonetheless, even with only five decades we get a statistically significant increase in the growth rate of 0.029 +/- 0.009 ppm/yr/yr.

We can also compute annual average CO2 concentration, and from those we can compute the annual growth in CO2 concentration:

Once again we see that the growth rate has increased; linear regression indicates that it’s increasing by about 0.026 +/- 0.009 ppm/yr/yr, so this result too establishes the increase of growth rate with statistical significance.

In short, there’s no doubt whatever that not only is atmospheric CO2 concentration growing, the growth rate itself is also growing. The rate of increase of CO2 since 2000 is about 2.1 ppm/yr, and the long-term growth rate of the growth rate is about 0.025 ppm/yr/yr.