CO 2 in our atmosphere is still increasing. In fact this year the annual average amount has passed 400 ppmv (part per million by volume) for the first time in a long time — at least a million years. The reason: we’re burning fossil fuels like oil, coal, natural gas. When we do, it turns that long-buried carbon into carbon dioxide, which ends up in the atmosphere. It’s as simple as that.



Lately, though, more and more are talking about reducing our emissions of CO 2 . So, how is planet earth doing? We haven’t stopped increasing atmospheric CO 2 , but is there any sign that at least we’ve slowed down?

Here’s CO2 concentration from the Mauna Loa atmospheric observatory:

Clearly there’s an annual cycle; we can see the planet “breathe” in and out each year. During spring and summer in the northern hemisphere (where most of the land is), plants grow, absorbing CO 2 from the atmosphere. During fall and winter they decay, releasing CO 2 back into the air.

So, the first thing I’ll do is remove the annual cycle to give a clearer picture of the changes which are not part of that annual cycle of plant growth and decay. Then I’ll compute yearly averages of such “de-seasonalized” data. We get this:

Yes, it’s still going up. But how fast? I’ll estimate it in two ways. For one, I’ll fit a smooth curve, and a piecewise-linear fit with changepoint analysis, and use those to estimate when and how the growth rate has changed. For an even simpler method, I’ll just take the difference in the amount from one year to the next. Both will give me estimates of how much CO 2 is increasing each year, which we can examine for growth rate changes.

Change-point analysis suggests three moments of rate change, at all of which the rate increased:

The changes are a lot easier to see if we first subtract a straight line from the data, showing “linearly de-trended” data and its changes. That gives us this:

This suggests that there’s no change in the growth rate after 2000 — at least, none we can confirm with confidence. We do note, however, that the growth rate has increased over time. This models it as sudden changes from one time span to another, but that’s not necessarily the way it’s really happening.

Here are the year-to-year rates, with a straight-line fit:

The increase from each year to the next, a.k.a. the growth rate, is itself increasing. It fluctuates from year to year, but the overall increase is evident (and “statistically significant”). The present rate of increase is a smidgen over 2 ppmv/year.

Let’s add the rate determined by a “smooth fit” to the data:

Nothing new here. As we saw with year-on-year data, the rate of increase itself is itself increasing. There are variations, but overall the rise in the growth rate has been consistent, and there’s no real evidence that the growth rate has stopped increasing, let along started to decline.

It seems that we know this much: CO 2 in the atmosphere is still increasing, and the rate of increase has itself been increasing. But it’s not yet clear whether that rate has changed since 2000.

My best estimate of the present rate comes from taking the monthly data since 2000, and doing a simultaneous fit of an annual cycle and a linear increase.

That estimates the current growth rate is 2.07 ppmv/year (+/- 0.04 ppmv/year).

The world is finally waking up to the fact that to avoid climate disaster, we need to reduce CO 2 emissions. But it seems not yet to have realized that what we really need to do is stop CO 2 increase. The frightening truth is that not only have we failed to stop CO 2 growth, we haven’t even slowed it down.

The more frightening truth is that as warming increases, we run the risk of triggering feedbacks in the carbon cycle. If the warming we’ve already brought about, or that soon to come, releases yet more CO 2 from sources other than fossil fuels, well then … the phrase that comes to mind is, “We’re fucked.”