Open Mind

A (brief) Tale of Three Sites

August 7, 2008 · 18 Comments

There’s been quite a bit of discussion about CO2 data lately. Most of it has centered around Anthony Watts’ nonsense and the recent revision to the Mauna Loa monthly averages. What’s been lacking is a look at what the data are telling us about the changing CO2 concentration of the atmosphere. So let’s take a “quickie” look at monthly average CO2 data from three different locations: Mauna Loa, Barrow (Alaska), and South Pole station.

Most of us have seen the graph of Mauna Loa CO2 data:

Superimposed on the steady increase of CO2 is an annual cycle of ups and downs. Data from Barrow, Alaska don’t cover as much time, not starting until mid-1973. They do show a larger annual cycle than is observed at Mauna Loa, but the long-term increase is similar:

South Pole data start even later, in late 1975. For this location the annual cycle is smaller than observed at Mauna Loa. But again, the long-term trend is similar:

We can see how similar the long-term trends are by removing the annual cycle and smoothing the data (on a 3-year time scale):

Although the trends are remarkably similar, the actual values are slightly different. Barrow tends to have the highest values, South Pole the lowest, and Mauna Loa in between. Here’s the difference in the smoothed values between Barrow and Mauna Loa, and between Mauna Loa and South Pole.

The differences are indeed small, between 1 and 3 ppmv.

The most pronounced difference between these locations is the size of the annual cycle; we can get a better view by examing a brief 5-year time span of the data:

Obviously Barrow shows by far the largest annual cycle, South Pole by far the smallest, and again Mauna Loa falls in between. We can use a wavelet transform to estimate the amplitude of the cycle (actually, the amplitude of a best-fit sinusoid to the data). Here’s the semi-amplitude, which is just half the amplitude:

There’s another important difference. For Barrow and Mauna Loa, CO2 concentration peaks during northern hemisphere winter/spring but for South Pole it peaks about six months later, during northern hemisphere summer/autumn.

This pretty thoroughly refutes an idea which some (who don’t really know what they’re talking about) have suggested, that the changes we’ve observed in CO2 concentration are due to changes in ocean temperature. Even the annual cycle, so they say, is due to ocean temperature changes, specifically that it’s controlled by temperature changes in the southern oceans. The argument is that during northern hemisphere summer it’s southern hemisphere winter, so the bulk of the oceans (which lie in the southern hemisphere) are cooler than during the opposite seaons, and due to the increased solubility of CO2 in colder water they draw CO2 from the atmosphere and reduce the global average atmospheric CO2 concentration. Six months later the seasonal cycle is reversed and the warmer southern hemisphere oceans release CO2 back to the atmosphere — hence the annual cycle in CO2. By tying CO2 changes to temperature change in the southern oceans, they hope to persuade people that it’s not human activity that’s the cause of CO2 increase, it’s temperature change.

But the observed data flatly contradict this. For one thing, Barrow is further from the southern oceans than Mauna Loa but shows a much larger annual cycle. But mainly, the South Pole is on a continent surrounded by the southern oceans, but its annual cycle doesn’t follow the prescribed pattern, it’s the opposite.

In fact the annual cycle in CO2 concentration is due to the uptake of CO2 by land plants during the growing season, and its return to the atmosphere when plant matter decays in the opposite season. That’s why it shows opposite phase in the two hemispheres — because the growing seasons are reversed in the two hemispheres. It also explains why the cycle is so much smaller in the south; there’s far less land in the southern hemisphere, so there are far less land plants to cause an annual cycle of CO2 changes.

I can understand why some people disbelieve in anthropogenic global warming; climate change is a complex subject and there’s a lot of cleverly crafted propaganda denying it. But those who disbelieve that humans are the cause of atmospheric CO2 growth have to go to great lengths to avoid seeing, or believing, the absolutely undeniable evidence that the cause is human activity. Such people aren’t skeptics at all — if they were, they’d investigate the outlandish claims of ocean-temperature control over CO2 (both its long-term growth and the seasonal cycle) and dismiss the idea as nutty. They’re simply in denial.

Categories: Global Warming

18 responses so far ↓

  • Doug Clover // August 7, 2008 at 5:25 am

    Dear Tamino

    First, thanks for your blogs on aspects of trend analysis I have learnt more from you than a whole year of undergrad econometrics.

    Second, you don’t have to go all the way to the south pole for some SH data.

    I hope this of interest

    kind regards


  • michel // August 7, 2008 at 6:45 am

    I thought that the proof that the rises were due to human agency was in the signature of the makeup of CO2 in the atmosphere. Is this not true? There was some way of distinguishing natural from fossil fuel combustion sources, and from this it was proven that human activity is the cause.

    Or has this subsequently been disproved?

    [Respoinse: No, the isotopic evidence is still true. But this post isn't about that.]

  • Duae Quartunciae // August 7, 2008 at 9:10 am

    I see you often use wavelet transforms for smoothing, especially when there is a periodic effect to be smoothed away. Is there a brief description of how these transforms work somewhere? Something that would let me code it into a spreadsheet? I’ve read a bit about it, but there seems to be an awful lot of choice about what wavelets to use and how to pick parameters.

    [Response: I don't think it would be very easy to do it in a spreadsheet; you might have to write an actual program. Also, it's well to have a good working knowledge of the behavior of wavelets before applying them for smoothing. I suggest some web searches for tutorials, and for free software. Good luck.]

  • Callan Bentley // August 7, 2008 at 10:56 am

    Well done. Gotta say, you’ve been putting up some excellent posts this week. Keep at it! And thanks!

  • J // August 7, 2008 at 12:38 pm

    Nice post, Tamino.

    I’m sure you already know this, but…. Looking at the Barrow data, you can see that the annual cycle is strengthening. See, for example, the increasing trend in your wavelet-transformed graph above. By my calculations, the amplitude of the [May-August] difference has increased by 0.08 ppmv per year at Barrow. This is much less visible at MLO and the South Pole, for obvious reasons.

    It also might be interesting to look at the “shape” (not just amplitude) of the annual cycle ….

    [Response: The amplitude of the annual cycle at MLO has also increased, but it may have been a "step change" rather than a steady trend. And the shape of the cycle at MLO has changed as well. Maybe I need to do *another* post about CO2...]

  • George Tobin // August 7, 2008 at 2:57 pm

    Is there a simple explanation why the rate of CO2 growth appears so linear? Given the continued growth of human output, warming of the permafrost, reduced ocean capacity etc shouldn’t all that make the increase accelerate more than it has?

    [Response: The graph isn't nearly as linear as visual inspection might suggest; remember we're only looking at 50 years. It has accelerated quite a bit in that time.

    Maybe I *do* need to do another post about CO2...]

  • Hank Roberts // August 7, 2008 at 3:16 pm

    Coby could borrow from this to improve “it’s the oceans” in his list of refuted bogus alternatives.

  • Dano // August 7, 2008 at 4:50 pm

    I say again: you have great skill at conceptualizing topics, distilling them, and reformulating them into usable forms.

    Keep up the good work, sir. And well-placed snark has a place in dialog or writing.



  • cat black // August 7, 2008 at 6:10 pm

    [amplitude change] Since the amplitude is a net product of plant uptake and decay releases, it is fair to predict that warming will increase both, but in particular the latter as formerly frozen carbon sources start to release their sequestered carbon. Likewise the amplitude change would be in the north in particular. Another tell-tale canary just dropped dead, perhaps.

  • swade016 // August 7, 2008 at 7:05 pm

    Kind of off topic, but can you recommend a good book on time-series analysis?

    [Response: Hmmm... What's your education level in mathematics?

    (maybe I should just recommend you wait 6 months or so ... I might finish mine by then)]

  • swade016 // August 7, 2008 at 7:46 pm

    BS Mathematics, MS Operations Research.

    Masters wasn’t very specialized because OR is all over the place, but I had 2 classes on stats theory (all the calculus crap like moments, etc…) and 3 on analysis (stochastic models, regressions, anova, etc…) Plus I’ve got all the linear algebra, diffEQ, vector analysis stuff…

    Also, if you announce your book on here, won’t we all know your *real* name then? :-)

  • george // August 7, 2008 at 11:31 pm

    I’ll second the comment of Doug.

    If your book is even half as good as the stuff you post here (or includes even half the stuff you post here!), it will be better than all of the textbooks I used in college.

    You have a real gift for making otherwise complicated stuff appear obvious.

    I’m sure there are some who wish you were not quite as good as you are at this!

  • Jim Arndt // August 8, 2008 at 2:48 am

    Hi Tammy,

    They had a nice response to Anthony. Glad to see the openness and making it clear right away. You are correct that it is no big deal. Now if for instance the PPM went from 385 to 383 in a year that would be significant. But just reacting to noise is like calling the Police if a car drives by with a loud stereo going. Not much you can do and no effect on the quality of life. My 2 Cents.

  • Curious // August 8, 2008 at 9:48 am

    Thanks a lot for all your anti-skeptic education. It is really helpful to spread the reality about science of climate change all over the world.

    Regarding antrhopogenic atmosferic CO2 growth, I cannot understand how can it be denied: natural reservoirs can only have one of three net effects: absorbing, equilibrium or sinking. If atmospheric growth is lower than our emissions, and the extra CO2 can only go to the natural reservoirs…then natural reservoirs must be net sinkers, and the remaining CO2 is due to our contribution. My impression is that skepticism at this point is a kind of disorienting obsession with labeling each CO2 molecule and checking if the one that remains in the atmosphers is ours…

    Thanks again for this informative blog.

  • Richard // August 11, 2008 at 4:36 am

    I do not disagree that humans have influenced the increase in CO2 levels. I just disagree that CO2 per se is the primary agent in global climate change.

  • Barton Paul Levenson // August 11, 2008 at 11:43 am

    Richard posts:

    I do not disagree that humans have influenced the increase in CO2 levels. I just disagree that CO2 per se is the primary agent in global climate change.

    It’s one of the primary agents, and it is the agent most responsible for the recent global warming.

  • Ray Ladbury // August 11, 2008 at 12:54 pm

    Richard, what other basic tenets of physics do you disagree with?

  • EliRabett // August 13, 2008 at 1:31 am

    There is a lag associated with the time that it takes (a couple of years) for atmospheric interchange across the equator. That pretty much accounts for the offset between the South Pole and M-L/Barrow.

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