A (brief) Tale of Three Sites

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.