Guest Post by Willis Eschenbach [See updated graph]
Inspired by some comments on another thread, I decided to see what I could find in the way of actual measurements of the amount of CO2 in the surface layer of the ocean. I found the following data on the Scripps Institute web site. What they did was drive around the ocean on four different cruises, measuring both the atmospheric CO2 levels and at the same time, the amount of CO2 in the surface seawater. Figure 1 shows those results:
Figure 1. All air-ocean simultaneous measurements from four Scripps cruises are shown as blue dots. The horizontal axis shows sea surface temperature. The vertical axis shows the difference between the CO2 in the overlying air, and the CO2 in the water. The red line is a lowess curve through the data. The paper describing the Scripps data and methods is here.
Now, I have to say that those results were a big surprise to me.
The first surprise was that I was under the impression that there was some kind of close relationship between the atmospheric CO2, and the CO2 in the surface seawater. I expected their values to be within maybe 5 ppmv of each other. But in fact, many parts of the ocean are 50 ppmv lower than the CO2 concentration of the overlying air, and many other parts of the ocean have 50 ppmv or more of CO2 than the CO2 in the air above.
The second surprise was the change in not only the size but even in the sign of the trendline connecting temperature and CO2 (red line in Figure 1). Compared to the CO2 level in the air, below about 17°C the seawater CO2 decreases with increasing temperature, at a rate of about -2 ppmv per °C.
Above about 17°C, however, the seawater CO2 content relative to the air increases fairly rapidly with temperature, at about +4 ppmv per °C.
To describe the situation in another way, when the water is cool, it contains less CO2 than the overlying air … but when the water is warm, it has more CO2 than the overlying air.
Say what? I gotta confess, I have little in the way of explanations or comprehension of the reason for that pattern … all suggestions welcome.
[UPDATE] By popular request, here is the same data, but in absolute rather than relative units and without the lowess curve.
Figure 2. As in Figure 1, but showing the CO2 content of the surface seawater directly. Atmospheric CO2 varied very little during the time of the measurements.
My main question in all of this is, how does the CO2 content of the seawater get to be up to 100 ppmv above the CO2 content of the overlying air? It seems to me that the driver must be biology … but I was born yesterday.