Carbon Dioxide and the Ocean
By Ed Caryl
In my last post, Figure 1 illustrated how much the annual carbon dioxide flux, the red trace, varied from year to year. Studies (here, here, and here, among others) have been done on the annual carbon flux, but they ignore this variation by using running averages, or by simply ignoring the data during El Niño periods. This seems like an effort to avoid actually learning something.
Figure 1 is a plot of the annual carbon flux into and out of the biosphere.
The shape of the annual carbon increase resembles the shape of the global sea surface temperature (HADSST3), especially after reliable CO2 measurements began by Keeling after March 1958. Several known events are visible. Counting backwards: the 1998 El Niño, the 1994-5 El Niño, Mt Pinatubo in 1991, the 1986-7 El Niño, Mt Ruiz in 1985, El Chichon eruption in 1982, the 1972-3 El Niño, etc. Every positive peak is an El Niño and every negative peak is associated with a major volcanic eruption.
As can be seen in Figure 1, there is no relationship between the fossil carbon emissions curve and the annual carbon increase curve. That is because all the fossil emissions carbon is taken up by the biosphere or by the oceans according to Henry’s Law, and then sequestered there. The carbon in the atmosphere is controlled by temperature. This has been described by Dr. Murry Salby in this presentations at Sydney and Hamburg. He compares the CO2 curve to the integral of temperature. Here, I am going the other way mathematically, taking the differential of the CO2 curve as temperature and comparing it to known temperature data, the HADSST3 data.
Figure 2 is a plot of the annual carbon increase in the atmosphere (the differential of the Mauna Loa data) and the HADSST3 sea surface temperature anomaly.
The spikes in the SST correspond to the spikes in CO2 increase, and they go in the same direction. As the ocean surface warms, it emits more CO2 (or takes in less). The balance changes with temperature. Why should this be?
Figure 3 is CO2 solubility in water: 0.08g/kg/degree C below 20C. Above 20C the solubility drops by half to about 0.04g/kg/°C.
The ocean surface area is 360 million sq. kilometers, or 360 trillion sq. meters. The top meter is 360 trillion tons of water. A change of temperature of one degree for cold water changes the solubility by:
360 X 1012 tons = 360 X 1015 kg X 0.08 g/kg = 28.8 X 1015 g CO2 or 28.8 Petagrams CO2. A tenth of a degree temperature change changes solubility by 2.88 Petagrams CO2. This is about 780 Gigatons carbon equivalent. (3.7 grams CO2 = 1 gram carbon.)
The tropical oceans are well above 20°C so the solubility there will be roughly half the above figure. The plots in figure 3 indicate about 1 Gigaton carbon change from a tenth of a degree temperature change. A scatter plot trend line will give us a more exact figure.
Figure 4a is a scatter plot of SST versus annual CO2 change. Figure 4b uses the linear trend formula from 4a to convert SST to the carbon equivalent.
Studies of the CO2 emission and absorption have shown that the tropical seas emit CO2 and the cold, sinking, north Pacific and Atlantic absorb CO2. This has even been mapped. This is all due to the variation in CO2 solubility with temperature.
Figure 5: Source link for the above figure and caption.
The sea surface CO2 partial pressure is always very close to the CO2 partial pressure in the atmosphere above it. The sea surface is always in equilibrium with the atmosphere. This means that as we add CO2 in burning fossil fuels, some is taken up by the land biosphere. The remainder CO2 is dissolved and added to the CO2 reservoir in the surface waters. The mixed layer in the ocean is the top 20 to 200 meters, depending on the amount of wave and current mixing. That mixed layer is about 1/50th of the ocean volume. It contains roughly the same amount of CO2 as the atmosphere, as dissolved inorganic carbon (DIC). The difference, whether the ocean is emitting CO2 or absorbing it, is driven by temperature. As can be seen in the above figures, an El Niño can drive 2 or 3 Gigatons of carbon into the atmosphere and a La Niña can take it right out again. The rise in CO2 is due to rising SST, not fossil fuel burning.
Figure 6 is a plot of figure 4b and the biosphere increase from the previous biosphere article here, subtracted from the annual carbon increase.
The above is a residual. But the curve looks familiar.
Figure 7 is Figure 6 with the average annual AMO index inverted.
Note on the map, figure 5, that the warm tropical seas emit CO2, and the cool northern seas absorb CO2. The AMO index is a temperature index for the North Atlantic. It is derived by subtracting the global SST from 60°N to 60°S from the total SST, or alternatively, the Atlantic temperate and tropical SST from the whole north Atlantic. This means that as the tropical ocean warms more than the average global ocean, this drives the AMO index negative. This shifts the CO2 solubility (figure 2) downward and to the right. That peak in the 1970’s occurs because the tropical oceans were warmer than the average during that period, emitting more CO2. Since that time, the difference has gone the other way, the biosphere is taking up increasing amounts of CO2, lowering the amount of CO2 left in the atmosphere. You can also see a one year lag between AMO and the carbon flux. This is because the AMO lags the tropical Pacific by about a year.
The point of all this is that temperature is driving CO2, not the other way around.
We have good measurements of atmospheric CO2 only since 1958. Before that time our measurements were at the mercy of whatever ice does to captured CO2. We have good global measurements of temperature only from 1979, the beginning of the satellite era. This means that all of our measurement periods are shorter than the natural cycles. We have hints only from surface and ship measurements that go back 120 years, that some of the natural cycles are ~60 years long. We are presently at a convergence and peak of several of those natural cycles. There are suggestions that we are past the peak of some longer solar cycles. I use the words “hints” and “suggestions” because of the large errors, lack of global coverage, and wishful thinking adjustments to these measurements. There are two possibilities. If CO2 drives temperature, then temperatures should continue to climb. If it doesn’t, then temperatures will fall, then, shortly, CO2 will fall also. Nature is in the process of demonstrating which is which. We can just watch.