Vostok Revisited: More on Iconic Figure #2
In my previous WU blog,
I showed the figure of carbon dioxide (CO2) and temperature from the Vostok ice core.
In the analysis of that figure I wrote: The periodicity is closely correlated with many of the orbital parameters of the Earth and the Sun. As many of you will note, this curve does not establish cause and effect. Further, higher scrutiny shows that the temperature increase begins before the carbon dioxide increase. This combination of the orbital parameters, the change in temperature, and the change in carbon dioxide provides a challenge for understanding. A plausible physical argument can be made that the greenhouse gases modulate, perhaps amplify, the radiative changes associated with the orbital parameters. Much of the change in the carbon dioxide would be associated with changes in ocean biology and chemistry.
I want to return to this analysis and the plausible role of the ocean. Here is a schematic figure that I drew of the ocean and the atmosphere and carbon dioxide. One of things that is most important when starting to examine a problem is to draw a figure.
Figure 1: Carbon dioxide at the ocean-atmosphere interface.
The top of the figure is the atmosphere and the bottom of the figure is the ocean. The wavy line in the middle is the ocean surface. I drew a dark dashed line below the ocean surface to represent the bottom of the mixed layer of the ocean. Like the atmosphere, the layer of the ocean closest to the surface is relatively well mixed. One factor that determines the depth of this layer is the wind speed at the surface of the ocean.
The green arrow labeled CO2 suggests that CO2 is transported back and forth between the ocean and atmosphere. The characteristic that is most important to the direction of the transport is the pressure of CO2. If the pressure of CO2 is higher in the atmosphere than in the ocean, then CO2 is transported into the ocean water. If the pressure of CO2 is lower in the atmosphere than in the ocean, then CO2 comes out of the water into the air. Remember that pressure is related to temperature. There are two other two-way arrows that I have drawn. The arrow labeled "tropics" represents the notion that in the tropics there is generally CO2 going from the ocean to the atmosphere. This is due to the warm ocean water. The arrow labeled "polar" represents the notion that at polar latitudes CO2 goes from the atmosphere to the ocean. This is the average situation.
I have drawn a bunch of arrows in the ocean; these are very important to the climate problem. Once in the ocean, the CO2 is "reactive." First, there is "chemistry," which converts the CO2 into carbonic acid. This effectively removes CO2 from the water, reduces the pressure of CO2, and hence, allows the ocean to take up more CO2. Thus, chemical conversion of CO2 allows the ocean to take up more and more of the atmospheric CO2. This mechanism is often called the "solubility pump," because CO2 is in a solution with ocean water. A consequence of the solubility pump is that the ocean becomes more acidic.
Second, there is biology. CO2 is used by plankton. In combination with calcium, exoskeletons and shells and bones are built. Again, carbon dioxide is removed from the ocean water, the pressure is reduced, and more CO2 can be transported from the air into the water. This is often called the biological pump.
For the sake of clarity, I needed space to draw the "biology" and "chemistry" arrows. The way they are drawn is not meant to represent a relationship to the tropics or the poles. They are just meant to explain what can happen to the CO2 dissolved in the ocean water.
There are two other elements in the figure I want to point out. These are the white arrow at the right of the figure and the artistically drawn cloud-like feature below the "plankton, shells, and bones." The arrow represents the transport of water out of the mixed layer into the deep ocean. This process occurs, primarily, in small regions of the polar oceans and seas. The cloud-like feature represents the "precipitation" of carbon, bound with calcium, as creatures die and sink.
None of the processes I described above go on without impacts or consequences. For instance, the acidity of the ocean will affect the ability of plankton to form their shells. Plus, much of the carbon that is transported down to the bottom of the ocean is slowly transported back to the surface, where it can return to the atmosphere. A small fraction remains as sediment on the bottom and gets incorporated back into the geological compartment of the Earth system. The mechanisms and paths described above are very good starting places.
Why did I go through all of this? Not so long ago many people viewed that the CO2 in the atmosphere would not be much of a problem because of the great capacity of the ocean. Two things: 1) While the capacity is large, there are consequences of putting more carbon into the ocean. 2) It takes a very long time for all of these oceanic processes to catch up with the excess CO2 we are putting into the atmosphere. The ocean might ultimately take it up, but not until there are significant changes to the atmosphere, the weather, the climate.
Back to that issue of the ice-age cycles and the warming starting before the CO2 starts to rise. Go to the figure above. Assume there is some source of warming that is not linked to CO2. If it leads to the ocean warming, then the CO2 in the ocean will start to be released into the atmosphere. The ocean will be less able to take up CO2. There will be transport of CO2 to the atmosphere. Once the CO2 is released into the atmosphere, it will make things warmer, and that will lead to more CO2 being released from the ocean. If the warming starts after a long period of cold, then there will be a lot of CO2 in the ocean to be released. This is a positive feedback. The links provided at the end go to earlier blogs where I have discussed other types of feedbacks. The general idea of the cycling is that the Sun-Earth orbital variability initiates the warming, the CO2 is released from the ocean, the positive feedback loop starts, and then this CO2 warming takes over. Why does it stop? (That's for later.)
Going back to the figure of the cycles from the last blog: What is different now than in the past few cycles? Plus what is the role of ice, especially sea ice.
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