<br>Interesting Hal. I magnified the charts and if I'm reading them right the higher CO2 concentrations are in the spring and the lower concentrations are in the fall. Hawaii is at 20 deg. N latitude, so this would fit with increases in oceanic plankton in the N hemisphere tracking with increasing sunlight in the spring & summer which would then draw down the CO2 (producing the lowest CO2 levels in the fall).
<br><br>I'd love to see graphs for the S. hemisphere -- perhaps Tahiti or Fiji -- they are almost as far south as Hawaii is North. Sydney or Cape Town might be good alternatives but they are a little further south than Hawaii is north.
<br><br>If the plankton in the arctic & antarctic is causing the annual fluctuation you would expect them to be relatively inverted (with a big caveat for N/S hemisphere atmosphere mixing) on a seasonal basis.<br><br>
Other than solar energy energy availability, the limits are micronutrients (which I've already discussed) and CO2 disolving into the ocean, which tends to be a function of wave, spray & rainfall action. Other than ships to distribute the iron & phosphorus over the ocean surface the other thing one might like is "sprayer" ships to take large volumes of ocean water & spray it into the atmosphere to increase its surface area and allow greater quantities of CO2 to be moved from the atmosphere into the oceans (in case the phytoplankton are carbon source limited). I suppose there may also be a nitrogen source limit as well but I haven't investigated that.
<br><br>But *until* one reaches a point where all of the oceans are a thick green "pea soup" or perhaps a red "tomato soup" you can generally assume that there is solar energy being wasted just heating the water rather than being harvested to produce useful biomass. If we were managing the planet properly we would be worried about an atmosphereic CO2 shortage decreasing land plant growth rates and excess atmospheric O2 increasing things like forest fires.