Measuring anthropogenic CO2

Oceans are intricate system, interrelated with other earth systems. And the biological, chemical and physical levels all play a role in the oceans carbon cycle. Therefore it is a difficult task to identify and isolate one variable to make future predictions. NASA concedes that there is much unknown about the dynamics in carbon uptake and release.

What complicates things further is that natural variability is also occurring (Riebeek 2008).

There is also geographic variability. In the Northern Hemisphere, oceans uptake carbon at all latitudes. In contrast, only the mid-latitudes in the Southern Hemisphere uptake carbon and there is outgassing from the oceans in the high latitudes (Gloor et al. 2003).

 

Estimated anthropogenic CO2 in the oceans. Source: Sabine et al. 2004

 

Besides, CO2 is cannot be traced, since it is transformed biologically (Gloor et al. 2003).

In such a context, how can we assess human-induced changes? Can we make reliable predictions?

Release of fossil fuel combustion to the atmosphere decreases the ratio of oxygen over nitrogen. This process is balanced by photosynthesis, which increases the ratio. Oceans, on the other hand, do not influence the ratio. Land storage of CO₂ then, is determined by the change in O₂/N₂ ratio. Ocean carbon storage is inferred from terrestrial storage: we subtract land storage from total CO2 uptake  (Sabine et al. 2004).

We can see that oceanic storage was not directly measured but inferred from terrestrial storage (which itself is a result from different equations). Does this induce a higher uncertainty in the amount of oceanic uptake? The less direct the measurement process the more likely errors will arise.

An alternative approach is to apply atmospheric, ocean models. They account for changes associated with photosynthesis processes (Gloor et al. 2003).

Although accurate precise results are hard to obtain, we can be confident in the long-term trends. Alternative methods confirm the fact that oceans are overall a net carbon sink although absorption is spatially distributed.