“How do you think CO2 is accumulating if there is only a 5-year residence time?”
It is true that the residence time is 5 years (or even less). But whoever wrote that part of the NIPCC report was confused and severely misinformed. The residence time applies to individual CO2 molecules. It is the average time that they remain in the atmosphere before moving to another reservoir. It has little relevance to the time that it takes for some imbalance to be restored. The latter is a function of partial pressure both in the oceans and in the atmosphere (and the balance with the biosphere).
For instance, when everything balances, there is no net exchange between the oceans and the atmosphere, but the residence time still is 5 years. In that case, whenever x molecules move from the atmosphere to the oceans, an approximately equal number of molecules move in the opposite direction, so there is no net change.
The time required for some fraction of a CO2 excess in the atmosphere to be captured by oceans (and/or terrestrial biosphere) depends on the difference between the fluxes, and it is much longer than the residence time and it depends very much on the differences in partial pressures.
