evidence: Now, let us turn to the carbon cycle and consider some of the flows in and out of the reservoirs. What is the residence time for the atmosphere? To determine this, we take the amount in the reservoir (750 Gt) and divide it by the sum of the inflows or the outflows. Let us take the outflow: 100 Gt C/yr for photosynthesis plus 90 Gt C/yr going into the oceans. The residence time is thus: [...] Residence time = ` tau = (38,973 Gt)/(0.6 Gt yr^-1) = ` 64,965 yearsThis result is much longer than the atmosphere, and what this tells us is that the carbon cycle has some parts that respond quickly, but other parts that respond very slowly; the very slow parts tend to put a damper on how quickly the other parts can change. In other words, if we suddenly inject carbon dioxide into the atmosphere, you might think that the short residence time of the atmosphere means that the excess CO2 can be removed very quickly, but because these reservoirs are linked together, it turns out that the ocean must return to its steady state before the atmosphere can get back to its steady state.