So you can argue that natural emissions are larger than human emissions. Sure. The respiration of plants and animals produce 8 times more $\text{CO}_2$ than humans, but plants take it back through photosynthesis.

Atmospheric $\text{CO}_2$ have varied widely through Earth's history, but in the last several thousands years it have been pretty stable, because all the sources were in balance with the sinks, leading to a very small net change in $\text{CO}_2$ concentration in the atmosphere and oceans (there in the form of carbonic acid).

However, this balance was disrupted by the industrial revolution and human emissions, mostly due to the use of fossil fuels.

Going back to the swimming pool analogy: evaporation might be tiny compared with the volume of water turned around by the filtering system, but give it long enough time and your swimming pool will be dry.

We added a source of carbon in the carbon cycle, and if we don't remove it or add an equivalent sink (carbon sequestration), the concentration of $\text{CO}_2$ in the atmosphere and carbonic acid in the oceans will keep rising.

Returning to your question, using the figures in the diagram, human emissions would account for ~3% of total emissions. But that doesn't matter at all. The point is that human emissions are the ones upsetting a system that was in balance, and are triggering an increase in atmospheric $\text{CO}_2$ concentrations and consequent ocean acidification.