Why is the CO2 concentration in Earth's atmosphere increasing? Use the bathtub analogy to explain your answer.

Now, I know you are thinking - 'That is a silly question, the answer is obviously human CO2 emissions'. However, that is only part of the answer - emissions (what type? what rates?) are only one part of the story. The contribution from human emissions fits into a complex system of carbon sinks and sources.

Bathtub simulation (MIT v.1): http://systemdynamics.mit.edu/ghg-exercise/welcome.htm

Play with the first exercise of the following 'stock and flow' simulation: MIT Stock and Flow Simulation v.1. The first page of the simulation uses a simple example of water flowing into and out of a bathtub to provide a nice illustration of 'stock' (the water in the bathtub) and 'flow' (the inputs and outputs from the bathtub)