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A bathtub is often used as an analogy for the accumulation of carbon dioxide (CO2) in the atmosphere.
CO2 is added to the atmosphere via the "faucet" and "drains" from the "bathtub" into "sinks" such as the ocean, plants and soil.
Prior to the industrial revolution, it drained out at the same rate as it flowed in. As a result, the level of CO2 and other greenhouse gases in the atmosphere remained steady. This pre-industrial level was around 280 parts per million (ppm). This may seem small but was just right to keep the global temperature in the comfortable range that allowed humanity and many animal and plant species to evolve.
During the industrial revolution we started burning fossil fuels such as coal and oil. These added more CO2 to the atmosphere. Even worse, the rate at which CO2 is being added is increasing and the rate at which it drains out has remained more or less the same. (Imagine what would happen if you left the faucets on your bathtub fully open. Soon you would have a flood.)
We have used CO2 in the graphic but it would be more accurate to use CO2e (carbon dioxide equivalent, which includes other greenhouse gases in addition to CO2.)
Many scientists have determined that a safe level of CO2e should be less than 350 ppm. However, some international agreements set a target of 450 ppm (which is now considered dangerous.)
Each year, the level of CO2e is rising by around 2 ppm. As of January, 2012 the accumulated CO2e in the atmosphere was around 393 ppm. For the current level click here.
For an extensive explanation of the bathtub model see http://climateinteractive.org/simulations/bathtub