concept

Mass balance

The accounting rule that matter is neither created nor destroyed: a reservoir's contents can change only by exactly the difference between what flows in and what flows out. It sounds like bookkeeping, and it is one of the sharpest investigative tools in the earth sciences. If you measure the sources, measure the reservoir, and the two do not agree, the conclusion is not that the measurement failed but that a flux has gone uncounted — and hunting that missing flux is how Earth's missing carbon sink was found, and how the oxygen loss inside Biosphere 2 was explained.