A good analogy to the methane cycle is a financial budget (and in fact atmospheric cycles are also called budgets).
In a financial budget, the reserve (balance) in your checking account (methane content of the atmosphere) is equal to
how much you earn (the methane source) and how much you spend (the methane sink). When you add it all up at the end of the month, you balance
(constrain) your checking account. Surprisingly, despite methane’s importance as the second most potent of the anthropogenic greenhouse gases,
not enough is known about the sources and sinks to get a good enough constraint on the methane cycle for us to predict how it will behave in the future.