All the time, I see confusion over the size the human contribution to CO2. Man emits a lot of CO2 each year, but it's still only about 2-3% the amount that Nature emits. Many people don't understand how Man can be responsible for the observed increase in CO2 when our emissions constitute 2-3% of natural emissions. This results from a misunderstanding of a basic concept called "stocks and flows." If you're one of the confused, don't worry - a study by Sterman (2002) found that even most MIT grad students have trouble grasping this concept intuitively. Here's the analogy:
Imagine a bathtub in which the faucet is turned on and the drain is open, and water is entering the tub from the faucet at the same rate it is leaving the tub through the drain. The tub is in equilibrium: the "flow" of water is entering and exiting the tub at the same rate, so the overall level of water in the tub (the "stock") does not change. This is like the natural carbon cycle: in equilibrium, CO2 enters and exits the biosphere at the same rate, so the overall level in the atmosphere doesn't change.
But back to the tub. Now imagine that you add a second faucet pouring water into the tub, but only 2% as much as the original faucet. Still, because there is now slightly more water entering the tub as draining out of it, the water level slowly increases. This is like the manmade contribution to CO2 emissions: even though ours are only 2-3% of the total, that 2-3% throws the system out of balance. We've disrupted the equilibrium, causing CO2 to accumulate in the atmosphere faster than natural processes can remove it, causing the total level of CO2 to rise.
So even though we are responsible for only a small fraction of the TOTAL CO2 that enters the atmosphere each year, we are responsible for 100% of what STAYS there.
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