Think of CO2 in the atmosphere as water in a bathtub. If you partly open the drain and run the tap at the same rate, the water level stays level. If you open the tap a bit more, the water level rises until it overflows.
As long as we keeping pouring CO2 into the air faster than nature drains it out, the CO2 level rises and the planet warms. Even a small imbalance, over time, makes a big difference.
Notes: Whether it's plumbing or climate, even a small imbalance over time can have large consequences. (When a CB editor recently left on vacation, unaware of a slow drip from his water heater, he returned two weeks later to a fully flooded basement! :))
Though humans add only a small amount "extra" CO2 to the natural carbon cycle, it's enough to tip the balance and, over time, cause the earth to heat up. Imagine a plumber saying, "Don't worry; enjoy your sabbatical! It's just one drop every few minutes!"
There are many variations to this classic climate metaphor, which was coined to clarify the distinction between the rate of annual emissions (the open tap), and the total concentration of CO2 in the air. For other variations, click on the tag "CO2 emissions" (in Advanced Search).
The challenge with this one is to phrase it succinclty. Though everybody knows what happens when you open the bathtub tap too far, the metaphor itself can get rather convoluted. (For a cartoon parody of getting tangled up in this extended metaphor, see Marc Roberts, cartoonist.)
Bite Source: various sources, including National Geographic
Image Source: here