If volcanoes keep erupting, and there is no reason why they should not, C02 will continue to be added to the atmosphere, and the atmosphere should trap more and more heat. The consequence should be that the Earth would end up like Venus - too hot to sustain life as we know it. If it has been coming out of volcanoes for a few billion years, we might wonder why the concentration is so small. That large discrepancy means that C02 dissolves in water much more easily than the other two gases. Think of water and air as two fluids.
Water molecules are much closer together than are the gas molecules in the air.
At the interface between water and air, the molecules are vibrating vigorously as they move around. From time to time, a water molecule leaps up and stays in the air. It drifts away as a molecule of water vapor, instead of liquid water. This is how water evaporates. On the other hand, from time to time, a gas molecule lands on the water surface and stays there.