The Earth’s atmosphere is like a big bathtub. As long as we pour water into a bathtub, the bathtub will continue to fill with water. Similarly, as long as we emit carbon dioxide into the atmosphere, the atmosphere will continue to fill with carbon dioxide.

Even if we reduce the flow of water into a bathtub, the bathtub will continue to fill with water. It just won’t fill as fast. Similarly, if we reduce the rate of emissions of carbon dioxide into the atmosphere, the atmosphere will continue to fill with carbon dioxide, just not as fast. Only by stopping the emissions of carbon dioxide into the atmosphere can we be assured that the buildup of carbon dioxide in the atmosphere will stop.

There is a drain in the atmosphere just like there is in a bathtub. The atmospheric drain is carbon dioxide being drained into “carbon sinks”. A carbon dioxide sink is a reservoir that accumulates and stores carbon dioxide. Currently about half of the carbon dioxide emitted into the atmosphere goes into two sinks, the oceans and plant growth.

Oceans are a carbon dioxide sink because water absorbs carbon dioxide. However, this additional carbon dioxide in the oceans is turning the water acidic and negatively impacting marine life.

Plants grow by taking in carbon dioxide through photosynthesis. The increased atmospheric concentration of carbon dioxide stimulates additional plant growth which increases the removal of carbon dioxide from the atmosphere. Trees appear to be the major users of this additional carbon dioxide. Even old growth trees continue to grow and increase in mass.

Even with these sinks, the amount of carbon dioxide emissions pouring into the atmosphere overwhelms the amount escaping into the sinks. Therefore, the atmosphere continues to fill with carbon dioxide.

There may also be limits to the capacity of these sinks to remove carbon dioxide from the atmosphere. The stimulating impact of increased atmospheric carbon dioxide on plant growth may wane. Likewise, absorption by the oceans may decrease or even reverse. The oceans may actually begin to emit carbon dioxide.

These facts demand that we move to “zero emissions” if we are serious about stopping global warming.

Don Hofstrand is a retired agricultural economist from Iowa State University Extension. During the last few years of his work life, he focused on renewable energy and climate change. He and his wife live in Mason City.