CO2 Formally Declared a Pollutant

Today in the New York Times:
The Environmental Protection Agency on Friday formally declared carbon dioxide and five other heat-trapping gases to be pollutants that threaten public health and welfare, setting in motion a process that for the first time in the United States will regulate the gases blamed for global warming.

What I don't understand is how this can be a defined a pollutant when it is also naturally occurring. There must be a measurement defining a level that has to be reached for CO2 to be a pollutant, but no one is stating what that is or even if they can measure it.

We all want to save energy and keep the planet clean. But I want to see more information about the standards and measurements used to make declarations like this. It just doesn't make sense that we jump from decreases our impact on the planet (good) to a necessary gas for life to function is a pollutant (bad). Huh?

I remember basic earth science studied in school stating CO2 was necessary for life on Earth. Has everyone forgotten this basic lesson?

From Merriam Webster, the definition for carbon dioxide