Its like this.

Assume some background level of any sort for any chemical. F'instance, methyl mercury is a normal part of the environment. Methyl mercury is usuually so low that it is pretty benign. But at some level, it becomes a problem and this is usually at a level that has a very low or near-zero toxicity. The problem is that methyl merucry accumulates in the food chain. The more organisms you eat that are predators, the more methyl mercury you'll accumulate. And so the prohibitions or warnings on swordfish and lake trout, etc., are not related necessarily to direct effects but rather to cumulative or collateral damage.

So CO2, also a natural part of the environment, may become bad at high concentrations. Not because of direct effects so much as indirect effects. One such indirect effect is as a greenhouse gas, which may accelerate global warming. Another is preferential uptake of CO2 by C-4 plant species which may hasten conversion of some subtropical or temperate plant communities to plant communities dominated by tropical species which are often C-4 species. These effects are in the 'active research' class. Meaning that science doesn't understand all the ramifications of the effects and so society hasn't passed value judgements on these effects. But the 'worst case scenario' that governs the "precautionary principle" would warn against assuming benign or beneficial effects of C02 until they're proven.

So, if you ask an environmental scientist for a sound-bite response, the scientist will probably call it 'pollution' and then add a 20-pound sack of caveats, which are promptly ignored by reporters and editors.

And that's the simple answer.

Bot