0 Parts per million

One of the sillier arguments against climate change, especially human-affected climate change, is to claim that since CO2 is a trace gas, it can't have any significant effect.  It's now about 395 parts per million in the atmosphere.

It happens that I'm taking some medicine at the moment.  100 mg of one, and 10 mg of another.  To make the math easy, let's say I weigh 100 kg.  mg is milligram, so there are 1000 of them per gram.  1000 grams is 1 kg.

That first medication represents 1 part per million.  The second is a mere 0.1 parts per million.  The body is a complex system.  Small amounts of things can be very important.  Climate is also a complex system.

Let's continue a little in this vein.  Daily nutrition requirements, which I'll take from http://en.wikipedia.org/wiki/Reference_Daily_Intake , include:

• 1000 mg Calcium --> 10 ppm
• 1000 mg Phosphorous --> 10 ppm
• 60 mg Vitamin C --> 0.6 ppm
• 15 mg Zinc --> 0.15 ppm
• 2 mg Manganese --> 0.02 ppm
• 80 micrograms Vitamin K --> 0.008 ppm
• 6 micrograms Vitamin B-12 --> 0.0006 ppm

If you'd like to avoid scurvy, rickets, pellagra, and a host of other illnesses, you have to treat concentrations far, far lower than 400 ppm as being important.