John,
Here is an article that I think your readers will find interesting.
It is a frequently asked question.
RealClimate: How do we know that recent CO2 increases are due to …
Related…
Study suggests rising CO2 in the past caused global warming
This article tracks the increase in atmospheric CO2 for the last 50 years. It is also quite interesting.
And this for the last 650,000 years:
Carbon Dioxide Rate is at Highest Level for 650000 Years
And this is just a general information article:
Carbon dioxide in Earth’s atmosphere – Wikipedia, the free …