If you know the answer, you can find the "evidence"
This is the main point of divergence when you think this true. An environmentalist whacko, when told this fact, and how human intervention might up this to 4.25% (my figure, only to serve to argue the point), will tell you that the additional 0.25% will thrust the entire climate out of whack. But then you need simply point out that volcanoes pump out equivalent, even greater amounts of CO2 (and pollutants) and the earth swallows it up, readjusting and stabilizing in only a few months.
Human involvement in the climate is anecdotal, at best.