Actually, life doesn't rely on the current balance of carbon in the atmosphere. It pre-existed the lay persons concept of a hospitable planet... prokaryotes... The definition of life sustaining conditions, is in fact more difficult to describe and quantify than Merritt Olsen has indicated. Without knowing how life began on this planet, how can we make conclusions on how life could exist elsewhere?

plants take in co2 and release oxygen.
we breathe in oxygen and exhale co2.
that's how it helps with your breathing.
when we cut down the plants, burn fuels that release co2, and continue exhaling, do you think it works as well?
the fact is that carbon dioxide isn't necessarily good or bad, it is part of a cycle. as part of a cycle that evolved between plants and other organisms, it is helpful for life. as part of a cycle between consumers and corporations, it unbalances that first cycle. whether or not the current global warming is due to human activities or natural changes, all you really have to do is stand next to a busy road for a couple hours to realize that this stuff isn't healthy, and will have a greater effect if continued.

This Cosmos series is the remastered one, which has some scenes replaced by more modern CGI and real-life film. So even though the original episode aired in 1980, the remastered episodes (including the 10-year updates from Sagan himself) include scenes of stuff like the Exxon Valdez.