Given the absolute necessity of atmospheric carbon dioxide for life on earth, one could argue that their true agenda is not merely the extinction of human life--it is the extinction of *all* life.
Without carbon dioxide in the air, plants have no source of carbon to convert into sugars, amino acids, and fats. Plants die as a result. Without plants, there is no organic material for any other living organism to incorporate into its own body. So everything else dies, as well.
If the left got its way with carbon dioxide, the extinction of life would not happen instantaneously, but over a period of years. The effects of carbon sequestration would start immediately, but the earth is so vast that by the time we really started to see the damage, an enormous amount of damage would have already occurred.
Almost all the CO2 on earth has nothing to do with humans. If collective humanity held its breath till it died, the effect would be a virtual nothing on global CO2.