This links in my post from yesterday - quite an old video, but a great demonstration of just how much we have changed the composition of the atmosphere.

The video shows how CO2 concentrations have changed - starting with the last few decades and then putting current levels of CO2 into the geological context.

The main sources of data are direct measurements of CO2 (flask measurements) and air bubbles from ice cores. The seasonal variation in CO2 is caused by the annual cycle of northern hemisphere forests - absorbing CO2 during the summer and releasing it during the winter.