A very interesting question!

November 29th, 2007 climate change, global warming

 Climate Audit - by Steve McIntyre » CO2 Levels

Being more in a mood to ponder rather than write, the linked post from Climate Audit certainly raises an interesting point.  Since the argument is that mankind is continually increasing the emissions of carbon dioxide, then why does the amount added vary from year to year?  Note the rate at which CO2 is being added has certainly increased - but why the ups and downs? Certainly something to ponder.