Recent investigations have shown that inconceivable catastrophic changes in the environment will take place if the global temperatures increase by more than 2° C (3.6° F). A warming of 2° C (3.6° F) corresponds to a carbon dioxide (CO2) concentration of about 450 ppm (parts per million) in the atmosphere. 

As of beginning of 2007, the CO2 concentration is already at 380 ppm and it raises on average 2 - 3 ppm each year, so that the critical value will be reached in approximately 20 to 30 years from now. See here
 for some graphs about the increasing concentration of greenhouse gases and the greenhouse gas emissions by sector.
 
Now I'm waiting for Spencer to get wheeled out...again