Sources: Long term data from the internationally recognized Vostok ice core samples: http://cdiac.esd.ornl.gov/trends/co2/vostok.htm (you can download the data at the top) More recent (industrial age) data: http://www.dar.csiro.au/publications/Pearman_1999a.htm. 2100 estimate from the Intergovernmental Panel on Climate Change. They estimate CO2 levels of between 540 and 970 in the year 2100, the average of which is about 750. Read about the scientific consensus here.