Real scientists base their conclusions on tested theories and data.  These are not "opinions" in the general usage of the word, they are conclusions.  If you would like to start looking at the data, one good lace to start is by looking up the Keeling Curve.  It shows the build up of carbon dioxide in the atmosphere.  The IPCC report also has a fairly comprehensive referance section if you care to understand the science. It includes a great deal of hard data.