5. Why do most scientists believe that global warming is anthropogenic (human-caused?)

   Ice core data for the last 650,000 years reveal that historic CO2 levels fluctuate between 180-300 parts per million (ppm).  During the 200 years since the start of the industrial revolution, 200 billion metric tons of CO2 have been released into our atmosphere.*  During that time, atmospheric CO2 levels have increased from 275 to 380 ppm.  

*That averages to 1 billion tons/year.  The current rate (2007) is 11-13 billion tons/year.