Prove that it IS more harmful. That's what you are supposed to do in a free society. You need a reason to make (or keep) something illegal, not to make something legal.
But just because you asked:
Deaths from the two substances. There are hundreds of alcohol overdose deaths each year, yet there has never been a marijuana overdose death in history. The consumption of alcohol is also the direct cause of tens of thousands of deaths in the U.S. each year.
The U.S. Centers for Disease Control reported 20,687 “alcohol-induced deaths” (excluding accidents and homicides) in 2003. In the same year, the CDC has no reports of “marijuana-induced deaths.”
Source: http://www.cdc.gov/nchs/fastats/alcohol.htm
In 2001, there were 331 alcohol overdose deaths and 0 marijuana overdose deaths.
Source: U.S. Centers for Disease Control (CDC). http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5337a2.htm
Alcohol is one of the most toxic drugs, and using just 10 times what one would use to get the desired effect can lead to death. Marijuana is one of – if not the – least toxic drugs, requiring thousands times the dose one would use to get the desired effect to lead to death. This “thousands times” is actually theoretical, since there has never been a recorded case of marijuana overdose.
Source: The American Scientist, the Magazine of Sigma Xi, the Scientific Research Society
Long-term marijuana use is far less harmful than long-term alcohol use.
Source: Iversen, Leslie. Current Opinion in Pharmacology. Volume 5, Issue 1, February 2005, Pages 69-72
There has never been a documented case of lung cancer in a marijuana-only smoker, and recent studies find that marijuana use is not associated with any type of cancer. The same cannot be said for alcohol, which has been found to contribute to a variety of long-term negative health effects, including cancers and cirrhosis of the liver.
Source: http://www.washingtonpost.com/wp-dyn/content/article/2006/05/25/AR2006052501729.html
Studies find alcohol use contributes to the likelihood of domestic violence and sexual assault and marijuana use does not.
Source: Fals-Stewart , William, James Golden, Julie A. Schumacher. Journal of Addictive Behaviors. 28, pages 1555-1574. Intimate partner violence and substance use: A longitudinal day-to-day examination. Research Institute on Addictions, University at Buffalo, State University of New York
Studies find alcohol use contributes to aggressive behavior and acts of violence, whereas marijuana use reduces the likelihood of violent behavior.
Source: Hoaken, Peter N.S., Sherry H. Stewart. Journal of Addictive Behaviors. 28, pages 1533-1554. Drugs of abuse and the elicitation of human aggressive behavior. Dept. of Psychology, University of Western Ontario. Dept. of of Psychiatry, Dalhousie University.