How do we know humans are causing climate change?

I just took a look at the comments on the El Paso Times Wed site after my column on the need for Copenhagen to accomplish something resembling a deal. Four comments, including one from a guy calling himself "Professor." All called climate warming a hoax and seemed to take this meteorological phenomenon as a personal insult. The "Professor," whose scientific credentials remain unclear, said carbon dioxide is a naturally occurring gas, not at all caused by human behavior.

So, how do we know climate change is caused by humans? It's true that carbon dioxide (which, by the way is only one of several greenhouse gases) occurs in nature. A lot of natural processes can impact the C02 levels, such as solar activity. However, carbon molecules that come from burning fossil fuel (which only we, humans, do) are heavier than other carbon molecules. Scientists have been measuring the weight of carbon molecules and can tell how the carbon was generated.

There are other ways scientists know humans are responsible for a great portion of greenhouse gases. But I don't want to blow the "Professor's" mind. For those who want to look further into it, check out the Union of Concerned Scientists, and the UK Science Museum.