Big, you say 'only a theory' as if it is of no consequence.
For starters, anthropogenic global warming is not a single theory. AGW is a broad term and is explained by a number of scientific theories.
A scientific theory is an accepted explanation of the world - with the emphasis on accepted. It's not just an idea that could be right or wrong. All accepted science is based on theories. There is a theory of gravity, for example.
In popular non-scientific parlance, theory is often used to equate with 'untested idea' - eg 'floating a theory'. That is not how the term theory is used in science, where it is more equivalent to the popular usage of 'fact'.
