For any conspiracy to succeed, there are several conditions that may be required: 

1. The participants must be as few as possible.
2. The conspiracy must be of as short a duration as possible.
3. The conspiracy must be extremely secretive.

Condition 3 relies on the first two, as indicated in the proverb, “Three can keep a secret if two of them are dead.”

Thus, the commonly held 9-11 conspiracy theory that many government officials under the Bush Administration were directly involved in the attacks on the World Trade Center and the Pentagon is far less credible than the idea that a dozen operatives of Al-Qaeda were responsible. Likewise, it is far more credible that Exxon and its operatives have been planting misleading claims about global warming in the popular press and various blogs over the past couple of decades than that thousands of scientists have been misleading people about global warming since 1896, when Swedish chemist Svante Arrhenius first identified the heat retaining properties of carbon dioxide (called “carbonic acid” in Arrhenius’ paper referred to below).

http://en.wikipedia.org/wiki/Svante_Arrhenius

http://www.globalwarmingart.com/images/1/18/Arrhenius.pdf

People make up conspiracy theories to explain what could be responsible for something they happen to dislike. The “theory” could be more properly considered a hypothesis in science. The problem comes when these people do not take the next step in the scientific method, which is to test the idea via observation or experiment. Instead, they proclaim the conspiracy theory as DOGMA and proceed to interpret all evidence according to that dogma, despite never finding any direct evidence to confirm the theory. Then they abandon all willingness to allow the claim to be disproven.

You can’t do science that way! Just because a theory explains something doesn’t mean it is true. You must ultimately rule out all other possibilities before stating something questionable to be FACT.