A dichotomy means a division into two mutually exclusive options. False dichotomies are imposing what seems to be a dichotomy on a continuum, or on something which is more complex, in that there are more than two options. Invoking false dichotomies is a tactic used to argue that if a person does not accept a position, then they must agree with the opposite. It is often also called the either-or fallacy, the excluded middle fallacy or the black and white fallacy.
The false dichotomy is used by climate change deniers. They state that carbon dioxide is either good or bad. They point out that carbon dioxide is not poisonous; it is plant food; it is in beer and soft drink. All of these things are good, they argue, therefore carbon dioxide cannot be harmful; therefore, climate change is not happening. The silliness of this is that these properties have little or nothing to do with the role of carbon dioxide as a greenhouse gas1. Another example is the misrepresentation of the relationship between elevated temperature and the concentration of carbon dioxide. Climate change deniers maintain that warming causes an increase in carbon dioxide and not the reverse, which they say the environmentalists believe. In fact, temperature and carbon dioxide are involved in a positive feedback cycle, so both are true. That is what makes climate change so dangerous.