Global warming is an increase in temperature in addition to the natural greenhouse effect.
The greenhouse effect interacts with other planet-wide influences such as the Milankovitch cycles in order to produce long-term climate movements.
This undermines efforts to combat global warming and there is a risk of an uncontrollable run away greenhouse effect. The error bars, particularly on the negative components, are currently the focus of much climate research in order to improve predictive climate models. A scientific consensus is reached when the vast majority of the scientists involved in a discipline broadly agree on the interpretation of the evidence pertaining to a specific scientific question.
When this occurs the case can be considered to have been demonstrated and the burden of proof then falls on those who would dispute the consensus.
Many of these claims are thrown into one big denialist soup.