Those who adhere to the global warming theory are using the Precautionary Principle as a reason to act. Their claims are that even if the science is not guaranteed as to the cause and effect of our emissions of CO2 that the Precautionary Principle dictates that we act to reduce our emissions. Thus it’s a default fallback position. That is, if AGW theory has a potential to be wrong, because we cannot have 100% certainty as to the effects of our emissions of CO2, then we must act anyway because the Precautionary Principle (PP) applies.

However, the definition of the Precautionary Principle is required in order to see if this fall back default position is justified. Surprisingly there is no specific definition of PP. Wikipedia has this:

The precautionary principle is a moral and political principle which states that if an action or policy might cause severe or irreversible harm to the public, in the absence of a scientific consensus that harm would not ensue, the burden of proof falls on those who would advocate taking the action.

It also notes that there are other defintions, and even four sub-definitions aimed at specific realms of society:

1. Scientific uncertainty should not automatically preclude regulation of activities that pose a potential risk of significant harm (Non-Preclusion PP). 
2. Regulatory controls should incorporate a margin of safety; activities should be limited below the level at which no adverse effect has been observed or predicted (Margin of Safety PP). 
3. Activities that present an uncertain potential for significant harm should be subject to best technology available requirements to minimize the risk of harm unless the proponent of the activity shows that they present no appreciable risk of harm (BAT PP). 
4. Activities that present an uncertain potential for significant harm should be prohibited unless the proponent of the activity shows that it presents no appreciable risk of harm (Prohibitory PP). 

In layman’s terms it is often touted as “better safe than sorry”.