Garnaut tackles this question in his report.
“Many would argue that the uncertainty requires a conservative rather than ambitious approach to mitigation. But what is conservative in a context where the possible outcomes include some that most humans today would consider catastrophic? Conservatism may in fact require erring on the side of ambitious mitigation. After all, prudent risk management would suggest that it is worth the sacrifice of a significant amount of current income to avoid a small chance of a catastrophic outcome.”
I can see his point.
A conservative would take out fire insurance even if they thought there was only a small chance of their house burning down.