Essentially because environmental economics and policy is full of uncertainty ........to invest in the environmental sustainability we need to have a precise picture of what we expect happening in the future,
The future can be determined in probabilistic terms . For example , in the case of global warming we can only predict difference scenarios on the basis of preconditions such as should the CO2 Omissions decrease significantly we will have an average temperature of suppose X . Should it not we will have Y etc . Now the question is what does Significantly mean here ? Can we build a policy based on such a great discrepancies ?