I know this in the same way that even though I don't know the exact credence to assign the probability of particular bad effects from global warming, I can confidently say that an increase by a factor of 1000 of the CO2 emissions would be a bad thing. This is not because I have done a simulation, but instead my beliefs are based on the assumption that while concerned experts might be wrong in the details, they are probably not wrong with a gap of 3 orders of magnitude.