It seems to be much more than 20 years, but does it even matter? We have no clue about the climate of the future, even if all the warming we attribute to our pollution is 100% correct, we still do not know if this will cause problems for future generation or help them cope with an ice age. Both is at best equality likely, the ice age is probably more likely because whatever causes it, is a much much bigger force than human pollution, there is probably not enough human accessible fossil CO2 to prevent an ice age.
