1950 is generally accepted by both AGW skeptics and proponents as a time before which human influence on climate was not discernable. That is, the warming trend

1950 is generally accepted by both AGW skeptics and proponents as a time before which human influence on climate was not discernable. That is, the warming trend before 1950 was natural, not human-caused. AGW skeptics assert this places a burden of proof on AGW proponents to show that warming since 1950 is beyond what could naturally occur.