Are 'Return Periods' being recalculated? If so, how?
One possible example could be the study of ice cores in Antarctica where scientists are trying to determine changes in concentrations of gases like carbon dioxide over a period of time . Such variations serve as indicators for other Climate related factors such as Temperature.



This record was a key contribution to climate science. One, it revealed how past CO2 levels compared to past temperatures. And since the data was directly derived from air bubbles trapped beneath hundreds of feet of ice, it also provided us with an exact measure for past atmospheres.

Secondly, and perhaps much more ominously, it showed us how very far beyond any climate comparable to that great span of time we’d already come.

102 ppm higher than at any time in the last 800,000 years

Humans have now pushed the CO2 boundary 102 parts per million higher than the context provided by the last 800,000 years. It’s kind of a big deal when you consider that a mere fluctuation of about 100 parts per million CO2 was enough, when combined with changes in orbital forcing, to set off feedbacks resulting in a 4 C temperature change globally (8 C change for the Antarctic environment) as ice age proceeded to interglacial and back.

source : World CO2 Averages Touch 402.2 PPM Daily Values 

Further read : Links between atmospheric carbon dioxide, the land carbon reservoir and climate over the past millennium