Limits of survival are set by climate, those long drifts of change which a generation may fail to notice. And it is the extremes of climate which set the pattern. Lonely, finite humans may observe climatic provinces, fluctuations of annual weather and, occasionally may observe such things as "This is a colder year than I've ever known. " Such things are sensible. But humans are seldom alerted to the shifting average through a great span of years. And it is precisely in this alerting that humans learn how to survive on any planet. They must learn climate.
Attributed to: Harq al-Ada
From Chapter 53 of Children of Dune by Herbert, Frank