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.