I saw an interesting note on the news a few days ago.
The "Old Farmers Almanac" is a publication available annually. One section talks about short term (yearly) and long tern (decades) weather trends. These trends are based on such scientific principles as (1) the bushiness of squirrel's tails, (2) the side of the tree a bear rubs his butt on, and (3) which direction the moss is pointing, etc.
To make a long story short, they (and their reviewers) claim a better than 90% accuracy and they said "forget global warming, prepare for a 50 year period of global cooling".
If you cut it to small you can always nail another piece on the end, but if you cut it to big... then what the hell you gonna do?
Steven