The explained variance of this residual yield by climate is generally about 50%, which is very close to the average I found for corn, soybeans, and winter wheat.

FORBES: Climategate II: An Open Letter to the Director of the National Center for Atmospheric Research

In fact, the Brinson, Hood and Beebower study a lot of advisors chant about demonstrated that 90% or more of the variance in returns among large pension plans was explained merely by their allocation to stocks, bonds and cash.

FORBES: Fake Diversification: Does Asset Allocation Work?