"In the first study of it's kind, scientists have determined that Americas winds are have slowed dramatically since their recorded peak in the 1970's. In some parts of the Midwest, average peak wind speeds have dropped as much as 10%."http://www.ecogeek.org/component/content/article/2806
And wind power has a cubic relationship to wind speed.. so that's 30% less power.
I was a bit sus about "average peak" which sounds a bit like "usually sometimes", and "mostly certain", but it turns out that it was a misquote of the source which reads "average and peak".
