Climate change. It never changed until we got here. 
I grew up on Lake Michigan.
The shore of Lake Michigan has grown and shrank since the last ice age over millenia as well as far before. This is well documented. I had two different houses on 2 different prehistoric beaches. One was some 5 miles from the current beach, the other 3 miles.
The shore has been up to (the hwy 131 Greenbay Road) thousands of years ago to the shores now that are some one mile further inland in places back in the 1940's and 1950's. In the 1960's, the lake reclaimed over one mile inland and destroyed homes and restaurants that are now under water. This ebb and flow has happened countless times over millenia long before Clovis and pre-Clovis Man arrived. If you check the area in Wisconsin near Kenosha/Racine you will find Petrified Springs Park, that shows a forest that had been flooded time and time again enough to show petrified tree stumps preserved in rock from millenia ago from Lake Michigan. They are above water now. There is no promise that it will be so in 10,000 years.
The tree stumps petrified in stone reflect the same patterns as stumps in California (giant redwoods) and other locations world wide that show that weather patterns are/have been greatly effected by SOLAR CYCLES than any other recorded scientific proof. The ebbs and flow of warming and cooling correspond with the established solar cycles (anywhere from 9 to 250 years in length but called by the misnomer "11 Year Sunspot Cycle"). There was a Maunder Minimum of Solar Activity in the 1700's. The Thames River froze solid in winter and some "summers" in colonial America where the Summer High's never got much above 80F and Winters seemed to never end -- crops suffered. This pattern is well documented world wide. It was referred to as the "Mini-Ice Age"...the "Maunder Minimum"...almost NO solar activity--sunspots very few and very far between.
I recently had cut a 200 year old oak on my property (broke my heart) that the shorn trunk showed obvious proof of annual rings in the stump showed every apparent solar cycle since before the US Civil War. The earlest rings showed times of cold years and cycles of long wet winters -- and then again in the 1930's -- solar activity that had extreme dry and hot years (about the same time as the Great Dust Bowl years.
