Bad year for blackberries

The empty lot near my house is always chock full of invasive Himalayan blackberry bushes, and each August my kids and I enjoy picking bucketful after bucketful without really trying.  But for some reason, this year doesn't seem nearly as bountiful, with smaller berries and long thorny branches.  We went to Luther Burbank park this afternoon and even there, the bushes didn't seem as loaded as the last few years. It's late in the season, so I expect the crop to have dwindled, but earlier in August wasn't all that much better.  

I'm not sure what to blame:  global warming, George Bush, the unseasonably cool summer, my imagination?