For me, the personal experience extends back to 1966 or '67. I have redone many trails in the Sierra Nevada Range and it seems warmer most of the time now. I can't compare sleeping bags because I know my first down bag was lighter and shorter than it should have been...I grew out of it by the time I was 13. Also, I have a dog as a heater now. But the mornings in the 60's and 70's were usually cold unless I had warm clothes on. By the 90's, I was wearing shorts all the time and many mornings were downright warm at elevations above 9000ft. On my last trip (end of Aug), we had two days where we had light snow flurries (and ice in the water bottles in the morning). Then it was back to relatively warm and we spent most of the time above 10000 and 11000 feet. I distinctly remember many more instances of frozen water bottles in the past. This is all very non-scientific and based on my bad memory. But partially because of these observations I am revamping my Sierra backpacking setup.