Hypothesis: with warm autumns, plants better remove and recycle the green chlorophyll from their leaves. If chlorophyll is completely removed, carotenoids and anthocyanins are fully visible. So, leaves appear more yellow and red than in cold autumns.
Experiment: shoot pictures in Zürich gardens (Zürich University)
More pictures of golden autumn in Zürich (Rietberg Park)
.. more (Raemistrasse) …
Conclusions: no previous data to compare! It was anyway a wonderful autumn day in Zürich 🙂