the Anthropocene

Anthropocene, sure, of course, but when did it start?

1945 has been proposed, because the advent of atomic bomb testing provides a geological marker of change detectable worldwide.

The Holocene, which we used to live in before deciding that those of us alive today really do have our own "cene", started some 11,600 years ago. It's divided into Ages, the Neolithic, Bronze, and Iron, capped by an Industrial Era.
None of those periods started in a particular year. None of those boundaries is anywhere near as sharp as we seem to want this one to be - except perhaps the Industrial Era. You could argue that the invention of the steam engine represents the start of that Era. But you could argue for other events too.
It's only because we're closer to it that people insist on a sharp boundary for the Anthropocene.
1945 doesn't make any sense to me. Surely the whole of the Industrial Era belongs in our new cene. Surely if we're defining the Anthropocene as the era where human activity is the main driving force of how the planet is, then you have to go back farther than that, to the Bronze Age and earlier. For thousands of years now, the influence of human activity in the clearing of forests, widespread proliferation of domesticated food animals, replacement of grasslands with cultivated crops, and the ocean acidification, soil erosion and atmospheric changes that were and still are consequent to that, places the beginning of the Anthropocene at around 5000 years.
More or less. Depends on where you look; depends on what you mean. There's no global geological event marking the start of this human-dominant period. In some places it's earlier than in others. Agriculture goes back 9000 years in Southwest Asia, but only 3000 in Central America. A million years from now, Geologists will not look at that as such a big difference. If only we had that perspective. In the meantime, it looks like the anthropocene will retain its lower-case, informal a.
There's no use in trying to pin a year on the change, and pinning it on 1945 is just hubris - Here we are not even a lifetime into our new Cene and we already know that that year was It? (Had the Cold War ended differently, perhaps! But it didn't.)
Welcome, fellow H. sapiens sapiens, to today.