Human impact on the environment
In a "Scientific Reports" publication, researchers propose 1965 as the start of the Anthropocene era. In that year, human nuclear weapons testing caused a noticeable spike in radiocarbon in the heartwood of the world's remotest tree, a Sitka spruce on Campbell Island, New Zealand. The general scientific community has already been using 1950 as the year "Before Present", when nuclear weapons began to significantly affect the reliability of radiocarbon dating of objects whose organic matter content formed after that epoch.
Sources: The Conversation