This article's from May but I haven't seen it before. Hopefully it hasn't already been posted.
What I get out of this, during the last Ice Age Europe was much colder and drier. It was a Steppe climate much like Central Asia and Siberia, the 'mammoth steppe'. Cannabis thrived in these conditions, probably Ruderalis. Short hot summers and long cold winters.
The climate changed 10,000 years ago. It got wetter and warmer. The mammoths, lions, and such went extinct. The steppe became forest and the cannabis retreated along with the Steppe back to Russia.
Which is ironic because by 9000 years ago agriculture was spreading from the Middle East to Europe. Europeans had to discover cannabis all over again 4000 years ago when hemp cultivation spread West to Europe.