Doesn't necessarily have to kill anyone, but should have substantial shock value through destruction of property or similar (i.e. significantly above the current gluing-to-things protests).

Longer-term version:

Terrorism is notoriously ill-defined. For this market I define it as the intentional use of violence to achieve a political goal, in this case a goal related to climate change. So actually blowing up a refinery is violence and counts, but sabotaging by drilling a hole in a pipe is not.