Here's a symbol known the world over, that some have used in climate action marches and gatherings. The mocking jay sign. It became associated with freedom-fighters everywhere when Myanmar protestors used it in recent years.
This is a symbol, simple and beautiful, that we might all know and should be with us a very long time. It represents the Holocene or Sixth Mass Extinction. The symbol is described on the Extinction Symbol website as (follows): "The circle signifies the planet, while the hourglass inside serves as a warning that time is rapidly running out for many species"