The Bulletin of the Atomic Scientists has published a doomsday clock for nuclear war for decades. The minute hand has been at two minutes to midnight for several years too, I believe. The trouble with going full-tilt for panic too soon is that one can’t get closer without resorting to Eleatic micro and pico-second intervals. If people establish a Doomsday clock for Global Warming it should not be at the panic place initially- give a little space to take note of worsening conditions.
Maybe these guys;The Union of Concerned Scientists have published a doomsday clock somewhere.