Set an alarm (clock or mobile phone) for 12.18pm. Be at the square by 12.10pm. When the clock on the Council House bongs a quarter past 12, freeze (as if asleep) wherever you are in the square, and then 'wake up' when the alarms go off & do a bit of mad running around shouting "WAKE UP!! SAVE YOURSELVES!...FLOODS!...FIRE!...FAMINE!...STORMS ARE BREWING!...SEAS LEVELS RISING!...HELP!...DO SOMETHING, WHILE YOU STILL HAVE THE CHANCE...DON'T PANIC!...etc" All very theatrical and melodramatic. Like Dad's Army! Bring banners etc highlighting that 350 ppb is the magic number.