Answer:
Gradual**
Step-by-step explanation:
This seems to be an opinion based question but i mean if you look back at history in the early ages or pre- industrialization the worls had very little pollution but gradually as factories cars and other things started being invented we as humans started releasing harmful substances into our atmosphere which began slowly craving away at the ozone layer and til date we are doing the same thing and are facing climate change/ global warming. The world did not suddenly face this problem.
I hope this makes sense