Both simple to implement:
- an alert tells you that the climate of the planet is changing.
- every few months the average temperature becomes a little higher or lower depending on which of the 2 random events happens.
- event stops when min or max possible temperature is reached, according to what the game considers the highest/lowest possible.
- randomly opposite event can still happen so you could go through a glaciation for some year but a global warming could strike and reverse the process of average temp changing.
- survive or escape!
I would tune these 2 events to become heavy change in at least 10 years but if you are in a boreal forest and global warming happens it would take 20 years to become 50°C on summer. Just to give an idea of how slow I imagine these things.
Hum...
I see what you mean, but maybe we shouldn't consider them "Event", because those are specifically meant to be short and never superpose.
Also, this is something that would influence the game experience entirely, contradicting biome.
So I would suggest to approach this as a "Storyteller".
- Cassandra Classic is for increasing challenge that incite you to END (https://ludeon.com/forums/index.php?topic=11275.msg112541#msg112541) the game.
- Phoebe Basebuilder is the same but with longer downtime
- Randy Random is for ... well constant randomness.
In my mind there should be more Storyteller specifically for scenario like that or "endless game". That way you could have one specifically for dramatic climate change.