Credit: u/FreshPrince987 on r/civ

Apocalypse is a game mode that ships with the Maya & Gran Colombia DLC as part of the New Frontier Pass. It takes the climate system introduced in Gathering Storm to the next level:
Hyper-real.

On one hand disasters might wreak havoc in your land but they also add fertility.
Floodplains become more fertile.
Volcanos enrich volcanic soil.
Blizzards and dust storms likewise add fertility to otherwise barren snow and desert tiles.

It's not all doom and gloom.

So instead of fighting climate, exploit it!

In Apocalypse mode forest fires are introduced.

Most notably these forest (and jungle) fires cause land to be fertilised. After a few turns the burnt forests and jungle regrow with additional food and production.

So civs with woods or jungle start bias can potentially exploit this. Or on arboreal or tropical maps.

So rule of thumb: Don't chop woods (or rain forest)!

Normally in the early game, chopping is a way to rush production. To build a settler, a wonder or whatever.

Apocalypse mode turns this go-to strategy on its head.

If you chop forests and jungles you are preventing forest and jungle fires.
By keeping nature intact, you invite insane late game tile yields.

Hyperyields.

Note: the asterisk symbol (*) means a yield of more than 12!

Originally posted by Billy Joel "We Didn't Start The Fire":

We didn't start the fire
It was always burning, since the world's been turning

Apocalypse mode introduces the Soothsayer.

You can use a soothsayer to create disasters.
But only one - the forest fire - will improve fertility, or tile yields.

Use your soothsayers on forest and rain forest to start fires to get the ball rolling on hyperyields.

At the start, locate the largest, most contiguous woods (or rain forest) to start a fire. The larger the forest or jungle the longer the burn resulting in the most yields.

Beware every time you use a soothsayer to create a disaster you are contributing to CO2 emissions.

Participate in the Appease The Gods competition. You will be rewarded with a soothsayer promotion. The winner gets a free additional soothsayer.

If you can't chop forests, then build your districts on marsh, floodplains, desert or snow. Alternatively, tiles with harvest-able resources, like copper or stone. Later on even rice, wheat and maize become comparatively good tiles to harvest and build on, if you run out of space.

But remember, whatever you do, in Apocalypse mode, don't chop your precious jungles and woods!

Note playing Vietnam: avoid building districts (apart from Preserves) on woods as they will burn. Build on marsh. Or on isolated rain forests.

Any strategic resources located on woods or rain forest must be chopped first before improving. Otherwise fires will burn the improvements making it a nightmare to constantly repair them.

Harvest bonus resources on woods and rain forests, like banana and deer. You don't need them.

Rewilding or planting woods is an opportunity to further exploit Apocalypse mode.

Prioritise corridors connecting patches of woods to make larger contiguous forests. (Note this cannot be done with rain forests as forest fires on woods do not burn rain forests and vice versa).

Playing Vietnam you will be able to plant forest with Medieval Faires.

With Conservation everybody can rewild.

Micro-movement.

Look at the square you intend to move your unit to in the next turn. If there are burning fires in hexes surrounding it, don't move there. You could potentially lose your unit.

Garrison units in city centres and Encampments. They will be safe from fires.

Vietnam, Kongo, Maori, Norway, Brazil, Canada, Russia and America.

• Europe is a good map for Apocalypse mode. North eastern Europe is arboreal. Russia's TSL.
  
• East Asia. Vietnam TSL.