ConsulCaesar: Could you post a screenshot of one of your cities so we can take a look?

My first thought was that you must be having a massive unhappiness problem. Unhappy citizens don't work (don't produce anything) but still eat. With enough unhappiness, this could turn into a famine. You can get a lot of sudden unhappiness with war weariness (wars that last too long) at high difficulty levels, if you defy a United Nations resolution, or if several other civilizations switch to the Emancipation civic but you don't.

Unhealthiness/pollution also reduces the food available in your cities, but I have never seen it become so bad that all cities struggle so much.

Global warming will turn random tiles into desert (=useless), but you should be at Apocalypse level if all your cities are shrinking down to population 1.

In a game as deep as Civ IV, maybe there's another reason. It should be visible in the city view.

trusteft: Sure thing.

This was a larger city.

For some reason the forum doesn't upload my pic. Here is from an external site.
https://i.postimg.cc/WpFNnpQS/city.jpg

ConsulCaesar: You have a huge amount of unhappiness. Even when extra happiness from the culture slider, there are so many unhappy citizens that they all seemed to go on strike (except that engineer which I guess is a free specialist from Mercantilism). Place the mouse cursor over the red face on the top and a tooltip will appear explain the origin of the 26 unhapiness.

P.S. The forum software limits the maximum size of pictures you can upload.

ConsulCaesar: "It's too crowded": one unhappy face per population point (with an adjustment for the difficulty level). That's not the problem

"The world considers you a villain": you have defied (voted "Never!!!") a United Nations resolution that would have otherwise passed. With a -25, it seems you have done this five times. The world (and yoir citizens) will forgive you if you vote "Yes" to any resolution that passes. It will also slowly fade away at some point.

The Apostolic Palace can also give this penalty; however, it is usually less severe as it is weighted by the number of religions present in the city.

trusteft: So I have to give in to UN voting to play the late game?

ConsulCaesar: Some of the resolutions are not that bad and even beneficial. But you can try and control the UN yourself: with enough population and/or some skillful diplomacy, you can become elected Secretary-General an be the one proposing the resolutions.

Alternatively, try to gain more happiness. Acquire luxury resources, adopt civics that raise your happiness, build the appropriate buildings, raise the culture slider (which comes with a happiness bonus).

trusteft: First of all, thanks for the answers.

Now, the rest are just bs. I have a huge empire, I believe if not all, most of my wars were not initiated by me, I have plenty of resources of all kinds, huge armies, good economy etc.
But because I "defy" UN resolutions my citizens refuse to work and die of hunger?
Such BS!

ConsulCaesar: On the bright side, no resolutions are permanent. If they are proposed again and "No" wins this time, then they are revoked. The forces civic changes are annoying, but by the late game, and especially with a big empire, you will probably want to run most of the bottom ones anyway. The one that adds a trade route per city benefits everybody, but it benefits bigger empires most.

Of course, you can always conquer the city where the United Nations wonder was built and raze it to the ground -no more resolutions.

And if you are definitely turned off by the whole mechanic, you can disable the Diplomatic victory thw beginning of the game. Neither the United Nations nor the Apostolic Palace will be possible to build.