Of all the unused tropes I think that of the world being too full of people comes up the most. I understand the reason for it, if there are starving people there must be a shortage of food right? Same for not having houses for everyone.
The problem is that that is not how economics works, nor does every country operate every farm at the same efficiency.
North America, Central America and the Caribbean, have about 6,050,697,738 acres of land. If we moved everyone to north America we would each have almost an acre, and about double that if you included South America.
We are not starved for land, and that does not count the truly desperate and expensive things like going underground or making islands.
*Before I start please note I am not a professional farmer or doctor or anything, nor do I care to get the math exactly right, I only want to get it close enough to prove my point*
If we used aeroponics (spraying roots with nutrient rich water rather then keeping them in soil) and planted sweet potatoes (very high calorie food), we could feed something like 49,000 people per square kilometer (link, link). We could feed more if you had artificial light and stacked the farms on top of each other.
Without any artificial light you could feed the earth on farming or about a fiftieth of south America. In reality you would want more for variety, but even so I think I have made my point. If we wanted to, we could live and farm in the America’s then use the rest of the world for industry and parks.
Of course we could go even more drastic and farm algae in the ocean, (the most efficient food source we know of). This would mean we would not need to use any land for farming at all.
That is the reality. The world will not starve nor will we run out of space anytime soon. That however does not mean that everyone is well fed and housed. You can of course have a class of people who starve for economic reasons even if the food to feed them exists, or at least the production capacity exists to do so.
Now if you still want to have a world food shortage as part of your plot, then I suggest you think about why food production would be cut by several orders of magnitude and stay cut.
Sunlight would be my go to in this situation. If something is stopping enough light from getting to the surface of the earth that would mean we would have to make up the difference artificially, and that takes a lot of power. If light was that low, we would also need power for many other things, such as heating up ect. Food would become very expensive and I could believe we would only produce enough to get by and no more. The same would apply to heat.
An obvious approach might be to lower the water supply but it does not work as well as you might think. The human body does not expend water, we just hold onto it for a bit then put it back into the environment. While not 100% efficient by far, its not like electricity that is made, used and then gone.
Also water is so important for so many things that we will always give it high priority. A lack of water could change the cost of certain foods but I can’t believe it would push us to starvation. If we ever reached that point we would have much worse problems to deal with.
Honestly, where there are any number of things that could cause a temporary dip in our food production, there are not that many that would decrease it long term. Even if large amounts of soil became non-viable there are ways around that. The only other one that comes to mind is if something stopped us from using machinery, like the sun starting to emit a continuous EMP.