These are probably the biggest factors in the world today, & they are interrelated. The data is immense, the analysis is complex, & conclusions are not certain. Some trends can be reversed, others seem to be unsustainable & heading for disaster. I have lots of charts, as they illustrate better than words. This is a philosophical overview.. for perspective.
Populations use food & energy. Energy can produce more food to sustain a population. Shortages in either food or energy will affect a population. These are all critical factors in human society.
A. Most of the increase in world population has been in developing countries, especially those in Africa. Unfortunately, food production has decreased there, & energy resources are few. The unstable political climate makes future improvement uncertain.
Population of the world by region:
B. Emigration. In past centuries, overcrowded conditions were mitigated by emigration. Europeans, especially, left the crowded cities of their homelands, & moved to north & south america, australia, & africa. Emigration is still going on, but open lands are more scarce, & nations are restricting it more. It no longer seems like a viable solution for overpopulation in a region.
C. Human resource. Large populations can provide cheap labor for manufacturing products. These goods can be exchanged for energy or food. I don't have manufacturing as a factor in this list, but it is significant. The only problem with it is if either food or energy are depleted, manufactured goods are secondary. The most important part of manufacturing is in the production & distribution of food.
The earth at night, showing the regions of high electricity output & generation.
B. Oil. From wiki: "Peak oil is the point in time when the maximum rate of petroleum extraction is reached, after which the rate of production is expected to enter terminal decline. Global production of oil fell from a high point in 2005 at 74 mb/d, but has since rebounded setting new records in both 2011 and 2012. There is active debate as to when global peak oil will occur, how to measure peak oil, and whether peak oil production will be supply or demand driven."
The idea is that at some time, oil output will decrease, causing shortages & increase in price. Poorer nations will not be able to compete with wealthier ones for the oil supplies, resulting in lower food production, & increasing famine. Fossil fuels are limited in supply, & they will eventually be exhausted.
This chart shows proven oil reserves, as of 2009.
Oil is very significant for modern society. It is the engine that generates the cheap, abundant food, that enables us to use our resources for luxury items & leisure pursuits. It fuels both production AND distribution of our food. Shortages & price increases in oil automatically translate to increases in food.
3. Food. Here is a global view of land use.
A. Arable land for farming is decreasing. Erosion, urbanization, & other factors have decreased the availability of cropland. Reliance on chemical fertilizers has depleted the soil, & contributed to erosion. Water shortages have also decreased arable cropland.
B. Improvements in methods, fertilization, hybridization, cheap fuel, & machinery have increased yields in the food supply steadily over the years, resulting in lower costs.
C. World food production. This chart shows global food production, by metric tons.
D. Water. Fresh water supplies in croplands are depleting. Natural droughts affect production, & lowering water tables will curtail future crop growth in those areas.
E. Problem areas. Africa. Most of the world has increased productivity, except for Africa. Since the 1990s, their food production has decreased per capita, & they have imported increasingly more food. They also have the worse population growth problem, outpacing their ability to feed new people. Farming methods, political upheaval, reliance on rainwater, & other factors have contributed to this problem. They also have the lowest development in the world, & the fewest energy resources. The combination of all these factors will likely spell continued poverty & famine for the continent. Much of the difference can be attributed to the massive population growth rate, which has outstripped their ability to feed themselves.
4. Analysis. These projections are not new. Doom & gloom predictors have warned of these things for millennia. But the population of the earth has increased exponentially, without the usual constraints of war, famine, & plague.. at least those factors are not slowing the growth dramatically.
"What most frequently meets our view (and occasions complaint), is our teeming population: our numbers are burdensome to the world, [The earth] scarcely can provide for our needs; as our demands grow greater, our complaints against nature's inadequacy are heard by all. The scourges of pestilence, famine, wars, and earthquakes have come to be regarded as a blessing to overcrowded nations, since they serve to prune away the luxuriant growth of the human race." ~Tertullian (c. 155-220)
Famine seems to be the last, the most dreadful resource of nature. The power of population is so superior to the power in the earth to produce subsistence for man, that premature death must in some shape or other visit the human race. The vices of mankind are active and able ministers of depopulation. They are the precursors in the great army of destruction; and often finish the dreadful work themselves. But should they fail in this war of extermination, sickly seasons, epidemics, pestilence, and plague, advance in terrific array, and sweep off their thousands and ten thousands. Should success he still incomplete, gigantic inevitable famine stalks in the rear, and with one mighty blow, levels the population with the food of the world. ~Thomas Robert Malthus (1766-1834)
Fears of overpopulation in years past were over incremental increases.. a few million or even billion was considered unsustainable, even though the full extent of the earth's resources was not known, at the time. Now, we have pretty much discovered the resources, & most of the 'good' farmland has been in production for centuries. Add to that the exponential increase in population, from better sanitation & hygiene, longer life spans, better medical care & nutrition, & the population factor seems to loom larger than in generations past.
The nations with both oil reserves AND productive cropland seem to be poised to grow in wealth & prosperity, as nations without one or both have to export their wealth to meet their needs. Some areas in Africa seemed destined to poverty, as they have neither productive cropland or energy sources. Europe has good food production, but has little oil. Much of the mid east has abundant oil reserves, but little food production. Nations like Brazil, the US, China, & Russia seem to have both, & their outlook is good for both energy & food.
This is a lot of information, & there are many other factors in human society, but these are key issues. Facts are useful in determining direction & perception. What we conclude from these facts may vary widely, but they form the basis for an informed opinion.