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Memories of the 28th Century

Sustainable population

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In addition to the pressure on farmland by the rising population, the amount of arable land that could be lost to oceans as sea level rises with further global warming is huge, and low-lying lands are some of the most fertile.
If all glacial ice melted, then it is estimated that sea level would rise 230 feet. Since the last glacial maximum, sea level has risen about 400 feet, but different sources give a variety of figures. It has been estimated that sea level will rise by about twenty feet in the next hundred years (that assumes that current trends continue). If current estimates for future sea level rise are correct, then we can expect large amounts of arable land to be covered by ocean. Ocean will cover large parts of Bangladesh, much of Southeast Asia, the Netherlands, South Florida, the Mississippi Delta, etc. will be inundated, along with coastal land around the world. Some of that land is highly productive, so less food will be produced. That will mean that the Earth will not be able to support as many people. It is quite possible that even more of the ice will be melted as a result of seal level rise. In the past, Antarctica was not covered with ice, until weather patterns changed after it separated from South America.
One change that happened when sea level rose after the last ice age was that North Africa became dry. It had been completely vegetated, except where Lake Chad covered it, and that is thought to be an oscillating change that has varied with the strength of monsoons.

But I am writing about how many people can live on Earth, and the above relate to some limiting factors, climate and available arable land. Additions to the arable land and subtractions from that could be very large, and those would change how many people can survive on Earth. There are large areas in Siberia that would be productive, if there were more global warming, or if the region moved south. Those additions would balance the decreases in arable land that would happen if sea level change will be what has been predicted.

At present, there are about 7.7 billion people, and many of those people are living in areas where survival is marginal. That is particularly true in India and in parts of Africa. Famines have been common in both areas for as long as there have been records, and the situation has not improved. Some parts of Africa are barely habitable now, but they supported many people well in the past, and the Sahara is only one such region. If population were lower, then such mismatches between population and the productively of land would be minor problems. Then there is the matter of local land use customs versus productively. Also, in Africa, the local culture usually opposes large land holdings, but when Zimbabwe was a colony, it produced enough for everyone in Africa and for export, but those plantations were broken up into individual farms, and Zimbabweans can’t even feed themselves.

These are just a few of the many factors that go into determining how many people the Earth can support. If everything went well, then the present population would be fine, but that would depend on sea level rise in the future and on other changes in climate. What was a good population one year might become impossible a few years later. If we take a pessimistic view and consider what it would be like, if everything went poorly, then we would consider the present population to be much too large, and note that there wouldn’t be enough arable land to support that many after sea level rise, desertification, and other unpleasant events; the maximum that could live on Earth might be less than a billion in that case.

In addition to the physical support of a given population, there are psychological considerations. Many animals have trouble tolerating high population densities; rats are one example, but similar experiments have not been done with humans. With humans, conclusions about population density have been taken from the real world, and it appears that humans do not tolerate high population density well.

My opinion is that a maximum of about three quarters of a billion would be best for all.

Sea level rise since last Glacial maximum
Projected future sea level rise.
Video of the Earth after all the ice melts, not very good
Antarctic’s icing
Sahara Desert- monsoon oscillation
Analysis of behavior in various densities =of population in the Netherlands