Yesterday, I posted my thoughts on the maximum sustainable human population, given current technology and land forms. But what if we got rid of those constraints and made some plausible guesses about future technology? How high can we go from 25 billion?
The biggest improvement would come from increasing crop yields. Yesterday's assumptions were a mostly vegetarian diet produced with modern organic yields. However, crop yields are slowly and steadily increasing, including organic at places like Rodale Intitute, a leading organic research farm. Additionally, if you combined organic with genetically modified organisms and otherwise got rid of some of the anti-scientific elements of the organic movement, it is easy to imagine increasing crop yields significantly.
There are other potential tricks up our sleeve as well. For example, climate change will have a mixed effect on crop yields, as CO2 fertilization battles it out with water stress and desertification. If we can mitigate the latter, crop yields for many staples could increase by 10-15%. Or we could get more wild, and used space-based reflectors to alter seasons, cooling the equatorial regions and warming the arctic areas in particular, perhaps by lengthening the evenings in the cooler half of the year. This would result in longer growing seasons (or multiple seasons) than is the current case, increasing yields. We could also engage in cloud seeding and increase rainfall in areas that are water-constrained. All in all, we might be able to increase crop yields by a third or half relative to modern organic practice, feeding an extra 8-12 billion, on current land.
Also, fungi are a possible food source that I did not mention yesterday. They can be grown underground using forest waste as their food source, providing us with just a bit more food. Another trick we might use is ocean fertilization, to increase the productivity of the oceans by seeding the relatively dead areas with the minerals that are constraining biological activity. Of course, land reclamation from the ocean is also possible in some places. This land could be used either for farming directly or more likely, used for living space, freeing up interior lands for farming. Synthetic foods are another real possibility, and may beat out photosynthesis on a total energy basis. If this were ever successful, the maximum population could be substantially higher. It is almost impossible to estimate at this point, however.
If solar PV efficiency increases (and it will), naturally less land will be used. So yesterday's land use assumptions were unnecessarily pessimistic, and the reality would be that some of the land assigned to PV could be reassigned to food production. Also note that we can (and do) get some of our energy from other renewable sources such as hydro, wind, tidal, and geothermal, which are more space efficient. Ignoring these made yesterday's estimates too high. Likewise, I assumed that the new population would live on the same land as we are currently using, but if we wanted to, we could go even higher density than that. It is certainly possible, and this would reclaim more land - often prime farm land - for food production. On top of all this, wide-scale terraforming is possible. Much of the marginal land that was assigned as "Other" or meadows or unfarmable forest is unfarmable precisely because it is hilly. We have bulldozers, and lots of time, making it possible to reclaim some of this land for productive use.
Oh, and there is no reason we cannot change ourselves. Using genetic engineering, we could make ourselves smaller and select for the most energy efficient among us, cutting our caloric needs dramatically.
Combining all this, I can certainly imagine something like 40 or even 50 billion people living on earth, in a sustainable manner, with a level of comfort similar to that of the citizens of modern industrial economies.