3) What are the physical limits to growth if there is unlimited energy?   Much discussion today revolves around energy sources and the need either to conserve energy or develop new sources of energy. Why?  Because an expanding global population, coupled with improving per capita economic development, requires more energy.  So the great focus is on developing more energy sources, or increasing efficiency, and not discussing global population, which is driving the need of more energy in the first place.
Perhaps surprisingly, I visualize that too much energy will ultimately be a greater problem to the natural state of the Earth than too little energy.  While high efficiency solar energy will likely offer the best low-impact energy source for humanity, the possibility also exists for nuclear fusion to take a major place in energy production – this, like solar or wind energy, would be virtually inexhaustible.  Note that even unlimited renewable energy would still require transport of energy from solar- or wind-rich regions to regions poor in such resources.  There would also be practical limits to solar or wind energy, such that the entire land surface of the earth could not be completely covered, otherwise food could not be grown etc.  But a many-fold increase in energy production is possible; covering the Sahara Desert with 75% efficient solar panels would (besides altering the climate of the region drastically) generate about  1000 TW – current instantaneous global electricity production is roughly 2 TW.  So a relatively small fraction of solar panel coverage over the Saharan desert could supply many times the current global need for electricity (assuming it could be then transported in some manner across the globe).What is the problem with having an abundance of energy?  It is that unlimited energy allows for unlimited development of the earth’s surface.  More energy for fertilizers, more energy for transforming hilly terrain to flat terrain for more efficient agriculture.  Desalinization of sea water for dry regions would allow for an expansion of agriculture in arid zones.  Essentially all human activities would increase in intensity if low-cost, unlimited energy were available.  It would be similar to an “Agricultural green revolution” but for energy.  And an increased food supply would of course allow for an increase in population to match the increase in available resources, like the previous green revolutions.4) How many people should the earth have?  Are there too few or too many?

 

The difference between maximum sustainable populations and the most desirable planetary population.  This topic is almost never discussed; UN population estimates for the future are magically taken as truth, oblivious to the different possible scenarios of energy and technology that might take place (BUT SEE REPORT NOTED ABOVE).  Will the human population of the planet magically peak at about 9 billion people and thereafter stabilize?  Decrease to a small number?  Go extinct?  There is no really convincing estimate of how population may fluctuate as a function of available energy, agriculture, social conditions, social pressures etc.  One thing that appears certain is that if truly sustainable energy production increases, then the planet’s population carrying capacity will certainly increase.  (A temporary increase in energy would allow for a temporary population spike but not an increase to a stable higher value.)  Of course, much like a thin person exposed to daily all-you-can-eat buffets, the global population might not increase if some social controls are in place, but historically that has not been the case.The concept of an “optimal population” for the planet Earth is a complicated one to understand or attempt to quantify.  While there is clearly an upper limit to the population of the earth (in some sense ultimately governed by the physical dimensions of the earth), there is also a lower limit – and this is not simply the minimum number required to reproduce without genetic inbreeding.  An advanced civilization requires a large degree of specialization, unlike the simplest agrarian-based or hunter-gatherer society, which can exist as small bands of individuals.  For a civilization to have aircraft transport, for example, it must have factory workers to make and repair aircraft, it must have pilots to fly the aircraft, schools to teach flying procedures, individuals to maintain and refuel the aircraft, people to control the boarding of the aircraft, people to build the runways (operate the heavy machinery to make large buildings) etc etc etc.  The more advanced the society the more the specialization required.  The same can be said for diversity of society – if only one kind of restaurant (or food type) is needed then less innovation and less specialization is needed.  “Quality of life” can often be quantified (correctly or not) by the availability of diversity in society – the number of museums, stores, restaurants that are available reflects to some measure the quality of life.

Since the quality of life reflects the degree of specialization, and since small populations cannot specialize much (everyone needs to be a farmer or hunter, since agricultural production is very inefficient due to lack of mechanization) it follows that the larger the population the higher the potential quality of life.  But is living in a city of 1 million inhabitants better than living in a city of 1000?  Many other factors, such as travel time to work, become important in larger communities.  And clearly there is an upper limit to a global population (maximum carrying capacity) for a given set of energy and agricultural production conditions.  So how do we determine the optimal population?  Will the optimal population change as a function of technological state of the civilization?  These are questions that have no clear or easy answer.

5) What characteristics of the physical earth system should humanity seek to preserve far into the future (as long as there remains civilization, or lacking an asteroid impact…)?  I suggest that there must be other constraints that must be used to provide the upper and lower bounds of the Earth’s population, so that certain conditions that are deemed essential will always be maintained indefinitely far into the future.  For example, preservation of original or existing biodiversity must be a strong constraint, so that a human population cannot threaten any established species.  Long-term maintenance of a certain proportion of the land surface in its original ecological “state”, however defined, might be another.  The preservation of existing surface landforms might be another item – this is one that is very rarely mentioned. For example, the mining of the earth’s surface for road construction materials, ores, or rocks for construction, changes the character of the landscape for a period close to geological time.  In some very humid environments the removal of limestone in a quarry for construction might not be evident as a human-induced disturbance on the landscape after a few hundred years, and sand and gravel removal from a riverbed may not be apparent after a few major flood events.  But an open pit copper mine in northern Chile or the western US will still be evident as a deep and artificial depression (human “graffitti”) of the topography thousands of years into the future.  Witness the pyramids of Giza after 4500 years – features that are much smaller than open pit copper mines.