An upper bound can be generated
somewhat easily. The sun continues to grow larger and larger, and
one day it will become so large, in what is called the red giant
stage, that it will consume the Earth. The Earth will have become
thermally uninhabitable long before then. This means that humanity,
if it wishes to continue to survive, must move somewhere away from
Earth before Earth's thermal death. This event is billions of years
in the future, and it is much more likely that some event will happen
before then to threaten the existence of humanity.
The title question might have several
answers, depending on the lifestyle situation of the people. Since
it began, human society has been progressing through different forms,
which were controlled by the development of technology. The earliest
days of human society were back in the Stone Age, many millennia ago.
Then someone invented a process for producing copper and then
bronze, which allowed some changes to society. Following these
developments, iron was discovered, probably simultaneously with
charcoal, which permits smelting at a higher temperature.
Agriculture was started somewhere in these early ages, and animals
became domesticated, both as food sources and as work animals. Wind
began to be used, with sails for water transportation and mills for
grinding grain. Flowing water was also harnessed for mills.
All this set the stage for the
discovery and utilization of fossil fuel energy, first coal, then oil
and finally natural gas. These fuels led to a large increase in
excess energy, and science and technology began a very serious period
of development. Technology led to the utilization of steam power,
and later electricity was invented. This led to a plethora of
inventions, including electronics and microelectronics, which led to
computers. The stream of inventions continues to flow strongly and
constantly. This involves the mining of a great many different ores
around the world and their transportation.
We might ask: How long can humanity
survive at the agricultural level, where more concentrated forms of
energy have disappeared? We might also ask: How long can humanity
continue to exist at the high standard of living we now possess? But
this ignores the obvious fact that society will continue to develop
new scientific breakthroughs and technological discoveries. The rate
of change is astounding, compared to earlier eras; it is unlikely
that society only one century from now will be very similar to what
it is now. This problem may seem unsolvable, but it is not. It is
fortunate for our discussion that science is not infinite, but
instead there are only a finite amount of discoveries that can be
made. As more are completed, diminishing returns sets in, until some
asymptotic value is reached. Science doesn't stop entirely, but
since almost everything will have already become known, there are
only some smaller details that need to be figured out. Thus, we can
also ask, how long can humanity survive at the era of asymptotic
technology?
Society might be capable of destroying
itself, so we need to ask these three questions under the presumption
that humanity has started thinking about its long term future and can
make the choices necessary to avoid damaging its future prospects or
even committing social suicide, as with a nuclear war or horribly
malevolent virus. Now the questions appear to be more nicely framed:
given that humanity adopts the goal of lasting as long as possible,
how long would this be for us, at an agricultural level, at the
current industrial and electronic level, and at the asymptotic level?
What exactly does it mean to make
choices to promote the long-term survival of humanity? It is
necessary to come up with a list of events that might put an end to
us and then see what mankind might do to avoid them. At the
agricultural level, having the soil become depleted and gradually
produce less and less food every year is one possible problem. This
problem is quite noticeable and could be coped with, over the course
of many decades, by the means already devised for preventing soil
death. Growing fewer crops is the simplest solution, and growing
non-edible restorative crops certain years is another. But there
must be enough food to feed the whole population, so the essential
answer is not to allow the population to exceed the number that can
be fed using sustainable practices, in the worst years of agricutural
productivity. It would also be worthwhile to develop storage for a
few years of food to allow humanity to get through the very worst
years, which might be years in which volcanic dust so fills the upper
atmosphere that sunlight is blocked and crops fail all over.
It would be most interesting if a few
groups of people who like accounting and agriculture would make the
calculations necessary to give us an indication of what the
population and storage numbers might be. Their results would depend
on some assumptions about transportation. If one region of Earth
suffered from, say, serious floods, could other regions supply them
for a few years by sailboat shipment of food and other recovery
supplies? This might affect the calculations. As a wild guess,
based on historical numbers, a half billion might be the maximum
headcount for our planet in these restricted circumstances. There
would also be a minimum number of people necessary to maintain all
the operations of this future agricultural society.
This level 1 future society would be
vulnerable to catastrophes. There are many, both geological and
astronomical. The one most familiar is that of a large asteroid that
somehow manages to score a direct hit on Earth. If it were large
enough, all large lifeforms would die out. The asteroid might
rupture the crust, leading to what is called a basalt flood, where
lava from the mantle flows to the surface for thousands of years,
polluting the atmosphere and killing off much plant life. Basalt
floods can also occur without the intrusion of any asteroids, and
there is evidence of them in many parts of the world. Another
possible catastrophe is a nearby supernova, sending enough gamma rays
to the surface of the Earth to sterilize everything. Less well known
would be the passage near our solar system of a black hole or star,
which might disrupt the stable orbit of Earth, causing it to move to
a different radius or change its eccentricity, rendering it
uninhabitable. A massive ice age could also lead to an end to human
society.
A level 2 society depends on having
sufficient energy to run much machinery and computation, enabling a
higher standard of living than can be accomplished with an
agricultural-only society that lives on sunlight alone. Fossil fuels
are what have brought us to this standard of living today, but they
will be depleted soon enough, probably a few centuries more at the
most. Nuclear
power does seem sufficient to replace fossil fuels, perhaps with
some optional additional contribution from what is called renewable
power. This includes hydropower, wind farms and solar panel acres.
It is not clear what will be the constraining resource in such a
society. Perhaps it will be thorium, perhaps lithium, perhaps rare
earths, perhaps something entirely different. These calculations
cannot be made at the present, as we do not know exactly how such a
society would be organized. The profligate use of resources would
certainly have come to a halt, but there would necessarily be some
resources used every year, and some one of them would run out sooner
or later.
Such a level 2 society would also be
subject to extermination by the same list of catastrophes that the
level 1 society would, but there is a saving grace. It may be
possible to establish self-sustaining colonies on other planets or
satellites, either within or outside of our solar system. This
project is even more hazy than the project of revising society to
conserve resources and be efficient at energy use. It might be
possible, but there could also be some unforeseen barrier to such
colonization. We are too immature scientifically to make a good call
on this question. However, the length of time that humanity can
survive is very dependent on the sustainability of such an insurance
colony. If the mean time between life-destroying catastrophes is a
few million years, having a colony somewhere would multiply that many
times over. It is not even necessary that the colony prepare to
reconstitute life on Earth after the catastrophe, as long as it can
continue to survive and someday establish an insurance colony of its
own. A recent book, “Looking
Back from Luna”, is built on the hypothetical possibility that
a sustainable colony can be established on the moon, and one theme of
the book is the discussion of these colonists about what they might
be able to do were a catastrophe to strike Earth, but not them.
The level 3 society takes some real
imagination to contemplate. This society would have gone through
several future technological revolutions. These would likely include
a genetics revolution, after which genetics would be as controllable
as software and would become industrialized like software. It would
also likely include a neurological revolution, where we learn how to
maximize both the non-verbal and verbal learning of humans, and
thereby increase their resulting intellectual capability. This would
have education and psychological developments as fundamental
components. There would also likely be an organizational revolution,
where politics is replaced by some form of governance that looks
forward to the long-term prospects of human society.
Even though there are many, many
options for organizing such a society, and few of them could be
determined by us now, they would all still be subject to the similar
limitations on energy and mineral resource use, just as a level 2
society would be. This means they would have similar expected
endurance as a very-high-tech society as level 2 would have, within a
factor of ten most probably, and would require the same option of a
self-sustaining colony as insurance against catastrophes.
So the answer to the title question is
much the same for all levels of society, and it is the mean time
before a catastrophe hits. Is this a million years? Can Earth's
resources be stretched to last this long? We are not even capable of
making this estimate except as a simple guess. It is clear, however,
that for levels 2 and 3, if the population becomes serious about
having humanity last for a tremendously long time, measured in
millions of years, figuring out how to establish colonies on other
planets, which would be self-sustaining for long periods, is perhaps
the most important project we can undertake.