The former US Secretary of Defense, Donald Rumsfeld made a famous statement about “things”:

As we know, there are known knowns. There are things we know we know. We also know there are known unknowns.
That is to say we know there are some things we do not know.
But there are also unknown unknowns, the ones we don’t know
we don’t know.

—Feb. 12, 2002, Department of Defense news briefing

Doomsday will probably come from some concatenation of unknowns from the collision of two or more predictable knowns but in some unanticipated and unknown way. The most likely type of problems of this type will be shortfalls of some previously known necessity. That is there will be something which is absolutely needed and which is realized is coming into short supply but where there is a hoped for replacement but when those replacements are requested they are found to be insufficient. When an effort is made to use some other resource for a fall back substitute it will too be found to be insufficient and thus a cascade of failures precipitates a progressive collapse. When this cascade of failure progresses and a realization of the impending collapse of alternatives becomes well appreciated it will cause desperate actions to be taken by some parties and then a general panic will overtake all parties. When this condition strikes the people in charge of some major weapons system the full blown effects of Doomsday will soon be upon us.

There are many things which could compound in this way to make this hideous mess and trigger the collapse but in the end it may be something even more trivial than the more obvious ones in the following list:

  1. Loss of topsoil
  2. Desertification
  3. Deforestation
  4. Toxic poisoning of drinking water
  5. Oceanic pollution
  6. Shrinking wetlands
  7. Overgrazing
  8. Loss of wilderness areas
  9. Species loss
  10. Shortage of firewood
  11. Exhaustion of oil reserves
  12. Exhaustion of mineral resources
  13. Siltation in rivers and estuaries
  14. Encroachment of human habitat on arable land
  15. Dropping water tables
  16. Erosion of the ozone layer
  17. CO2 accumulation
  18. Global warming
  19. Rising sea levels
  20. Nuclear wastes
  21. Acid rain
  22. Human and plant disease

This list is based in part upon, Paul Demeny Population and Development Review 1190 Vol. 16 p. 416.

All of these environmental problems are already upon us to some degree and all of them are aggravated by more human population. They are long term, multi-decade, problems and they are resistant to quick fixes. They must be healed by Earth’s natural processes. Each of them has a deleterious effect upon each of the others and there are probably unforseen interactions between them which will prove worse than any one taken seperately. The political interactions will also affect everyone and be interactive and when these are compounded with economic complications which affect the world economy there may be chaos which is unrelated to the items on the list but which aggravates something and generates a general progressive collapse. The unknown unknowns are in the interactions between these known problems but when they occur together they will create very real effects.