You can now see an excellent visualization of global catastrophic risks estimates produced in the Ragnarök series here.

In 1959, Richard Feynman pointed out that nanometre‐scale machines could be built and operated, and that the precision inherent in molecular construction would make it easy to build multiple identical copies. This raised the possibility of manufacturing at ever increasing speeds, in which production systems could rapidly and cheaply increase their productive capacity. This in turn suggested the possibility of destructive runaway self‐replication.

As Eric Drexler, a nanotech pioneer, first warned in Engines of Creation in 1986 (pg. 146),

In a mature form, molecular nanotechnology would enable the construction of bacterium-scale self-replicating mechanical robots that can feed on dirt or other organic matter. Such replicators could eat up the biosphere or destroy it by other means such as by poisoning it, burning it, or blocking out sunlight.

Plants with ‘leaves’ no more efficient than today’s solar cells could out‐compete real plants, crowding the biosphere with an inedible foliage. Tough omnivorous “bacteria” could out‐compete real bacteria: They could spread like blowing pollen, replicate swiftly, and reduce the biosphere to dust in a matter of days. A person of malicious intent in possession of this technology might cause a catastrophe on Earth by releasing such nanobots into the environment.

Such self-replicating systems, if not countered, could make the earth largely uninhabitable. Other potential risks include ecological and health disasters resulting from nano-pollutants, the use of misuse of nanotechnology weaponry, and, given the general-purpose character of nanotech, possibly much more.

Moreover, the technology to produce a destructive nanobot seems considerably easier to develop than the technology to create an effective defense against such an attack (a global nanotech immune system, an “active shield”). Regulation might also be hard. Nanotech doesn’t require rare radioactive isotopes or large, easily identifiable manufacturing plants, as does production of nuclear weapons.

Although only small portion of scientists might currently be working to develop self-replicating nanotech, a recent study done for NASA's Institute for Advanced Concepts by General Dynamics Advanced Information Systems suggests that a useful self-replicating machine could be less complex than a Pentium 4 chip, and uncovered no road blocks to extending macroscale systems to microscale and then to nanoscale self-replicating systems. Drexler points out that much of recent surprising progress comes from disparate fields, and isn't labelled generally "nanotechnology".

In the headline question to this series, we defined a global catastrophe as a 10% decrease in the world population in any period of 5 years.

If a global catastrophe happens before 2100, will it be principally due to the deployment nanotechnology?

The question resolves positively if a global nanotechnology catastrophe occurs that claims at least 10% in any period of 5 years or less before 2100, and resolves as ambiguous if no global catastrophe occurs. Years are here defined as consecutive calendar years.