Weapons Of Extermination (WOEs) are those designed to kill species. When human intelligence and research are aimed at destroying humans, not as individuals, but as large genetic groups, there exists the possibility of being too successful and designing something which will kill all humans.
In the past, genocidal efforts have been made with rabbits in Australia and with other unwanted invasive species, such as the kudzu vine (Pueraria lobata), Andean Pampas grass (Cortaderia jubata), and yellow starthistle (Centaurea solstitialis).
In a balanced environment, all species are attempting a relative crowding of their own species members for food and mates, but they are also crowding other species with overlapping needs in more absolute terms for those same necessities of life. However, sometimes an alien species with an unusual advantage, developed elsewhere, can invade a locality and totally crowd out many otherwise healthy species, creating a local havoc.
Humans are the most successful of the invasive species, and they have crowded out and replaced with their own bodies and with their domesticated species many entire stable environments. Also, by their pollution they have made the entire Earth’s ecosystem different, which is disruptive to otherwise stable local environments. This has generally been good for humans and their numbers have grown prodigiously. As I am a human I find this growth to be a positive thing, but we have been so successful that we are consuming our base. In the short run of say another ten years or so this overconsumption is probably sustainable, but in the long run, which may be as short as forty years and unlikely to be so long as eighty years, collapse is inevitable. A huge population when it runs short of some necessity which cannot be economically replaced will suddenly consume all of the item, and some sort of disaster will ensue. That collapse will happen even without a major war, but it will probably precipitate a major war, which will make it even worse.
In the 19th century there was the intentional destruction in the US of some competitor species such as the passenger pigeon, the American bison, coyotes, wolves, and Grizzly bears. This was usually done on an individual basis, by shooting, trapping or poisoning, etc. This type of exterminating behavior, carried out in a widespread and sustained way, can lead to the extinction of a species, but it is very difficult. Humans have exterminated lots of species unintentionally, but the only species to have been intentionally exterminated is the smallpox virus. And now we’re attempting to eradicate polio.
The WOEs that are applicable to humans are probably limited at present to a Doomsday war where nearly all of the atomic bombs are exploded over cities. The bombs don’t kill everyone but the smoke of the burning cities creates so much smoke that crops fail for a growing season and perhaps for years. With only two months of food in storage at any time, the survivors would quickly eat it up, and there would be nothing left for anyone. However, even in this grim scenario there would probably be a few pockets of a small number of survivors who just happened to be near a large storage-depot of food.
There is the possibility of a Cobalt bomb, which is a standard Hydrogen bomb with a casing of cobalt. When the H-bomb explodes it converts the cobalt into a radioactive isotope which is long lived enough to create several years of sterilization effects. These effects would be planet-wide, but it would take quite a few bombs to sterilize the whole planet. There isn’t much reason for anyone to use them because it simply kills everything they hold dear. And yet, their use isn’t impossible; after all, some people commit suicide, so these types of weapons must be absolutely eliminated. Fortunately, very few people will ever have possession of these types of weapons. It has been remarkably difficult even for countries to develop conventional atomic bombs, and a cobalt bomb would require these atomic bombs to trigger them and much more.
What is perhaps the most worrisome are biological weapons, because a single person with the right knowledge, equipment, time, money and genocidal inclinations might be able to fabricate a deadly disease. A new disease might be undefended against by the human body and yet be quite contagious. If this disease had some ubiquitous co-host, such as rodents or flies which carried the disease but didn’t die from it, then this new disease might be sustained even when there were no humans around. And yet, when there were humans around they would contract the disease and die. That terrible combination of effects of a disease might be capable of humanicide. I hope not, but in our new world of instant transmission of new knowledge it might become a group effort and be accomplished. I am very enthusiastic about free access of knowledge, but these WOES are a case where it would be counterproductive for all humanity.