What can we do now to help one hundred people survive here on Earth when all of the other people have perished? This is a disaster level DISS~13. This few people surviving for some several years after the disaster, and finding themselves in a world that is in very poor condition (but possibly getting better rapidly) would still be at serious risk of extinction. They would probably die out if they didn’t have substantial aid from remnants of our current civilization. If they didn’t have access to previous human learning, and wisdom to help them they would soon become wild animals. Natural mammal populations that currently fall to a population below five hundred are considered so endangered as to be near extinction, and require careful nurturing. This tiny human population would be facing additional hazards by entering a world that it was not adapted to through many generations of their ancestors’ social, and genetic experience.
It is difficult to imagine how or where such a small population could survive because some disaster that was so very bad that it killed six point seven billion people would seem likely to kill every last one of them. In fatal accident analysis a multiple death calculation is sometimes used. For example, a person in the World Trade Center who was hit by the terrorist plane would be killed by the impact, but also they would have been killed by the fire, and also by the smoke, and also by the fall when the building collapsed, and also by the collapsing building crushing them, etc. etc. In a world so devastated that it killed nearly everyone it is near impossible that one hundred would survive. To have killed so very many it would have to have killed most of them multiple times, and if there were so many ways to have been killed it is difficult for some small number to survive. It is more likely that one billion, or one million would survive than one hundred.
This scenario is for 100 survivors even though it is very unlikely. For one hundred to have survived they would have to have had an underground shelter with the ability to grow food, and keep growing it for several years. The mere storage of food, even if there were quite a large quantity, would not be enough because it would eventually run out, and it is going to run out faster the larger the population having access to it. To grow food underground requires power, power that is transformed into light. It is the created light that is used to grow the food. It is going to be a rare circumstance that provides several years of reliable continuous electricity of greater than two kilowatts per person to very large underground rooms. There are a multitude of other problems as well, such as a huge supply of light bulbs to convert the electricity to natural light for the plants to convert to food. Perhaps some atomic reactor or hydroelectric or ground oil electric plant could provide power this long without maintenance, but it would have to be a preexisting system or one easily set up at the Southern hemisphere in the last weeks. See: Lifehavens – A list of potential refuges for humanity.