Any disaster DISS~10 (death of 6.5 billion people) that dropped human population to 100 million (about 1.5% of current population) people would probably strike most heavily in the Northern Hemisphere. The most probable cause, an atomic war, would devastate the north more severely with its primary blast effects but the secondary effects, fallout and sun obscuring dust, would also be more severe in the next months and years. After the first month the lack of food, food distribution, and work toward food creation would exacerbate the problems in the coming years bringing about a continuing famine. Even those few remaining farmers attempting to maintain a crop production might find it impossible, and could even die trying while exposing themselves doing the necessary outdoor work. We citified humans tend to worry about the possibility of exposure to radiation, but we can escape most of its effects by going into protected places at least while sleeping or while not traveling. Plants and animals, on the other hand, have no options, and are exposed constantly to radiation dosages and other toxic environmental exposures which are only marginally deadly to humans, but become deadly to them, because they are totally exposed in these very stressed outdoor situations. The death of the wildlife, of the crops and of the domestic animals then comes back with yet another round of deadly force over the next months. Conditions grow worse for the survivors, as there is no replacement for the crops or the animals, and stored foods are consumed, or when uncared for rot. Therefore, in the Northern Hemisphere the disaster continues even for the well prepared survivors who had food stores set aside. The ensuing famine itself increases the likelihood of further conflict and it is said that 80% of past wars were caused by or at least were aggravated by food shortages, which were induced by weather problems, in this case it would be a nuclear winter.
The situation in this 100 million surviving scenario is much better in the Southern Hemisphere and that is where nearly all of those who are survivors would be. Only the most well prepared people in the North would survive even the first month whereas the southerners would have some time to prepare for the worst of the disaster. Most of the radiation, and dust are created in the north, and would have to cross the equator before becoming a problem for people in the south. The north to south exchange rate of air is probably less than 10% per year, and so most of the radioactive fallout from the north would have expended its toxic effects before it came across the equator. Only those bombs which were exploded in the South would be a life threatening problem and hopefully there are not enough targets in the south for the fallout to become life exterminating except locally downwind from the specific targets.
The above scenario is likely to play out in a disaster of that type, and so the question becomes, “What can be done, at this time, to help humanity survive and reestablish itself?” One thing that probably cannot be done is to set aside enough food to sustain a population of that size for very long, and so preparations for establishing extra fields of food crops, instead of power crops or export nonfood crops such as coffee, should be set up as quickly as possible, and specific fields should be identified for this purpose in the preparations. Fuels necessary to raise crops should be instantly allocated to those channels which are to become essential, and all non essential uses minimized. After a year it would become obvious what the post disaster distribution needed to be but initially it should be to secure food production.
The Lifehaven program should be instituted now and perhaps it would be best if most of these should be placed in remote locations in the Southern Hemisphere such as Easter Island, Tasmania, Fiji, New Zealand, Samoa, Cook Islands, Antarctica . Once operational these could be maintained as self supporting world storage sites for the world’s information, and banking data as well as seed banks, DNA data and complete ecosystem storage vaults. These Lifehavens might not be opened in a DISS~10 event except to get some small percentage of seeds for regenerating obliterated areas, which would be immediately replaced when the first crop was brought in. The reason to keep the Lifehavens intact even after the disaster is because there may be further collapse of population even to extinction outside of the Lifehavens.
At present we can’t expect any government to fund any of the Lifehavens, because they cost money, and governments are sensitive to being laughed at, and are too shortsighted to realize that war is possible, and when war is possible they allocate all of their resources to its demands. However, the setting up of the plans for a DISS~10 is no more than setting up contingency plans in these southern locations, and so it is little more than bureaucratic drawing up of paper plans. Perhaps some portion of the World Bank could allocate a few of its people to set this up, perhaps in their spare time. The basic system only needs to be done once, and after that it is only maintenance of the records. Or perhaps one of the foundations could be persuaded that humanity is worth saving.