200 million megaton of explosions will not kill the biosphere. The current nuclear arsenal will not kill all humans and the pattern of nuclear explosions for a nuclear war between the largest nuclear powers will not destroy civilization, let alone kill all people or even half of all people. The greatest risks from a total nuclear war are from fire and starvation and not from the radiation or the blasts.
The historic high for megatonnage was reached in 1960 with nearly
20,500 megatons (that’s 20 billion tons, or 40 trillion pounds, of TNT)—the equivalent of about 1,400,000 Hiroshimas. Today the total is about one-tenth the 1960 level, or about 2,000 megatons, or 140,000 Hiroshimas. The 2000 megaton total is less than four times the above ground nuclear tests that have been performed and only six times than the peak years for nuclear testing 1961-1962 when 340 megatons were exploded.
Here is a link that has a list of the estimated nuclear weapons for each nation by name of weapon and tonnage.
Steps can and should be taken to sustain agriculture in case of a rapid climate change for any reason. A variant of the strategic seed reserve would be useful to hold short growing season seeds for distribution in case of some incident that shortens the growing season. The seeds should be distributed into repositories around the world so that they are easily accessible in spite of any significant disruption.
The longer science of nuclear war effects and Battlestar Galactica is here