Re-allocating the NASA budget to these other worthy causes would disappear in the noise of cost overruns. Spending to start a human presence in space would open the way to resources far beyond what's available on Earth, and would provide these resources without pollution of Earth. We could turn the Earth back into the wilderness it was 1000 years ago, while providing for a greater human population than present. The threat of destroying the ozone layer by introduction of artificial CFCs was discovered by another program to study the interactions of complex chemicals in Vesus' upper atmosphere, as reported by early Mariner space probe data. The threat of flooding the surface in UV by not having an ozone layer was found out by studying data returned from Mars.
Nuclear winter is the same effect whether caused by war, asteroid impact, or volcanism.
This was found out by modelling Mars atmosphere data from Mariner 9. The upper clouds were too warm, while the surface was too cold.