aerosol amazon standard identification number american geophysical union atomic bombings of hiroshima and nagasaki carl sagan city civil defense climate comet cresson kearny dobson unit doomsday device earth eurasia fire freeman dyson global climate model goddard institute for space studies gram greenhouse effect gulf war impact winter journal of geophysical research mars nasa north america nuclear war survival skills nuclear summer nuclear warfare nuclear weapon oak ridge national laboratory optical depth ozone propaganda scenario science magazine smoke soot stratosphere supervolcano the national interest ton troposphere united states national academy of sciences volcanic winter world meteorological organization world war ii