question: What would happen to the Southern Hemisphere if a catastrophic nuclear exchange happened in the Northern Hemisphere?
comment: Radioactivity from the 2056 nuclear explosions over 7 decades have already contaminated exposed steel worldwide so we have resorted to salvage ["low-background steel"]( URL_0 ) from WW2 shipwrecks for high quality instruments. A full out nuclear war with several thousands of atmospheric detonations within hours would hit us much harder and render huge portions of land in the NH dangerously contaminated. Survivors would have to live without a functioning infrastructure probably for many years and endure famines and diseases and suffer long term damage, like an increasing rate of cancer due to radiation. Potentially an extinction scenario for humans and other species. The effects due to the dust blasted into the atmosphere and the radioactive fallout would disperse and reach the Southern Hemisphere somewhat later. Several studies try to simulate the outcome. The nuclear winter might not happen - they coined the term nuclear autum, though. (smh) tl;dr: Avoid nuclear war at all costs.
passages: Few prevailing winds cross the Equator and effects of nuclear explosions in the Northern Hemisphere might send less fallout to the Southern Hemisphere.
  
  The five southern NWFZs together cover all land in the Southern Hemisphere except East Timor, still in the process of joining ASEAN, and Atlantic and Indian Ocean islands belonging to non-NWFZ countries in the box (map) bounded by 60° S, 20° W, and 115° E, which combined have less than 8000 km² of land area:
  
  BULLET::::- St. Helena and its dependencies Ascension Island and Tristan da Cunha, a British overseas territory
  
  BULLET::::- Bouvet Island, a Norwegian territory
   BULLET::::- 1883 eruption of Krakatoa, which caused approximately 1 kelvin of global cooling for 2 years due to sulfate emissions.
  
  BULLET::::- Dalton Minimum, 1790 to 1830, a period of prolonged solar minimum activity, resulting in Earth receiving lower insolation values.
  
  BULLET::::- Doomsday device
  
  BULLET::::- Global dimming, global reduction in ground insolation, due to the atmospheric injection of aerosols from various sources.
  
  BULLET::::- Impact winter
  
  BULLET::::- Laki, 1783 eruption of an Icelandic volcano which produced continentally localized cooling for 1–2 years.
  
  BULLET::::- List of states with nuclear weapons
   Another flag officer then explains to Condor that the world has taken a lot of abuse in its time, and even the damage done today would be absorbed and things would eventually go back to normal. However, if there is another nuclear exchange it could spell disaster for the planet, including human extinction from nuclear winter or the destruction of vital parts of the atmosphere.
   In 1981, William J. Moran began discussions and research in the National Research Council (NRC) on the airborne soil/dust effects of a large exchange of nuclear warheads, having seen a possible parallel in the dust effects of a war with that of the asteroid-created K-T boundary and its popular analysis a year earlier by Luis Alvarez in 1980. An NRC study panel on the topic met in December 1981 and April 1982 in preparation for the release of the NRC's "The Effects on the Atmosphere of a Major Nuclear Exchange", published in 1985.
   Nuclear famine
  
  Nuclear famine is a hypothesized famine considered a potential threat following global or regional nuclear exchange. It is thought that even subtle cooling effects resulting from a regional nuclear exchange could have a substantial impact on agriculture production, triggering a food crisis amongst the world's survivors.
   Section::::Potential climatic precedents.
  
  Similar climatic effects to "nuclear winter" followed historical supervolcano eruptions, which plumed sulfate aerosols high into the stratosphere, with this being known as a volcanic winter.
  
  Similarly, extinction-level comet and asteroid impacts are also believed to have generated impact winters by the pulverization of massive amounts of fine rock dust. This pulverized rock can also produce "volcanic winter" effects, if sulfate-bearing rock is hit in the impact and lofted high into the air, and "nuclear winter" effects, with the heat of the heavier rock ejecta igniting regional and possibly even global forest firestorms.
   The Latin American, African, South Pacific and Southeast Asian zones also cover most land in the tropics, but not some Northern Hemisphere areas south of the Tropic of Cancer. Most tropical land outside of NWFZs is in India and the Arabian Peninsula.
  
  Little of the land area covered by the five southern Nuclear-Weapon-Free Zones extends north of the Tropic of Cancer: only northern Mexico, northern Bahamas, northern Myanmar, and North Africa. However, the Central Asian and Mongolian zones are entirely in the North Temperate Zone.
  
  Section::::Northern Hemisphere.
   Compared to climate change for the past millennium, even the smallest exchange modeled would plunge the planet into temperatures colder than the Little Ice Age (the period of history between approximately 1600 and 1850 AD). This would take effect instantly, and agriculture would be severely threatened. Larger amounts of smoke would produce larger climate changes, and for the 150 teragrams (Tg) producing a true nuclear winter (1 Tg is 10 grams), making agriculture impossible for years. In both cases, new climate model simulations show that the effects would last for more than a decade.
   Section::::By region and country.:Asia.:Australia.
   Much of the speculative research on nuclear war-induced climate change to date focuses on a hypothetical, large-scale nuclear exchange between modern day Russia and the United States. However, the post-Cold War world also includes a number of other nuclear-armed countries — such as India, Pakistan, and North Korea — that are currently engaged in de facto or frozen armed conflicts with their neighbors. In comparison to "global" nuclear war, a regional conflict between nations with relatively small nuclear arsenals would likely produce less dramatic climate effects. Nonetheless, it has been argued that global cooling resulting from such a conflict could have large-scale impacts on agriculture and food supply systems worldwide.
   Section::::Climate engineering.
   Section::::Global famine due to regional nuclear conflict.
   Regional nuclear conflicts could also inflict significant damage to the ozone layer. A 2008 study found that a regional nuclear weapons exchange could create a near-global ozone hole, triggering human health problems and impacting agriculture for at least a decade. This effect on the ozone would result from heat absorption by soot in the upper stratosphere, which would modify wind currents and draw in ozone-destroying nitrogen oxides. These high temperatures and nitrogen oxides would reduce ozone to the same dangerous levels we now experience below the ozone hole above Antarctica every spring.
  
  Section::::Effects of nuclear war.:Nuclear famine.
   If a nuclear explosion did occur, it occurred within the 3,000-mile-wide (4,800 km diameter) circle covering parts of the Indian Ocean, the South Atlantic, the southern tip of Africa, and a small part of Antarctica.
  
  Section::::Possible responsible parties.:Israel.
   In the United States National Research Council (NRC) book "Long-Term Worldwide Effects of Multiple Nuclear-Weapons Detonations" published in 1975 it states that a nuclear war involving 4,000 Mt from "present arsenals" would probably deposit much less dust in the stratosphere than the Krakatoa eruption, judging that the effect of dust and oxides of nitrogen would probably be slight climatic cooling which "would probably lie within normal global climatic variability, but the possibility of climatic changes of a more dramatic nature cannot be ruled out".
   Section::::Consequences.
  
  Section::::Consequences.:Climatic effects.
   BULLET::::- 1961 - The magnetopause, boundary between magnetosphere and the solar wind, is observed by Explorer 12. The measurements confirm predictions made in 1931 by Chapman and Ferraro.
  
  BULLET::::- 1962 - In July, a U.S. H-bomb test (Project Starfish) above the central Pacific Ocean creates a radiation belt of high-energy electrons, parts of which remain until 1967. The new belt creates aurora at Samoa and unexpectedly knocks out 3 artificial satellites.
   Section::::Nuclear power in NWFZ states.
  
  Four NWFZ countries have nuclear plants to generate electricity.
  
  South Africa formerly had a nuclear weapons program which it terminated in 1989.
  
  Argentina and Brazil are known to operate uranium enrichment facilities. Countries that had enrichment programs in the past include Libya and South Africa, although Libya's facility was never operational. Australia has announced its intention to pursue commercial enrichment, and is actively researching laser enrichment.
  
  Argentina and Brazil also have plans to build nuclear submarines.
  
  Section::::Protocols for non-member states.
   In the section of this 1975 NRC book pertaining to the issue of fireball generated NOx and ozone layer loss therefrom, the NRC present model calculations from the early-to-mid 1970s on the effects of a nuclear war with the use of large numbers of multi-megaton yield detonations, which returned conclusions that this could reduce ozone levels by 50 percent or more in the northern hemisphere.
   Section::::In popular culture.
  
  The 1957 novel "On the Beach" by Nevil Shute is about a nuclear war that released so much radioactive fallout that all the life in the Northern Hemisphere disappeared, while the Southern Hemisphere awaited a similar fate. The American government voiced a criticism of this premise—a threat of extinction from nuclear war—because they did not have enough nuclear weapons to cause human extinction.
   A study presented at the annual meeting of the American Geophysical Union in December 2006 found that even a small-scale, regional nuclear war could disrupt the global climate for a decade or more. In a regional nuclear conflict scenario where two opposing nations in the subtropics would each use 50 Hiroshima-sized nuclear weapons (about 15 kiloton each) on major population centers, the researchers estimated as much as five million tons of soot would be released, which would produce a cooling of several degrees over large areas of North America and Eurasia, including most of the grain-growing regions. The cooling would last for years, and, according to the research, could be "catastrophic".
   BULLET::::- "On the 8th Day" – Nuclear winter documentary (1984) filmed by the BBC and available on Internet video hosting websites; chronicles the rise of the hypothesis, with lengthy interviews of the prominent scientists who published the nascent papers on the subject.
  
  Section::::Media.
  
  BULLET::::- "": A book co-authored by Carl Sagan in 1984 which followed his co-authoring of the TTAPS study in 1983.
  
  BULLET::::- "Threads": A 1984 docu-drama that Carl Sagan assisted in an advisory capacity.
   Section::::Global famine due to regional nuclear conflict.:Vulnerable populations.
  
  The International Physicians for the Prevention of Nuclear War (IPPNW) reported in 2013 that more than two billion people would be at risk of starvation in the event of a limited nuclear exchange, such as one that could occur between India and Pakistan, or by the use of even a small number of the nuclear weapons held by the US and Russia.
   Section::::Effects on biosphere.
   One of the major results of TTAPS' 1990 paper was the re-iteration of the team's 1983 model that 100 oil refinery fires would be sufficient to bring about a small scale, but still globally deleterious nuclear winter.
  
  Following Iraq's invasion of Kuwait and Iraqi threats of igniting the country's approximately 800 oil wells, speculation on the cumulative climatic effect of this, presented at the World Climate Conference in Geneva that November in 1990, ranged from a nuclear winter type scenario, to heavy acid rain and even short term immediate global warming.
  