Nuclear holocaust

=== Nuclear winter ===
{{Main|Nuclear winter}}
In the early 1980s, scientists began to consider the effects of smoke and soot arising from burning wood, plastics, and petroleum fuels in nuclear-devastated cities. It was speculated that the intense heat would carry these particulates to extremely high altitudes where they could drift for weeks and block out all but a fraction of the sun's light.<ref name="Britannica">{{cite web|title=Nuclear winter|url=https://www.britannica.com/topic/nuclear-winter|website=Encyclopædia Britannica|access-date=13 February 2016}}</ref> A landmark 1983 study by the so-called TTAPS team ([[Richard P. Turco]], [[Owen Toon]], Thomas P. Ackerman, [[James B. Pollack]] and [[Carl Sagan]]) was the first to model these effects and coined the term "nuclear winter."<ref>{{cite journal |title=Nuclear Winter: Global Consequences of Multiple Nuclear Explosions |journal=Science |volume=222 |issue=4630 |pages=1283–92 |date=23 December 1983 |pmid=17773320 |doi=10.1126/science.222.4630.1283|bibcode = 1983Sci...222.1283T |last1=Turco |first1=R. P. |last2=Toon |first2=O. B. |last3=Ackerman |first3=T. P. |last4=Pollack |first4=J. B. |last5=Sagan |first5=C. |s2cid=45515251 }}</ref>

More recent studies make use of modern global circulation models and far greater computer power than was available for the 1980s studies. A 2007 study examined the consequences of a global nuclear war involving moderate to large portions of the current global arsenal.<ref name="Robock2007">{{cite journal|last1=Robock|first1=Alan|last2=Oman|first2=Luke|last3=Stenchikov|first3=Georgiy L.|title=Nuclear winter revisited with a modern climate model and current nuclear arsenals: Still catastrophic consequences|journal=Journal of Geophysical Research|date=2007|volume=112|issue=D13107|page=14|doi=10.1029/2006JD008235|url=http://climate.envsci.rutgers.edu/pdf/RobockNW2006JD008235.pdf|access-date=13 February 2016|bibcode=2007JGRD..11213107R|doi-access=free}}</ref> The study found cooling by about 12–20&nbsp;°C in much of the core farming regions of the US, Europe, Russia and China and as much as 35&nbsp;°C in parts of Russia for the first two summer growing seasons. The changes they found were also much longer-lasting than previously thought, because their new model better represented entry of soot aerosols in the upper stratosphere, where precipitation does not occur, and therefore clearance was on the order of 10 years.<ref name="ToonandRobock2010" /> In addition, they found that global cooling caused a weakening of the global hydrological cycle, reducing global [[Precipitation (meteorology)|precipitation]] by about 45%.

The authors did not discuss the implications for agriculture in depth, but noted that a 1986 study which assumed no food production for a year projected that "most of the people on the planet would run out of food and starve to death by then" and commented that their own results show that, "This period of no food production needs to be extended by many years, making the impacts of nuclear winter even worse than previously thought."<ref name="Robock2007" />

In contrast to the above investigations of global nuclear conflicts, studies have shown that even small-scale, regional nuclear conflicts 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 [[Atomic bombings of Hiroshima and Nagasaki|Hiroshima]]-sized nuclear weapons (about 15 kilotons each) on major populated centres, 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.<ref name="ScienceDaily">[https://www.sciencedaily.com/releases/2006/12/061211090729.htm Regional Nuclear War Could Devastate Global Climate], Science Daily, December 11, 2006</ref><ref name="RobockRegional2007"/><ref name="ToonandRobock2010"/> The cooling would last for years, and according to the research, could be "catastrophic". Additionally, the analysis showed a 10% drop in average global precipitation, with the largest losses in the low latitudes due to failure of the monsoons.

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.<ref name=Mills2008>{{cite journal |title=Massive global ozone loss predicted following regional nuclear conflict |journal=Proc. Natl. Acad. Sci. U.S.A. |date=2008 |pmid=18391218 |pmc=2291128 |doi=10.1073/pnas.0710058105 |last1=Mills |first1=M. J. |last2=Toon |first2=O. B. |last3=Turco |first3=R. P. |last4=Kinnison |first4=D. E. |last5=Garcia |first5=R. R. |volume=105 |issue=14 |pages=5307–12 |bibcode = 2008PNAS..105.5307M |doi-access=free }}[http://acd.ucar.edu/~mmills/pdf/2008MillsPNAS_MassiveOzoneLoss.pdf as PDF] {{webarchive|url=https://web.archive.org/web/20160304100404/http://acd.ucar.edu/~mmills/pdf/2008MillsPNAS_MassiveOzoneLoss.pdf |date=2016-03-04 }}</ref> 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 that are experienced below the ozone hole above Antarctica every spring.<ref name="ToonandRobock2010"/>