Editing Nuclear winter (section)

=== 2019 ===
2019 saw the publication of two studies on nuclear winter that build on previous modeling and describe new scenarios of nuclear winter from smaller exchanges of nuclear weapons than have been previously simulated.

As in the 2007 study by Robock ''et al.'',<ref name="Robock"/> a 2019 study by Coupe ''et al.'' models a scenario in which 150 Tg of black carbon is released into the atmosphere following an exchange of nuclear weapons between the United States and Russia where both countries use all of the nuclear weapons [[New START|treaties]] permit them to.<ref name=":0">{{Cite journal|last1=Coupe|first1=Joshua|last2=Bardeen|first2=C.|last3=Robock|first3=A.|last4=Toon|first4=O.|date=2019|title=Nuclear Winter Responses to Nuclear War Between the United States and Russia in the Whole Atmosphere Community Climate Model Version 4 and the Goddard Institute for Space Studies ModelE|journal=Journal of Geophysical Research: Atmospheres|volume=124|issue=15|pages=8522–8543|doi=10.1029/2019JD030509|bibcode=2019JGRD..124.8522C|s2cid=200047350}}</ref> This amount of black carbon far exceeds that which has been emitted in the atmosphere by all volcanic eruptions in the past 1,200 years but is less than the asteroid impact which caused a mass extinction event 66 million years ago.<ref name=":0" /> Coupe ''et al.'' used the "[[Whole Atmosphere Community Climate Model|whole atmosphere community climate model]] version 4" (WACCM4), which has a higher resolution and is more effective at simulating [[aerosol]]s and stratospheric chemistry than the ModelE simulation used by Robock ''et al''.<ref name=":0" />

The WACCM4 model simulates that black carbon molecules increase to ten times their normal size when they reach the stratosphere. ModelE did not account for this effect. This difference in black carbon particle size results in a greater [[optical depth]] in the WACCM4 model across the world for the first two years after the initial injection due to greater [[Absorption (chemistry)|absorption]] of sunlight in the stratosphere.<ref name=":0" /> This will have the effect of increasing stratospheric temperatures by 100K and result in ozone depletion that is slightly greater than ModelE predicted.<ref name=":0" /> Another consequence of the larger particle size is accelerating the rate at which black carbon molecules fall out of the atmosphere; after ten years from the injection of black carbon into the atmosphere, WACCM4 predicts 2 Tg will remain, while ModelE predicted 19 Tg.<ref name=":0" />

The 2019 model and the 2007 model both predict significant temperature decreases across the globe, however the increased [[Optical resolution|resolution]] and particle simulation in 2019 predict a greater temperature anomaly in the first six years after injection but a faster return to normal temperatures. Between a few months after the injection to the sixth year of anomaly, the WACCM4 predicts cooler global temperatures than ModelE, with temperatures more than 20K below normal leading to freezing temperatures during the summer months over much of the northern hemisphere leading to a 90% reduction in agricultural growing seasons in the midlatitudes, including the midwestern United States.<ref name=":0" /> WACCM4 simulations also predict a 58% reduction in global annual precipitation from normal levels in years three and four after injection, a 10% higher reduction than predicted in ModelE.<ref name=":0" />

Toon ''et al.'' simulated a nuclear scenario in 2025 where [[India]] and [[Pakistan]] engage in a nuclear exchange in which 100 urban areas in Pakistan and 150 urban areas in India are attacked with nuclear weapons ranging from 15 kt to 100 kt and examined the effects of black carbon released into the atmosphere from [[Air burst|airburst]]-only detonations.<ref name=":1"/> The researchers modeled the atmospheric effects if all weapons were 15 kt, 50 kt, and 100 kt, providing a range where a nuclear exchange would likely fall into given the recent nuclear tests performed by both nations. The ranges provided are large because neither India nor Pakistan is obligated to provide information on their nuclear arsenals, so their extent remains largely unknown.<ref name=":1" />

Toon ''et al.'' assume that either a [[firestorm]] or [[conflagration]] will occur after each detonation of the weapons, and the amount of black carbon inserted into the atmosphere from the two outcomes will be equivalent and of a profound extent;<ref name=":1" /> in Hiroshima in 1945, it is predicted that the firestorm released 1,000 times more energy than was released during the nuclear explosion.{{sfn|Toon|Turco|Robock|Bardeen|2007}} Such a large area being burned would release large amounts of black carbon into the atmosphere. The amount released ranges from 16.1 Tg if all weapons were 15 kt or less to 36.6 Tg for all 100 kt weapons.<ref name=":1" /> For the 15 kt and 100kt range of weapons, the researchers modeled global precipitation reductions of 15% to 30%, temperature reductions between 4K and 8K, and ocean temperature decreases of 1K to 3K.<ref name=":1" /> If all weapons used were 50 kt or more, [[Hadley cell]] circulation would be disrupted and cause a 50% decrease in precipitation in the American midwest. [[Primary sector of the economy|Net primary productivity]] (NPP) for oceans decreases from 10% to 20% for the 15 kt and 100 kt scenarios, respectively, while land NPP decreases between 15% and 30%; particularly affected are midlatitude agricultural regions in the United States and Europe, experiencing 25-50% reductions in NPP.<ref name=":1" /> As predicted by other literature, once the black carbon is removed from the atmosphere after ten years, temperatures and NPP will return to normal.<ref name=":1" />