Editing Desertification (section)

==Causes==
{{See also|Deforestation#Causes}}

=== Immediate causes ===
The immediate cause of desertification is the loss of most vegetation. This is driven by a number of factors, alone or in combination, such as drought, climatic shifts, tillage for agriculture, overgrazing and [[Deforestation and climate change|deforestation]] for fuel or construction materials. Though vegetation plays a major role in determining the [[soil biology|biological composition of the soil]], studies have shown that, in many environments, the rate of erosion and runoff decreases exponentially with increased vegetation cover.<ref name=":8">{{cite book|author=Geeson, Nichola|title=Mediterranean desertification: a mosaic of processes and responses|publisher=John Wiley & Sons|year=2002|isbn=978-0-470-84448-9|page=58|url=https://books.google.com/books?id=G_0qg0f49GQC&pg=PA58|display-authors=etal|access-date=2016-05-16|archive-date=2023-07-30|archive-url=https://web.archive.org/web/20230730161049/https://books.google.com/books?id=G_0qg0f49GQC&pg=PA58|url-status=live}}</ref> Unprotected, dry soil surfaces blow away with the wind or are washed away by flash floods, leaving infertile lower soil layers that bake in the sun and become an unproductive hardpan.

=== Influence of human activities ===
Early studies argued one of the most common causes of desertification was overgrazing, over consumption of vegetation by cattle or other livestock.<ref>{{Cite journal |last=Charney |first=J. G. |date=April 1975 |title=Dynamics of deserts and drought in the Sahel |url=https://onlinelibrary.wiley.com/doi/10.1002/qj.49710142802 |journal=Quarterly Journal of the Royal Meteorological Society |language=en |volume=101 |issue=428 |pages=193–202 |doi=10.1002/qj.49710142802 |bibcode=1975QJRMS.101..193C |access-date=2022-05-24 |archive-date=2023-07-30 |archive-url=https://web.archive.org/web/20230730161050/https://rmets.onlinelibrary.wiley.com/doi/10.1002/qj.49710142802 |url-status=live }}</ref> However, the role of local overexploitation in driving desertification in the recent past is controversial.<ref name=":6">{{Cite journal |date=2016 |editor-last=Behnke |editor-first=Roy |editor2-last=Mortimore |editor2-first=Michael |title=The End of Desertification? |url=https://doi.org/10.1007/978-3-642-16014-1 |journal=Springer Earth System Sciences |language=en |doi=10.1007/978-3-642-16014-1 |isbn=978-3-642-16013-4 |s2cid=132424053 |issn=2197-9596}}</ref> Drought in the Sahel region is now thought to be principally the result of seasonal variability in rainfall caused by large-scale [[sea surface temperature]] variations, largely driven by natural variability and anthropogenic emissions of aerosols (reflective [[Particulates|sulphate particles]]) and greenhouse gases.<ref name=":3">{{Cite journal |last=Biasutti |first=Michela |date=July 2019 |title=Rainfall trends in the African Sahel: Characteristics, processes, and causes |journal=WIREs Climate Change |language=en |volume=10 |issue=4 |pages=e591 |doi=10.1002/wcc.591 |issn=1757-7780 |pmc=6617823 |pmid=31341517|bibcode=2019WIRCC..10E.591B }}</ref> As a result, changing ocean temperature and reductions in [[sulfate]] emissions have caused a re-greening of the region.<ref name=":3" /> This has led some scholars to argue that agriculture-induced vegetation loss is a minor factor in desertification.<ref name=":6" />

Human population dynamics have a considerable impact on overgrazing, over-farming and deforestation, as previously acceptable techniques have become unsustainable.<ref>{{Cite journal|last1=Epule|first1=Terence Epule|last2=Peng|first2=Changhui|last3=Lepage|first3=Laurent|date=February 2015|title=Environmental refugees in sub-Saharan Africa: a review of perspectives on the trends, causes, challenges and way forward|journal=GeoJournal|volume=80|issue=1|pages=79–92|doi=10.1007/s10708-014-9528-z|bibcode=2015GeoJo..80...79E |s2cid=154503204|issn=0343-2521}}</ref>

There are multiple reasons farmers use [[intensive farming]] as opposed to [[extensive farming]] but the main reason is to maximize yields.<ref name=":52">{{Cite web |date=2019-08-06 |title=Explainer: Desertification and the role of climate change |url=https://www.carbonbrief.org/explainer-desertification-and-the-role-of-climate-change |url-status=live |archive-url=https://web.archive.org/web/20220210001559/https://www.carbonbrief.org/explainer-desertification-and-the-role-of-climate-change |archive-date=2022-02-10 |access-date=2019-10-22 |website=Carbon Brief |language=en}}</ref> By increasing productivity, they require a lot more fertilizer, pesticides, and labor to upkeep machinery. This continuous use of the land rapidly depletes the nutrients of the soil causing desertification to spread.<ref>{{Cite web |last=Nations |first=United |title=World Day to Combat Desertification and Drought |url=https://www.un.org/en/observances/desertification-day |access-date=2023-09-11 |website=United Nations |language=en}}</ref><ref>{{Cite web|url=https://www.britannica.com/topic/intensive-agriculture|title=Intensive agriculture|website=Encyclopedia Britannica|language=en|access-date=2019-11-19|archive-date=2008-06-24|archive-url=https://web.archive.org/web/20080624184604/http://www.britannica.com/eb/article-9042533/intensive-agriculture|url-status=live}}</ref>

=== Natural variations ===
Scientists agree that the existence of a desert in the place where the Sahara desert is now located is due to natural variations in [[Solar irradiance|solar insolation]] due to [[Axial precession|orbital precession]] of the Earth.<ref>{{Cite journal |last1=Tierney |first1=Jessica E. |last2=Pausata |first2=Francesco S. R. |last3=deMenocal |first3=Peter B. |date=2017-01-06 |title=Rainfall regimes of the Green Sahara |journal=Science Advances |language=en |volume=3 |issue=1 |pages=e1601503 |bibcode=2017SciA....3E1503T |doi=10.1126/sciadv.1601503 |issn=2375-2548 |pmc=5242556 |pmid=28116352}}</ref> Such variations influence the strength of the West African Monsoon, inducing feedback in vegetation and dust emission that amplify the cycle of wet and dry Sahara climate.<ref>{{Cite journal |last1=Pausata |first1=Francesco S. R. |last2=Messori |first2=Gabriele |last3=Zhang |first3=Qiong |date=2016-01-15 |title=Impacts of dust reduction on the northward expansion of the African monsoon during the Green Sahara period |journal=Earth and Planetary Science Letters |language=en |volume=434 |pages=298–307 |bibcode=2016E&PSL.434..298P |doi=10.1016/j.epsl.2015.11.049 |issn=0012-821X |doi-access=free}}</ref> There is also a suggestion the transition of the Sahara from savanna to desert during the mid-[[Holocene]] was partially due to overgrazing by the cattle of the local population.<ref name="Humans as Agents in the Termination of the African Humid Period">{{cite journal |last1=K. Wright |first1=David |last2=Rull |first2=Valenti |last3=Roberts |first3=Richard |last4=Marchant |first4=Rob |last5=Gil-Romera |first5=Graciela |date=26 January 2017 |title=Humans as Agents in the Termination of the African Humid Period |journal=Frontiers in Earth Science |volume=5 |pages=4 |bibcode=2017FrEaS...5....4W |doi=10.3389/feart.2017.00004 |doi-access=free}}</ref>

=== Climate change ===
{{See also|Effects of climate change on the water cycle|Effects of climate change on biomes}}

Research into desertification is complex, and there is no single metric which can define all aspects. However, more intense climate change is still expected to increase the current extent of drylands on the Earth's continents: from 38% in late 20th century to 50% or 56% by the end of the century, under the "moderate" and high-warming [[Representative Concentration Pathway]]s 4.5 and 8.5. Most of the expansion will be seen over regions such as "southwest North America, the northern fringe of Africa, southern Africa, and Australia".<ref name=":52"/>

Drylands cover 41% of the earth's land surface and include 45% of the world's agricultural land.<ref name="Burrell">{{Cite journal |last1=Burrell |first1=A. L. |last2=Evans |first2=J. P. |last3=De Kauwe |first3=M. G. |date=2020 |title=Anthropogenic climate change has driven over 5 million km2 of drylands towards desertification |journal=Nature Communications |language=en |volume=11 |issue=1 |page=3853 |doi=10.1038/s41467-020-17710-7 |issn=2041-1723 |pmc=7395722 |pmid=32737311 |doi-access=free}} [[File:CC-BY icon.svg|50px]] Text was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License]</ref> These regions are among the most vulnerable ecosystems to [[Climate change|anthropogenic climate]] and [[land use change]] and are under threat of desertification. An observation-based attribution study of desertification was carried out in 2020 which accounted for climate change, [[Climate variability and change|climate variability]], [[CO2 fertilization effect|CO<sub>2</sub> fertilization]] as well as both the gradual and rapid ecosystem changes caused by land use.<ref name="Burrell" /> The study found that, between 1982 and 2015, 6% of the world's drylands underwent desertification driven by unsustainable land use practices compounded by anthropogenic climate change. Despite an average global greening, anthropogenic climate change has degraded 12.6% (5.43 million km<sup>2</sup>) of drylands, contributing to desertification and affecting 213 million people, 93% of who live in [[Developing country|developing economies]].<ref name="Burrell" />