Editing Little Ice Age (section)

==Possible causes==
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[[File:Radiative-forcings.svg|thumb|upright=1.4|The relative effect of various forcings]]
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Scientists have tentatively identified seven possible causes of the Little Ice Age: [[orbital forcing|orbital cycles]], decreased [[solar variation|solar activity]], increased volcanic activity, altered [[ocean current|ocean current flows]],<ref name=":0">{{cite journal |last1=Wanamaker |first1=Alan D. |last2=Butler |first2=Paul G. |last3=Scourse |first3=James D. |last4=Heinemeier |first4=Jan |last5=Eiríksson |first5=Jón |last6=Knudsen |first6=Karen Luise |last7=Richardson |first7=Christopher A. |year=2012 |title=Surface changes in the North Atlantic meridional overturning circulation during the last millennium |journal=[[Nature Communications]] |volume=3 |issue=1 |pages=899 |bibcode=2012NatCo...3..899W |doi=10.1038/ncomms1901 |pmc=3621426 |pmid=22692542 }}</ref> fluctuations in the human population in different parts of the world causing [[reforestation]] or [[deforestation]], and the inherent variability of global climate.

===Orbital cycles===
{{main|Milankovich cycles}}
[[Orbital forcing]] from cycles in the Earth's orbit around the Sun has for the past 2,000 years caused a long-term northern hemisphere cooling trend, which continued through the Middle Ages and the Little Ice Age. The rate of [[Arctic]] cooling is roughly 0.02&nbsp;°C per century.<ref name="bello">{{Cite journal |last1=Kaufman |first1=D. S. |last2=Schneider |first2=D. P. |last3=McKay |first3=N. P. |last4=Ammann |first4=C. M. |last5=Bradley |first5=R. S. |last6=Briffa |first6=K. R. |last7=Miller |first7=G. H. |last8=Otto-Bliesner |first8=B. L. |author-link8=Bette Otto-Bliesner |last9=Overpeck |first9=J. T. |last10=Vinther |first10=B. M. |last11=Abbott |first11=M. |last12=Axford |first12=M. |last13=Bird |first13=Y. |last14=Birks |first14=B. |last15=Bjune |first15=H. J. B. |year=2009 |title=Recent Warming Reverses Long-Term Arctic Cooling |url=http://www.geo.umass.edu/climate/papers2/Kaufman2009a.pdf |journal=Science |volume=325 |issue=5945 |pages=1236–1239 |bibcode=2009Sci...325.1236K |citeseerx=10.1.1.397.8778 |doi=10.1126/science.1173983 |pmid=19729653 |s2cid=23844037 |last0=Harshit |first0=H. P. |author32=Arctic Lakes 2k Project Members |last16=Briner |first16=A. E. |last17=Cook |first17=J. |last18=Chipman |first18=T. |last19=Francus |first19=M. |last20=Gajewski |first20=P. |last21=Geirsdottir |first21=K. |last22=Hu |first22=A. |last23=Kutchko |first23=F. S. |last24=Lamoureux |first24=B. |last25=Loso |first25=S. |last26=MacDonald |first26=M. |last27=Peros |first27=G. |last28=Porinchu |first28=M. |last29=Schiff |first29=D. |last30=Seppa |first30=C. |last31=Seppa |first31=H.}}<br />{{cite web |date=3 September 2009 |title=Arctic Warming Overtakes 2,000 Years of Natural Cooling |url=http://www2.ucar.edu/news/846/arctic-warming-overtakes-2000-years-natural-cooling |url-status=dead |archive-url=https://web.archive.org/web/20110427235538/http://www2.ucar.edu/news/846/arctic-warming-overtakes-2000-years-natural-cooling |archive-date=27 April 2011 |access-date=19 May 2011 |publisher=UCAR}}<br />{{cite magazine |last=Bello |first=David |date=4 September 2009 |title=Global Warming Reverses Long-Term Arctic Cooling |url=http://www.scientificamerican.com/article.cfm?id=global-warming-reverses-arctic-cooling |magazine=Scientific American |access-date=19 May 2011}}</ref> That trend could be extrapolated to continue into the future and possibly lead to a full ice age, but the 20th-century [[instrumental temperature record]] shows a sudden reversal of that trend, with a rise in global temperatures attributed to [[greenhouse gas emissions]].<ref name="bello" />

===Solar activity===
{{Main|Solar activity|Solar activity and climate}}
[[File:Sunspot Numbers.png|thumb|upright=1.8|The Maunder Minimum in a 400-year history of sunspot numbers]]
[[File:Little Ice Age and Sunspots.png|thumb|upright=1.8|Sunspot number compared with Northern Hemisphere (NH) temperature anomaly. The upper panel shows 11-year smoothed group sunspot numbers from telescopic observations and the sunspot number derived from carbon-14 cosmogenic isotope abundances in tree trunks. The lower panel shows the Northern Hemisphere (NH) temperature anomaly (relative to the 1990 level) from a wide variety of paleoclimate proxies: the black line is the mean value, and the colors give the uncertainty probability distribution. The blue dots are the instrumental record. The dashed lines mark the start and end of the Little Ice Age (LIA) defined by the (NH) temperature anomaly level −0.16 degrees Celsius.<ref name="owens1"/><ref name="lock1"/>]]
Solar activity includes any disturbances on the Sun such as sunspots and solar flares associated with the variable magnetic field of the solar surface and solar atmosphere (corona). Because [[Alfvén's theorem]] applies, the coronal magnetic field is dragged out into the heliosphere by the [[solar wind]]. Irregularities in this [[heliospheric magnetic field]] shield Earth from galactic [[cosmic ray]]s by scattering them, which allows scientists to track solar activity in the past by analyzing both the carbon-14 or beryllium-10 isotopes generated by cosmic rays hitting the atmosphere and which are deposited in terrestrial reservoirs such as tree rings and ice sheets. In the intervals 1400–1550 (the [[Spörer Minimum]]) and 1645–1715 (the [[Maunder Minimum]]) there were very low recorded levels of solar activity and they are both within, or at least overlapped with, the LIA for most definitions. However, solar activity deduced from cosmogenic isotopes was as high between the Spörer Minimum and the Maunder Minimum as it was in about 1940,<ref>{{Cite journal |last1=Usoskin |first1=I. G. |date=March 2017 |title=A history of solar activity over millennia |journal=[[Living Reviews in Solar Physics]] |language=en |volume=15 |issue=3 |page=3 |bibcode=2017LRSP...14....3U |doi=10.1007/s41116-017-0006-9 |s2cid=195340740 |doi-access=free|arxiv=0810.3972 }}</ref> yet this interval is also within the LIA. Hence any relationship between solar activity and the LIA is far from a simple one.

A drop in solar activity circa 1230 AD as measured by biogenic silica corrected ignition residue (IR-<sub>BSi</sub>) has been suggested by one study as a forcing potentially responsible for initiating the LIA, with the authors noting that this drop in solar output preceded the onset of significant volcanism.<ref>{{Cite journal |last1=Kokfelt |first1=U. |last2=Muscheler |first2=R. |date=March 2013 |title=Solar forcing of climate during the last millennium recorded in lake sediments from northern Sweden |url=http://journals.sagepub.com/doi/10.1177/0959683612460781 |journal=[[The Holocene]] |language=en |volume=23 |issue=3 |pages=447–452 |doi=10.1177/0959683612460781 |bibcode=2013Holoc..23..447K |s2cid=128814633 |issn=0959-6836 |access-date=11 November 2023}}</ref>

A study by Dmitri Mauquoy et al. confirmed that at the beginning of the Spörer Minimum, the carbon-14 production rate rose rapidly.<ref>{{Cite journal|last1=Mauquoy|first1=Dmitri|last2=van Geel|first2=Bas|last3=Blaauw|first3=Maarten|last4=van der Plicht|first4=Johannes|date=1 January 2002|title=Evidence from northwest European bogs shows 'Little Ice Age' climatic changes driven by variations in solar activity|url=https://doi.org/10.1191/0959683602hl514rr|journal=[[The Holocene]]|language=en|volume=12|issue=1|pages=1–6|doi=10.1191/0959683602hl514rr|bibcode=2002Holoc..12....1M|s2cid=131513256|issn=0959-6836}}</ref> These authors argued this rise coincided with a sharp drop in temperatures deduced from European peat bogs. This temperature drop is also seen in mean northern hemisphere temperatures deduced from a wide variety of paleoclimate indicators but the timing of the onset of the Spörer Minimum is actually some 50 years earlier.<ref name="owens1"/> A 50-year response lag is possible but is not consistent with subsequent variations in inferred solar activity and average northern hemisphere temperature.<ref name="owens1"/> For example, the peak in solar activity between the Spörer Minimum and the Maunder Minimum is 50 years after the only peak in average northern hemisphere temperature that it could be associated with.

A study by [[Judith Lean]] in 1999 also pointed to a relationship between the Sun and the Little Ice Age. Her research found that there was a 0.13% total [[solar irradiance]] (TSI) increase (1.8&nbsp;Wm{{sup|−1}}) over 1650–1790 which could have raised the temperature of the Earth by 0.3&nbsp;°C. In the calculated correlation coefficients of the global temperature response to their reconstruction of the solar forcing over three different periods, they found an average coefficient of 0.79 (i.e. 62% of the variation could be explained by the TSI) which indicates a possible relationship between the two components. Lean's team also formulated an equation in which the temperature change is 0.16&nbsp;°C increase in temperature for every 0.1% increase in total solar irradiance.<ref>{{Cite journal|date=1 January 1999|title=Evaluating sun–climate relationships since the Little Ice Age|url=https://www.sciencedirect.com/science/article/abs/pii/S1364682698001138|journal=[[Journal of Atmospheric and Solar-Terrestrial Physics]]|language=en|volume=61|issue=1–2|pages=25–36|doi=10.1016/S1364-6826(98)00113-8|issn=1364-6826|last1=Lean|first1=Judith|author-link=Judith Lean|last2=Rind|first2=David|bibcode=1999JASTP..61...25L}}</ref> However, the main problem with quantifying the longer-term trends in TSI lies in the stability of the absolute radiometry measurements made from space, which has improved since the pioneering work of Judith Lean discussed above, but still remains a problem.<ref>{{Cite journal |last1=Kopp |first1=G. |date=April 2014|title=An assessment of the solar irradiance record for climate studies|url=https://www.swsc-journal.org/articles/swsc/abs/2014/01/swsc130036/swsc130036.html |journal=J. Space Weather and Space Climate |language=en |volume=4|pages=A14 |doi=10.1051/swsc/2014012|bibcode=2014JSWSC...4A..14K |doi-access=free }}</ref><ref>{{Cite journal |last1=Kopp |first1=G. |date=July 2016|title=Magnitudes and timescales of total solar irradiance variability |journal=Journal of Space Weather and Space Climate |language=en |volume=6 |pages=A30 |doi=10.1051/swsc/2016025|arxiv=1606.05258 |bibcode=2016JSWSC...6A..30K |s2cid=55902879 }}</ref> Analysis comparing trends in modern observations of TSI and cosmic ray fluxes shows that the uncertainties mean that it is possible that TSI was actually higher in the Maunder Minimum than present-day levels, but uncertainties are high with best estimates of the difference between the modern-day TSI and the Maunder-Minimum TSI in the range ±0.5&nbsp;Wm{{sup|−1}} but with a 2&nbsp;σ uncertainty range of ±1&nbsp;Wm{{sup|−1}}.<ref>{{Cite journal |last1=Lockwood |first1=M. |last2= Ball |first2=W. |date=May 2020 |title=Placing limits on long-term variations in quiet-Sun irradiance and their contribution to total solar irradiance and solar radiative forcing of climate |url=https://centaur.reading.ac.uk/90804/ |journal=[[Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences]] |language=en |volume=476 |issue=2238 |pages=20200077 |issn=1364-5021 |doi=10.1098/rspa.2020.0077|pmid=32831591 |pmc=7428030 |bibcode=2020RSPSA.47600077L }}</ref>

At the center of the LIA, during the [[Spörer Minimum]] and the [[Maunder Minimum]], sunspots were minimal and cosmogenic isotope deposition (carbon-14 and beryllium-10) was increased in these minima as a result. However, detailed studies from multiple paleoclimate indicators show that the lower Northern Hemisphere temperatures in the Little Ice Age began before the start of the [[Maunder Minimum]] but after the start of the Spörer Minimum and persisted until after the Maunder Minimum (and even after the much weaker Dalton Minimum) had ceased. The return to more active solar conditions between these two grand solar minima had no obvious effect on either global or Northern Hemisphere temperatures. The [[Central England Temperature]] provide evidence that low solar activity may have contributed to the LIA through the increased occurrence of cold winters, at least in Europe,<ref>{{Cite journal |last1=Lockwood |first1=M. |last2= Harrison |first2=R. G. |last3=Woollings |first3=T. |last4=Solanki |first4=S. K. |date=2010 |title=Are cold winters in Europe associated with low solar activity? |journal=[[Environmental Research Letters]] |language=en |volume=5 |issue=2 |pages=024001 |doi=10.1088/1748-9326/5/2/024001|bibcode=2010ERL.....5b4001L |s2cid=10669151 |doi-access=free }}</ref> but colder summers are more correlated with volcanic activity.<ref name="lock1"/> Comparison of TSI records with Greenland ice core δ<sup>18</sup>O trends suggests that solar activity only accounted for 55% of the observed trend variance.<ref>{{Cite journal |last1=Fischer |first1=H. |last2=Werner |first2=M. |last3=Wagenbach |first3=D. |last4=Schwager |first4=M. |last5=Thorsteinnson |first5=T. |last6=Wilhelms |first6=F. |last7=Kipfstuhl |first7=J. |last8=Sommer |first8=S. |date=1998-05-15 |title=Little Ice Age clearly recorded in northern Greenland ice cores |journal=[[Geophysical Research Letters]] |language=en |volume=25 |issue=10 |pages=1749–1752 |doi=10.1029/98GL01177 |bibcode=1998GeoRL..25.1749F |s2cid=128608360 |doi-access=free }}</ref> Numerical climate modelling indicates that volcanic activity was the greater driver of the overall lower temperatures in the LIA, as seen in a variety of paleoclimate proxies.<ref name="owens1"/>

===Volcanic activity===
{{Main|Little Ice Age volcanism}}
In a 2012 paper, Miller ''et al.'' link the Little Ice Age to an "unusual 50-year-long episode with four large sulfur-rich explosive eruptions, each with global sulfate loading >60 Tg" and notes that "large changes in [[solar irradiance]] are not required."<ref name="miller2012" />

Throughout the LIA, there was heightened volcanic activity.<ref name="Robock1979">{{cite journal |last1=Robock |first1=Alan |date=21 December 1979 |title=The "Little Ice Age": Northern Hemisphere Average Observations and Model Calculations |url=https://www.science.org/doi/10.1126/science.206.4425.1402 |journal=[[Science (journal)|Science]] |volume=206 |issue=4425 |pages=1402–1404 |bibcode=1979Sci...206.1402R |doi=10.1126/science.206.4425.1402 |pmid=17739301 |s2cid=43754672 |access-date=11 September 2023}}</ref> When a [[volcano]] erupts, its ash reaches high into the atmosphere and can spread to cover the whole earth. The ash cloud blocks out some of the incoming solar radiation, which leads to [[volcanic winter|worldwide cooling]] for up to two years after an eruption. Also emitted by eruptions is [[sulfur]] in the form of [[sulfur dioxide]]. When sulfur dioxide reaches the [[stratosphere]], the gas turns into [[sulfuric acid|sulfuric]] and [[sulfurous acid]] particles, which reflect the Sun's rays. That further reduces the amount of radiation reaching the Earth's surface.

A recent study found that an especially severe tropical volcanic eruption in 1257, possibly [[1257 Samalas eruption|Mount Samalas]] (pre-caldera edifice of the active Rinjani) near [[Mount Rinjani]], both in [[Lombok]], Indonesia, followed by three smaller eruptions in 1268, 1275, and 1284, did not allow the climate to recover. That may have caused the initial cooling, and the [[1452/1453 mystery eruption]] triggered a second pulse of cooling.<ref name="miller2012">{{cite journal |last1=Miller |first1=Gifford H. |last2=Geirsdóttir |first2=Áslaug |last3=Zhong |first3=Yafang |last4=Larsen |first4=Darren J. |last5=Otto-Bliesner |first5=Bette L. |author-link5=Bette Otto-Bliesner |last6=Holland |first6=Marika M. |author-link6=Marika Holland |last7=Bailey |first7=David A. |last8=Refsnider |first8=Kurt A. |last9=Lehman |first9=Scott J. |last10=Southon |first10=John R. |last11=Anderson |first11=Chance |last12=Björnsson |first12=Helgi |last13=Thordarson |first13=Thorvaldur |date=30 January 2012 |title=Abrupt onset of the Little Ice Age triggered by volcanism and sustained by sea-ice/ocean feedbacks |url=https://www.sciencedaily.com/releases/2012/01/120130131509.htm |journal=[[Geophysical Research Letters]] |volume=39 |issue=2 |pages=n/a |bibcode=2012GeoRL..39.2708M |citeseerx=10.1.1.639.9076 |doi=10.1029/2011GL050168 |s2cid=15313398}}</ref> The cold summers can be maintained by sea-ice/ocean feedbacks long after volcanic aerosols are removed.

Other volcanoes that erupted during the era and may have contributed to the cooling include [[Billy Mitchell (volcano)|Billy Mitchell]] (c.&nbsp;1580), [[Huaynaputina]] (1600), [[Mount Parker (Cotabato)|Mount Parker]] (1641), [[Long Island (Papua New Guinea)]] (ca.&nbsp;1660), and [[Laki]] (1783).<ref name=aspects/> The [[1815 eruption of Mount Tambora|1815 eruption of Tambora]], also in Indonesia, blanketed the atmosphere with ash, and the following year came to be known as the [[Year Without a Summer]],<ref>{{Cite web|url=https://thewire.in/the-sciences/is-the-meghalayan-event-a-tipping-point-in-geology|title=Is the Meghalayan Event a Tipping Point in Geology?|website=The Wire}}</ref> when [[frost]] and snow were reported in June and July in both [[New England]] and Northern Europe.

===Ocean circulation===
[[File:Ocean circulation conveyor belt.jpg|thumb|upright=1.6|Thermohaline circulation or Oceanic conveyor belt illustrated]]
In the early 2000s, a slowing of [[thermohaline circulation]] was proposed as an explanation for the LIA,<ref name="Broecker00">{{cite journal |last1=Broecker |first1=Wallace S. |year=2000 |title=Was a change in thermohaline circulation responsible for the Little Ice Age? |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |volume=97 |issue=4 |pages=1339–1342 |bibcode=2000PNAS...97.1339B |doi=10.1073/pnas.97.4.1339 |jstor=121471 |pmc=34299 |pmid=10677462 |doi-access=free}}</ref><ref name=":0"/><ref>{{cite web |url=https://science.nasa.gov/headlines/y2004/05mar_arctic.htm |title=A Chilling Possibility – NASA Science |publisher=Science.nasa.gov |access-date=24 June 2013 |url-status=dead |archive-url=https://web.archive.org/web/20100317001142/http://science.nasa.gov/headlines/y2004/05mar_arctic.htm |archive-date=17 March 2010  }}</ref> specifically, through the weakening of the [[North Atlantic Gyre]].<ref>{{Cite journal |last1=Lapointe |first1=Francois |last2=Bradley |first2=Raymond S. |date=2021-12-17 |title=Little Ice Age abruptly triggered by intrusion of Atlantic waters into the Nordic Seas |journal=[[Science Advances]] |volume=7 |issue=51 |pages=eabi8230 |doi=10.1126/sciadv.abi8230 |issn=2375-2548 |pmc=8673760 |pmid=34910526 |bibcode=2021SciA....7.8230L }}</ref><ref name="Hopkin2006">{{cite web |author=Hopkin, Michael |date=29 November 2006 |title=Gulf Stream weakened in 'Little Ice Age' |url=http://www.bioedonline.org/news/news.cfm?art=2966 |access-date=1 February 2019 |website=BioEd Online}}</ref> The circulation could have been interrupted by the introduction of a large amount of fresh water into the North Atlantic and might have been caused by a period of warming before the LIA that is known as the [[Medieval Warm Period]].<ref name=Fagan/><ref name="Villanueva2009">{{cite news |last=Villanueva |first= John Carl |title=Little Ice Age |work= Universe Today |url=http://www.universetoday.com/guide-to-space/earth/little-ice-age/ |date=19 October 2009 |access-date=22 September 2010}}</ref><ref>{{cite journal |last1=Pittenger |first1= Richard F. |last2=Gagosian |first2= Robert B. |title=Global Warming Could Have a Chilling Effect on the Military |url=http://handle.dtic.mil/100.2/ADA422382  |archive-url=https://web.archive.org/web/20120531100736/http://handle.dtic.mil/100.2/ADA422382  |url-status=dead  |archive-date=31 May 2012  |format=PDF |journal=Defense Horizons  |volume= 33  |date=October 2003 |access-date=22 September 2010 }}</ref> Some researchers have thus classified the LIA as a [[Bond event]].<ref>{{cite book |last1=Banerji |first1=Upasana S. |url=https://www.sciencedirect.com/book/9780323900850/holocene-climate-change-and-environment |title=Holocene Climate Change and Environment |last2=Padmalal |first2=D. |publisher=Elsevier |year=2021 |isbn=978-0-323-90085-0 |editor-last1=Kumaran |editor-first1=Navnith |location= |pages=293–339 |chapter=12 – Bond events and monsoon variability during Holocene—Evidence from marine and continental archives |doi=10.1016/B978-0-323-90085-0.00016-4 |s2cid=244441781 |access-date=9 September 2023 |editor-last2=Damodara |editor-first2=Padmalal |chapter-url=https://www.sciencedirect.com/science/article/abs/pii/B9780323900850000164}}</ref> In 2005 there was some concern that a [[shutdown of thermohaline circulation]] could happen again as a result of the present warming.<ref name="Leake2005">{{cite news| url=http://www.timesonline.co.uk/tol/news/uk/article520013.ece | work=The Times | location=London | title=Britain faces big chill as ocean current slows | first=Jonathan | last=Leake | date=8 May 2005 | access-date=11 May 2010 | url-status=dead | archive-date=February 8, 2007 | archive-url=https://web.archive.org/web/20070208064001/http://www.timesonline.co.uk/tol/news/uk/article520013.ece}}</ref><ref>{{Scientific American Frontiers|15| 5 |title=Hot Planet – Cold Comfort |url=https://www.pbs.org/saf/1505/index.html |network=PBS |date=16 February 2005 |transcript=Only a Little Ice Age |transcript-url=https://www.pbs.org/saf/1505/segments/1505-3.htm}}</ref>

More recent research indicates that the overall Atlantic Meridional Overturning Circulation may already be weaker now than it was during the LIA,<ref name="RahmstorfBox2015">{{cite journal |last1=Rahmstorf |first1=Stefan |last2=Box |first2=Jason E. |last3=Feulner |first3=Georg |last4=Mann |first4=Michael E. |last5=Robinson |first5=Alexander |last6=Rutherford |first6=Scott |last7=Schaffernicht |first7=Erik J. |year=2015 |title=Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation |url=https://eprints.ucm.es/32657/1/robinson10postprint.pdf |archive-url=https://web.archive.org/web/20160909080138/http://eprints.ucm.es/32657/1/robinson10postprint.pdf |archive-date=2016-09-09 |url-status=live |journal=[[Nature Climate Change]] |volume=5 |issue=5 |pages=475–480 |bibcode=2015NatCC...5..475R |doi=10.1038/nclimate2554 |issn=1758-678X }} {{Closed access}} [http://wedocs.unep.org/bitstream/handle/20.500.11822/17802/Exceptional_twentiethcentury_slowdown_in_Atla.pdf PDF in UNEP Document Repository] {{Webarchive|url=https://web.archive.org/web/20190712144829/http://wedocs.unep.org/bitstream/handle/20.500.11822/17802/Exceptional_twentiethcentury_slowdown_in_Atla.pdf |date=12 July 2019 }}</ref><ref name="Thorn2018">{{Cite journal |last1=Thornalley |first1=David JR |display-authors=etal  |date=11 April 2018 |title=Anomalously weak Labrador Sea convection and Atlantic overturning during the past 150 years |url=https://www.nature.com/articles/s41586-018-0007-4 |journal=[[Nature (journal)|Nature]] |volume=556 |issue=7700 |pages=227–230 |doi=10.1038/s41586-018-0007-4 |pmid=29643484 |bibcode=2018Natur.556..227T |s2cid=4771341 |access-date=3 October 2022}}</ref> or perhaps even over the past millennium.<ref>{{Cite journal |last1=Caesar |first1=L. |last2=McCarthy |first2=G.D. |last3=Thornalley |first3=D. J. R. |last4=Cahill |first4=N. |last5=Rahmstorf |first5=S. |date=25 February 2021 |title=Current Atlantic Meridional Overturning Circulation weakest in last millennium |url=https://www.nature.com/articles/s41561-021-00699-z |journal=Nature Geoscience |volume=14 |issue=3 |pages=118–120 |doi=10.1038/s41561-021-00699-z |bibcode=2021NatGe..14..118C |s2cid=232052381 |access-date=3 October 2022}}</ref> While there is still a robust debate about the present-day AMOC strength,<ref>{{Cite journal |last1=Kilbourne |first1=Kelly Halimeda |last2=et |first2=al. |date=17 February 2022 |title=Atlantic circulation change still uncertain |url=https://www.nature.com/articles/s41561-022-00896-4 |journal=[[Nature Geoscience]] |volume=15 |issue=3 |pages=165–167 |doi=10.1038/s41561-022-00896-4 |bibcode=2022NatGe..15..165K |s2cid=246901665 |access-date=3 October 2022|hdl=2117/363518 |hdl-access=free }}</ref><ref>{{Cite journal |last1=Caesar |first1=L. |last2=McCarthy |first2=G. D. |last3=Thornalley |first3=D. J. R. |last4=Cahill |first4=N. |last5=Rahmstorf |first5=S. |date=17 February 2022 |title=Reply to: Atlantic circulation change still uncertain |url=https://www.nature.com/articles/s41561-022-00897-3 |journal=[[Nature Geoscience]] |volume=15 |issue=3 |pages=168–170 |bibcode=2022NatGe..15..168C |doi=10.1038/s41561-022-00897-3 |s2cid=246901654 |access-date=3 October 2022}}</ref><ref>{{Cite journal |last1=Latif |first1=Mojib |last2=Sun |first2=Jing |last3=Visbeck |first3=Martin |last4=Bordbar |date=25 April 2022 |title=Natural variability has dominated Atlantic Meridional Overturning Circulation since 1900 |journal=[[Nature Climate Change]] |volume=12 |issue=5 |pages=455–460 |doi=10.1038/s41558-022-01342-4|bibcode=2022NatCC..12..455L |s2cid=248385988 |doi-access=free }}</ref> these findings make the link between AMOC and the LIA unlikely. However, some research instead suggests that a far more localized disruption of the [[North Atlantic Current|North Subpolar Gyre]] convection was involved in the LIA.<ref>{{Cite journal |last1=Arellano-Nava |first1=Beatriz |last2=Halloran |first2=Paul R. |last3=Boulton |first3=Chris A. |last4=Scourse  |first4=James |last5=Butler |first5=Paul G. |last6=Reynolds |first6=David J. |last7=Lenton |first7=Timothy  |date=25 August 2022 |title=Destabilisation of the Subpolar North Atlantic prior to the Little Ice Age |journal=[[Nature Communications]] |volume=13 |issue=1 |page=5008 |doi=10.1038/s41467-022-32653-x |pmid=36008418 |pmc=9411610 |bibcode=2022NatCo..13.5008A |s2cid=251842966 }}</ref> This is potentially relevant for the near future, as a minority of [[climate model]]s project a permanent collapse of this convection under some scenarios of future [[climate change]].<ref>{{Cite journal |last1=Swingedouw |first1=Didier |last2=Bily |first2=Adrien |last3=Esquerdo |first3=Claire |last4=Borchert |first4=Leonard F. |last5=Sgubin |first5=Giovanni |last6=Mignot |first6=Juliette |last7=Menary |first7=Matthew |date=2021 |title=On the risk of abrupt changes in the North Atlantic subpolar gyre in CMIP6 models |url=https://nyaspubs.onlinelibrary.wiley.com/doi/10.1111/nyas.14659 |journal=[[Annals of the New York Academy of Sciences]] |volume=1504 |issue=1 |pages=187–201 |bibcode=2021NYASA1504..187S |doi=10.1111/nyas.14659 |pmid=34212391 |s2cid=235712017|hdl=10447/638022 |hdl-access=free }}</ref><ref>{{Cite journal |last1=Armstrong McKay |first1=David |last2=Abrams |first2=Jesse |last3=Winkelmann |first3=Ricarda |last4=Sakschewski |first4=Boris |last5=Loriani |first5=Sina |last6=Fetzer |first6=Ingo |last7=Cornell |first7=Sarah |last8=Rockström |first8=Johan |last9=Staal |first9=Arie |last10=Lenton |first10=Timothy |date=9 September 2022 |title=Exceeding 1.5°C global warming could trigger multiple climate tipping points |url=https://www.science.org/doi/10.1126/science.abn7950 |journal=[[Science (journal)|Science]] |language=en |volume=377 |issue=6611 |pages=eabn7950 |doi=10.1126/science.abn7950 |issn=0036-8075 |pmid=36074831 |s2cid=252161375 |hdl=10871/131584|hdl-access=free }}</ref><ref>{{Cite web |last=Armstrong McKay |first=David |date=9 September 2022 |title=Exceeding 1.5°C global warming could trigger multiple climate tipping points – paper explainer |url=https://climatetippingpoints.info/2022/09/09/climate-tipping-points-reassessment-explainer/ |access-date=2 October 2022 |website=climatetippingpoints.info |language=en}}</ref>

===Decreased human populations===
Some researchers have proposed that human influences on climate began earlier than is normally supposed (see [[Early anthropocene]] for more details) and that major population declines in Eurasia and the Americas reduced that impact and led to a cooling trend.

==== The Black Death in Europe ====
The [[Black Death]] is estimated to have killed 30% to 60% of the [[demographics of Europe|European population]].<ref>{{Cite book|url=https://books.google.com/books?id=YiHHnV08ebkC&pg=PA21|title=A pest in the land: new world epidemics in a global perspective|last=Austin Alchon|first=Suzanne|publisher=University of New Mexico Press|year=2003|isbn=978-0-8263-2871-7|page=21}}</ref> In total, the plague may have reduced the world population from an estimated 475 million to 350–375 million in the 14th century.<ref>{{cite web|url=https://www.census.gov/data/tables/time-series/demo/international-programs/historical-est-worldpop.html|title=Historical Estimates of World Population|publisher=Census.gov|access-date=28 April 2019}}</ref> It took 200 years for the world population to recover to its previous level.<ref>{{cite journal|last=Jay|first=Peter|date=17 July 2000|title=A Distant Mirror|url=http://www.time.com/time/world/article/0,8599,2050585,00.html|journal=TIME Europe|volume=156|issue=3|archive-url=https://web.archive.org/web/20080725005418/http://www.time.com/time/europe/magazine/2000/0717/peter.html|archive-date=25 July 2008|url-status=dead|access-date=25 January 2018}}</ref> [[William Ruddiman]] ''et al.'' proposed that those large population reductions in Europe, East Asia, and the Middle East caused a decrease in agricultural activity that allowed [[reforestation]] to cause additional [[carbon dioxide removal|carbon dioxide uptake]] from the atmosphere, leading to LIA cooling.<ref name=Holocene_20110801/>

==== Mongol invasions ====
A 2011 study by the [[Carnegie Institution]]'s Department of Global Ecology asserts that the [[Mongol invasions and conquests]], which lasted almost two centuries, contributed to global cooling by [[Destruction under the Mongol Empire|depopulating vast regions]] and replacing cultivated land by carbon-absorbing forest.<ref>{{cite web |url=https://carnegiescience.edu/news/war-plague-no-match-deforestation-driving-co2-buildup|title=War, Plague No Match For Deforestation In Driving CO2 Buildup|author=<!--Not stated-->|date=20 January 2011|publisher=Carnegie Institution for Science|access-date=8 December 2019}}</ref><ref name= "Q10">{{Cite Q | Q106515792}}</ref>

==== Destruction of native populations and biomass of the Americas ====
William Ruddiman further hypothesized that a [[population history of the indigenous peoples of the Americas|reduced population in the Americas]] after European contact started in the 16th century could have had a similar effect.<ref name="Ravilious2006">{{Cite news |last=Ravilious |first=Kate |title=Europe's chill linked to disease |date=27 February 2006 |publisher=BBC |url=http://news.bbc.co.uk/1/hi/sci/tech/4755328.stm}}</ref><ref name="Ruddiman2003">{{cite journal |last1=Ruddiman |first1=William F. |year=2003 |title=The Anthropogenic Greenhouse Era Began Thousands of Years Ago |journal=[[Climatic Change (journal)|Climatic Change]] |volume=61 |issue=3 |pages=261–293 |citeseerx=10.1.1.651.2119 |doi=10.1023/B:CLIM.0000004577.17928.fa |bibcode=2003ClCh...61..261R |s2cid=2501894}}</ref> In a similar vein, Koch and others in 1990 suggested that as European conquest and disease brought by Europeans killed as many as 90% of Indigenous Americans, around 50 million hectares of land may have returned to a wilderness state, causing increased carbon dioxide uptake.<ref>David Graeber and David Wendgrow, "The Dawn of Everything" (New York: Farrar, Straus and Giroux, 2021) p. 258.</ref> Other researchers have supported depopulation in the Americas as a factor and have asserted that humans cleared considerable amounts of forest to support agriculture there before the arrival of Europeans brought on a population collapse.<ref name=ams1>{{Cite journal |doi=10.1175/EI157.1 |bibcode=2006EaInt..10k...1F |title=Evidence for the Postconquest Demographic Collapse of the Americas in Historical CO2 Levels |journal=[[Earth Interactions]] |volume=10 |issue=11 |pages=1 |last1=Faust |first1=Franz X. |last2=Gnecco |first2=Cristóbal |last3=Mannstein |first3=Hermann |last4=Stamm |first4=Jörg |year=2006 |url=https://elib.dlr.de/43708/1/Faust_et_al_2006.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://elib.dlr.de/43708/1/Faust_et_al_2006.pdf |archive-date=2022-10-09 |url-status=live }}</ref><ref>Richard J. Nevle ''et al''., "Ecological-hydrological effects of reduced biomass burning in the neotropics after A.D. 1500," ''Geological Society of America Meeting'', Minneapolis MN, 11 October 2011. [http://gsa.confex.com/gsa/2011AM/finalprogram/abstract_196092.htm abstract] {{Webarchive|url=https://web.archive.org/web/20190815074531/https://gsa.confex.com/gsa/2011AM/finalprogram/abstract_196092.htm|date=15 August 2019}}. Popular summary: "[http://www.sciencenews.org/view/generic/id/335168/title/Columbus_arrival_linked_to_carbon_dioxide_drop Columbus' arrival linked to carbon dioxide drop: Depopulation of Americas may have cooled climate]," ''Science News,'' 5 November 2011. (access date 2 January 2012).</ref>

Richard Nevle, Robert Dull and colleagues further suggested not only that anthropogenic forest clearance played a role in reducing the amount of carbon sequestered in [[Neotropical realm|Neotropical]] forests but also that human-set fires played a central role in reducing biomass in Amazonian and Central American forests before the arrival of the Europeans and the concomitant spread of diseases during the [[Columbian exchange]].<ref>{{Cite journal|last1=Nevle|first1=Richard J.|last2=Bird|first2=Dennis K.|date=7 July 2008|title=Effects of syn-pandemic fire reduction and reforestation in the tropical Americas on atmospheric CO2 during European conquest|journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]]|volume=264|issue=1|pages=25–38|doi=10.1016/j.palaeo.2008.03.008|issn=0031-0182|bibcode=2008PPP...264...25N}}</ref><ref name="Dull 755–771">{{Cite journal |last1=Dull |first1=Robert A. |last2=Nevle |first2=Richard J. |last3=Woods |first3=William I. |last4=Bird |first4=Dennis K. |last5=Avnery |first5=Shiri |last6=Denevan |first6=William M. |date=31 August 2010 |title=The Columbian Encounter and the Little Ice Age: Abrupt Land Use Change, Fire, and Greenhouse Forcing |journal=Annals of the Association of American Geographers |volume=100 |issue=4 |pages=755–771 |doi=10.1080/00045608.2010.502432 |issn=0004-5608 |s2cid=129862702}}</ref><ref name="Holocene_20110801">{{Cite journal |last1=Nevle |first1=R. J. |last2=Bird |first2=D. K. |last3=Ruddiman |first3=W. F. |last4=Dull |first4=R. A. |date=1 August 2011 |title=Neotropical human–landscape interactions, fire, and atmospheric CO2 during European conquest |journal=[[The Holocene]] |language=en |volume=21 |issue=5 |pages=853–864 |bibcode=2011Holoc..21..853N |doi=10.1177/0959683611404578 |issn=0959-6836 |s2cid=128896863}}</ref> Dull and Nevle calculated that reforestation in the tropical biomes of the Americas alone from 1500 to 1650 accounted for net [[carbon sequestration]] of 2–5 [[petagram|Pg]].<ref name="Dull 755–771"/> Brierley conjectured that the European arrival in the Americas caused mass deaths from epidemic disease, which caused much abandonment of farmland. That caused much forest to return, which sequestered more CO<sub>2</sub>.<ref name="Brierley  et al 2019"/> A study of sediment cores and soil samples further suggests that CO<sub>2</sub> uptake via reforestation in the Americas could have contributed to the LIA.<ref name="Bergeron2008">{{Cite news |last=Bergeron|first=Louis |title=Reforestation helped trigger Little Ice Age, researchers say |date=17 December 2008 |publisher=Stanford News Service |url=http://news-service.stanford.edu/news/2009/january7/manvleaf-010709.html}}</ref> The depopulation is linked to a drop in CO<sub>2</sub> levels observed at [[Law Dome]], Antarctica.<ref name=ams1/>

However, the hypothesis is criticized on the grounds that the kind of [[agroforestry]] practiced by the pre-Columbian farmers in South America didn't actually result in a large-scale deforestation typical for modern agriculture so the reforestation shouldn't have had such a huge effect.<ref>{{Cite journal |last1=Watling |first1=Jennifer |last2=Iriarte |first2=José |last3=Mayle |first3=Francis E. |last4=Schaan |first4=Denise |last5=Pessenda |first5=Luiz C. R. |last6=Loader |first6=Neil J. |last7=Street-Perrott |first7=F. Alayne |last8=Dickau |first8=Ruth E. |last9=Damasceno |first9=Antonia |last10=Ranzi |first10=Alceu |date=2017-02-21 |title=Impact of pre-Columbian "geoglyph" builders on Amazonian forests |journal=Proceedings of the National Academy of Sciences |language=en |volume=114 |issue=8 |pages=1868–1873 |doi=10.1073/pnas.1614359114 |doi-access=free |issn=0027-8424 |pmc=5338430 |pmid=28167791|bibcode=2017PNAS..114.1868W }}</ref>

===Population increases at mid- to high latitudes===
It is suggested that during the Little Ice Age, increased deforestation had enough effect on the Earth's [[albedo]] (reflectiveness) to cause regional and global temperature decreases. Changes in albedo were caused by widespread deforestation at high latitudes, which exposed more snow cover and thus increased reflectiveness of the Earth's surface, as land was cleared for agricultural use. The theory implies that over the course of the Little Ice Age, enough land was cleared to make deforestation a possible cause of climate change.<ref>{{cite journal |last1=Ellis |first1=Erle C. |last2=Kaplan |first2=Jed O. |last3=Fuller |first3=Dorian Q. |last4=Vavrus |first4=Steve |last5=Klein Goldewijk |first5=Kees |last6=Verburg |first6=Peter H. |year=2013 |title=Used planet: A global history |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |volume=110 |issue=20 |pages=7978–7985 |bibcode=2013PNAS..110.7978E |doi=10.1073/pnas.1217241110 |pmc=3657770 |pmid=23630271 |doi-access=free}}</ref>

It has been proposed that the Land Use Intensification theory could explain this effect. The theory was originally proposed by Ester Boserup and suggests that agriculture advances only as the population demands it.<ref>{{cite journal |last1=Turner |first1=B. L. |last2=Fischer-Kowalski |first2=Marina |date=2010 |title=Ester Boserup: An interdisciplinary visionary relevant for sustainability. |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |volume=107 |issue=51 |pages=21963–21965 |bibcode=2010PNAS..10721963T |doi=10.1073/pnas.1013972108 |pmc=3009765 |pmid=21135227 |doi-access=free}}</ref> Furthermore, there is evidence of rapid population and [[agricultural expansion]], which could warrant some of the changes observed in the climate during this period.

This theory is still under speculation for multiple reasons: primarily, the difficulty of recreating climate simulations outside of a narrow set of land{{clarify|date=April 2022}} in those regions; so that one cannot rely on data to explain sweeping changes or to account for the wide variety of other sources of climate change globally. As an extension of the first reason, climate models including this period have shown increases and decreases in temperature globally.<ref>{{cite journal |last1=Pitman |first1=A. J. |last2=Noblet-Ducoudre |first2=N. |last3=Cruz |first3=F. T. |last4=Davin |first4=E. L. |last5=Bonan |first5=G. B. |last6=Brovkn |first6=V. |last7=Claussen |first7=M. |last8=Delire |first8=C. |last9=Ganzeveld |first9=L. |last10=Gayler |first10=V. |last11=Can den Hurk |first11=B. J. J. M. |last12=Lawrence |first12=P. J. |last13=van der Molen |first13=M. K. |last14=Muller |first14=C. |last15=Reick |first15=C. H. |date=2009 |title=Uncertainties in climate responses to past land cover change: First results from the LUCID intercomparison study |journal=Geophysical Research Letters |volume=36 |issue=L14814 |pages=L14814 |bibcode=2009GeoRL..3614814P |doi=10.1029/2009GL039076 |s2cid=15504757 |doi-access=free |last16=Seneviratne |first16=S. I. |last17=Strengers |first17=B. J. |last18=Voldoire |first18=A.|hdl=11858/00-001M-0000-0011-F8CF-9 |hdl-access=free }}</ref> That is, climate models have shown deforestation as neither a singular cause for climate change nor a reliable cause for the global temperature decrease.

===Inherent variability of climate===
Spontaneous fluctuations in global climate might explain the past variability. It is very difficult to know what the true level of variability from internal causes might be given the existence of other forces, as noted above, whose magnitude may not be known. One approach to evaluating internal variability is the use of long integrations of coupled ocean-atmosphere [[global climate model]]s. They have the advantage that the external forcing is known to be zero, but the disadvantage is that they may not fully reflect reality. The variations may result from [[chaos theory|chaos]]-driven changes in the oceans, the atmosphere, or interactions between the two.<ref name="melissa">{{cite journal |last1=Free |first1=Melissa |last2=Robock |first2=Alan |year=1999 |title=Global warming in the context of the Little Ice Age |journal=[[Journal of Geophysical Research]] |volume=104 |issue=D16 |pages=19,057 |bibcode=1999JGR...10419057F |doi=10.1029/1999JD900233 |doi-access=free}}</ref> Two studies have concluded that the demonstrated inherent variability was not great enough to account for the Little Ice Age.<ref name=melissa/><ref name="B2006">{{cite journal |last1=Hunt |first1=B. G. |year=2006 |title=The Medieval Warm Period, the Little Ice Age and simulated climatic variability |url=https://www.researchgate.net/publication/225165835 |journal=[[Climate Dynamics]] |volume=27 |issue=7–8 |pages=677–694 |bibcode=2006ClDy...27..677H |doi=10.1007/s00382-006-0153-5 |s2cid=128890550 |access-date=11 September 2023}}</ref> The severe winters of 1770 to 1772 in Europe, however, have been attributed to an anomaly in the [[North Atlantic oscillation]].<ref>{{cite magazine |title=Hungern und handeln |language=de |first=Dominik |last=Collet |magazine=[[Damals]] |year=2020 |issue=6 |pages=72–76}}</ref>