Editing Little Ice Age (section)

===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"/>