Editing Earth's magnetic field (section)

=== Secular variation ===
{{Main|Geomagnetic secular variation}}
[[File:Earth Magnetic Field Declination from 1590 to 1990.gif|thumb|Estimated declination contours by year, 1590 to 1990 (click to see variation)]]
[[File:Geomagnetic axial dipole strength.svg|thumb|Strength of the axial dipole component of Earth's magnetic field from 1600 to 2020]]

Changes in Earth's magnetic field on a time scale of a year or more are referred to as ''secular variation''. Over hundreds of years, magnetic declination is observed to vary over tens of degrees.<ref name="MMMch2" /> The animation shows how global declinations have changed over the last few centuries.<ref name="declination">{{cite journal |last1=Jackson |first1=Andrew |last2=Jonkers |first2=Art R. T. |last3=Walker |first3=Matthew R. |title=Four centuries of Geomagnetic Secular Variation from Historical Records |journal=Philosophical Transactions of the Royal Society A |volume=358 |pages=957–990 |year=2000 |jstor=2666741 |issue=1768 |bibcode=2000RSPTA.358..957J |doi=10.1098/rsta.2000.0569|citeseerx=10.1.1.560.5046 |s2cid=40510741 }}</ref>

The direction and intensity of the dipole change over time. Over the last two centuries the dipole strength has been decreasing at a rate of about 6.3% per century.<ref name="MMMch2" /> At this rate of decrease, the field would be negligible in about 1600 years.<ref name="GSC">{{cite web |url=http://nrhp.focus.nps.gov/ |title=Secular variation |work=Geomagnetism |publisher=Canadian Geological Survey |year=2011 |access-date=18 July 2011 |archive-date=25 July 2008 |archive-url=https://web.archive.org/web/20080725123211/http://nrhp.focus.nps.gov/ |url-status=dead }}</ref> However, this strength is about average for the last 7 thousand years, and the current rate of change is not unusual.<ref name="Constable_dipole">{{Cite book|last=Constable|first=Catherine|title=Encyclopedia of Geomagnetism and Paleomagnetism|publisher=Springer-Verlag|year=2007|isbn=978-1-4020-3992-8|editor-last=Gubbins|editor-first=David|location=|pages=159–161|chapter=Dipole Moment Variation|doi=10.1007/978-1-4020-4423-6_67|author-link=Catherine Constable|editor2-last=Herrero-Bervera|editor2-first=Emilio}}</ref>

A prominent feature in the non-dipolar part of the secular variation is a ''westward drift'' at a rate of about 0.2° per year.<ref name="GSC" /> This drift is not the same everywhere and has varied over time. The globally averaged drift has been westward since about 1400 AD but eastward between about 1000 AD and 1400 AD.<ref name="Dumberry">{{cite journal |last1=Dumberry |first1=Mathieu |last2=Finlay |first2=Christopher C. |title=Eastward and westward drift of the Earth's magnetic field for the last three millennia |journal=Earth and Planetary Science Letters |volume=254 |issue=1–2 |pages=146–157 |year=2007 |doi=10.1016/j.epsl.2006.11.026 |bibcode=2007E&PSL.254..146D |url=http://www.epm.geophys.ethz.ch/~cfinlay/publications/dumberry_finlay_epsl07.pdf |access-date=2013-10-22 |archive-url=https://web.archive.org/web/20131023062443/http://www.epm.geophys.ethz.ch/~cfinlay/publications/dumberry_finlay_epsl07.pdf |archive-date=2013-10-23 |url-status=dead }}</ref>

Changes that predate magnetic observatories are recorded in archaeological and geological materials. Such changes are referred to as ''paleomagnetic secular variation'' or ''paleosecular variation (PSV)''. The records typically include long periods of small change with occasional large changes reflecting [[geomagnetic excursion]]s and reversals.<ref name="TauxeCh1">{{harvnb|Tauxe|1998|loc=Chapter 1}}</ref>

A 1995 study of lava flows on [[Steens Mountain]], Oregon appeared to suggest the magnetic field once shifted at a rate of up to 6° per day at some time in Earth's history, a surprising result.<ref name="nature.com" /> However, in 2014 one of the original authors published a new study which found the results were actually due to the continuous thermal demagnitization of the lava, not to a shift in the magnetic field.<ref name="epsl.com" />

In July 2020 scientists report that analysis of simulations and a recent observational field model show that maximum rates of directional change of Earth's magnetic field reached ~10° per year – almost 100 times faster than current changes and 10 times faster than previously thought.<ref>{{cite news|title=Simulations show magnetic field can change 10 times faster than previously thought|language=en|work=phys.org|url=https://phys.org/news/2020-07-simulations-magnetic-field-faster-previously.html|access-date=16 August 2020}}</ref><ref>{{cite journal|last1=Davies|first1=Christopher J.|last2=Constable|first2=Catherine G.|date=6 July 2020|title=Rapid geomagnetic changes inferred from Earth observations and numerical simulations|journal=Nature Communications|language=en|volume=11|issue=1|page=3371|doi=10.1038/s41467-020-16888-0|pmid=32632222|pmc=7338531|bibcode=2020NatCo..11.3371D|issn=2041-1723|doi-access=free}}</ref>