Editing Ice sheet (section)

==In geologic timescales==
[[File:Schannwell 2024 Heinrich events.png|thumb|A reconstruction of how Heinrich events would have likely proceeded, with the Laurentide ice sheet first growing to an unsustainable position, where the base of its periphery becomes too warm, and then rapidly losing ice until it is reduced to sustainable size<ref name="Schannwell2024">{{Cite journal|last1=Schannwell |first1=Clemens |last2=Mikolajewicz |first2=Uwe |last3=Kapsch |first3=Marie-Luise |last4=Ziemen |first4=Florian |date=5 April 2024 |title=A mechanism for reconciling the synchronisation of Heinrich events and Dansgaard-Oeschger cycles |journal=Nature Communications|language=en |volume=15 |issue=1 |page=2961 |doi=10.1038/s41467-024-47141-7 |pmid=38580634 |pmc=10997585 |bibcode=2024NatCo..15.2961S }}</ref>]]
Normally, the transitions between glacial and interglacial states are governed by [[Milankovitch cycles]], which are patterns in [[insolation]] (the amount of sunlight reaching the Earth). These patterns are caused by the variations in shape of the Earth's orbit and its angle relative to the Sun, caused by the gravitational pull of other planets as they go through their own orbits.<ref>{{cite journal |last1=Kerr |first1=Richard A. |date=14 July 1978 |title=Climate Control: How Large a Role for Orbital Variations? |url=https://www.jstor.org/stable/1746691 |journal=Science |volume=201 |issue=4351 |pages=144–146 |doi=10.1126/science.201.4351.144 |jstor=1746691 |pmid=17801827 |bibcode=1978Sci...201..144K |access-date=29 July 2022|url-access=subscription }}</ref><ref>{{cite web |title=Why Milankovitch (Orbital) Cycles Can't Explain Earth's Current Warming |last=Buis |first=Alan |date=27 February 2020 |url=https://climate.nasa.gov/ask-nasa-climate/2949/why-milankovitch-orbital-cycles-cant-explain-earths-current-warming/ |publisher=NASA |access-date=29 July 2022}}</ref>

For instance, during at least the last 100,000 years, portions of the ice sheet covering much of North America, the [[Laurentide Ice Sheet]] broke apart sending large flotillas of icebergs into the North Atlantic. When these icebergs melted they dropped the boulders and other continental rocks they carried, leaving layers known as [[ice rafted debris]]. These so-called [[Heinrich events]], named after their discoverer [[Hartmut Heinrich]], appear to have a 7,000–10,000-year [[Periodic function|periodicity]], and occur during cold periods within the last interglacial.<ref>{{cite journal |last1=Heinrich |first1=Hartmut |title=Origin and Consequences of Cyclic Ice Rafting in the Northeast Atlantic Ocean During the Past 130,000 Years |journal=Quaternary Research |date=March 1988 |volume=29 |issue=2 |pages=142–152 |doi=10.1016/0033-5894(88)90057-9 |bibcode=1988QuRes..29..142H |s2cid=129842509 }}</ref>

Internal ice sheet "binge-purge" cycles may be responsible for the observed effects, where the ice builds to unstable levels, then a portion of the ice sheet collapses. External factors might also play a role in forcing ice sheets. [[Dansgaard–Oeschger event]]s are abrupt warmings of the northern hemisphere occurring over the space of perhaps 40 years. While these D–O events occur directly after each Heinrich event, they also occur more frequently – around every 1500 years; from this evidence, paleoclimatologists surmise that the same forcings may drive both Heinrich and D–O events.<ref>{{cite book |doi=10.1029/GM112p0035 |chapter=The North Atlantic's 1–2 kyr climate rhythm: Relation to Heinrich events, Dansgaard/Oeschger cycles and the Little Ice Age |title=Mechanisms of Global Climate Change at Millennial Time Scales |series=Geophysical Monograph Series |year=1999 |last1=Bond |first1=Gerard C. |last2=Showers |first2=William |last3=Elliot |first3=Mary |last4=Evans |first4=Michael |last5=Lotti |first5=Rusty |last6=Hajdas |first6=Irka |last7=Bonani |first7=Georges |last8=Johnson |first8=Sigfus |volume=112 |pages=35–58 |doi-broken-date=2024-11-26 |isbn=978-0-87590-095-7 }}</ref>

''Hemispheric asynchrony in ice sheet behavior'' has been observed by linking short-term spikes of methane in Greenland ice cores and Antarctic ice cores. During [[Dansgaard–Oeschger event]]s, the northern hemisphere warmed considerably, dramatically increasing the release of methane from wetlands, that were otherwise tundra during glacial times. This methane quickly distributes evenly across the globe, becoming incorporated in Antarctic and Greenland ice. With this tie, paleoclimatologists have been able to say that the ice sheets on Greenland only began to warm after the Antarctic ice sheet had been warming for several thousand years. Why this pattern occurs is still open for debate.<ref>{{cite journal |last1=Turney |first1=Chris S. M. |last2=Fogwill |first2=Christopher J. |last3=Golledge |first3=Nicholas R. |last4=McKay |first4=Nicholas P. |last5=Sebille |first5=Erik van |last6=Jones |first6=Richard T. |last7=Etheridge |first7=David |last8=Rubino |first8=Mauro |last9=Thornton |first9=David P. |last10=Davies |first10=Siwan M. |last11=Ramsey |first11=Christopher Bronk |last12=Thomas |first12=Zoë A. |last13=Bird |first13=Michael I. |last14=Munksgaard |first14=Niels C. |last15=Kohno |first15=Mika |last16=Woodward |first16=John |last17=Winter |first17=Kate |last18=Weyrich |first18=Laura S. |last19=Rootes |first19=Camilla M. |last20=Millman |first20=Helen |last21=Albert |first21=Paul G. |last22=Rivera |first22=Andres |last23=Ommen |first23=Tas van |last24=Curran |first24=Mark |last25=Moy |first25=Andrew |last26=Rahmstorf |first26=Stefan |last27=Kawamura |first27=Kenji |last28=Hillenbrand |first28=Claus-Dieter |last29=Weber |first29=Michael E. |last30=Manning |first30=Christina J. |last31=Young |first31=Jennifer |last32=Cooper |first32=Alan |title=Early Last Interglacial ocean warming drove substantial ice mass loss from Antarctica |journal=Proceedings of the National Academy of Sciences |date=25 February 2020 |volume=117 |issue=8 |pages=3996–4006 |doi=10.1073/pnas.1902469117 |pmid=32047039 |pmc=7049167 |bibcode=2020PNAS..117.3996T |doi-access=free }}</ref><ref>{{cite journal |last1=Crémière |first1=Antoine |last2=Lepland |first2=Aivo |last3=Chand |first3=Shyam |last4=Sahy |first4=Diana |last5=Condon |first5=Daniel J. |last6=Noble |first6=Stephen R. |last7=Martma |first7=Tõnu |last8=Thorsnes |first8=Terje |last9=Sauer |first9=Simone |last10=Brunstad |first10=Harald |title=Timescales of methane seepage on the Norwegian margin following collapse of the Scandinavian Ice Sheet |journal=Nature Communications |date=11 May 2016 |volume=7 |issue=1 |pages=11509 |doi=10.1038/ncomms11509 |pmid=27167635 |pmc=4865861 |bibcode=2016NatCo...711509C }}</ref>

===Antarctic ice sheet during geologic timescales===
{{excerpt|Antarctic ice sheet#Situation during geologic time scales}}

===Greenland ice sheet during geologic timescales===
{{excerpt|Greenland ice sheet#Geological history}}