Editing Ice sheet (section)

===Boundary conditions===

[[Image:Larsen B collapse.jpg|thumb|The collapse of the [[Larsen B]] ice shelf had profound effects on the velocities of its feeder glaciers.]]

[[Image:Ice flow controls.jpg|thumb|Accelerated ice flows after the break-up of an ice shelf]]

As the margins end at the marine boundary, excess ice is discharged through ice streams or [[outlet glacier]]s. Then, it either falls directly into the sea or is accumulated atop the floating [[Ice shelf|ice shelves]].<ref name="IPCC_AR6_AnnexVII">IPCC, 2021: Annex VII: [https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_AnnexVII.pdf Glossary] [Matthews, J.B.R., V. Möller, R. van Diemen, J.S. Fuglestvedt, V. Masson-Delmotte, C.  Méndez, S. Semenov, A. Reisinger (eds.)]. In [https://www.ipcc.ch/report/ar6/wg1/ Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 2215–2256, doi:10.1017/9781009157896.022.</ref>{{Rp|page=2234}} Those ice shelves then [[Ice calving|calve]] icebergs at their periphery if they experience excess of ice. Ice shelves would also experience accelerated calving due to basal melting. In Antarctica, this is driven by heat fed to the shelf by the [[circumpolar deep water]] current, which is 3&nbsp;°C above the ice's melting point.<ref name=Walker2007>{{cite journal |last1=Walker |first1=Dziga P. |last2=Brandon |first2=Mark A. |last3=Jenkins |first3=Adrian |last4=Allen |first4=John T. |last5=Dowdeswell |first5=Julian A. |last6=Evans |first6=Jeff |title=Oceanic heat transport onto the Amundsen Sea shelf through a submarine glacial trough |journal=Geophysical Research Letters |date=16 January 2007 |volume=34 |issue=2 |pages=L02602 |doi=10.1029/2006GL028154 |bibcode=2007GeoRL..34.2602W |s2cid=30646727 |url=http://nora.nerc.ac.uk/id/eprint/1199/1/grl22452.pdf }}</ref>

The presence of ice shelves has a stabilizing influence on the glacier behind them, while an absence of an ice shelf becomes destabilizing. For instance, when [[Larsen B]] ice shelf in the [[Antarctic Peninsula]] had collapsed over three weeks in February 2002, the four glaciers behind it - [[Crane Glacier]], [[Green Glacier]], [[Hektoria Glacier]] and [[Jorum Glacier]] - all started to flow at a much faster rate, while the two glaciers (Flask and Leppard) stabilized by the remnants of the ice shelf did not accelerate.<ref name=Scambos2004>{{cite journal |last1=Scambos |first1=T. A. |title=Glacier acceleration and thinning after ice shelf collapse in the Larsen B embayment, Antarctica |journal=Geophysical Research Letters |date=2004 |volume=31 |issue=18 |pages=L18402 |doi=10.1029/2004GL020670 |bibcode=2004GeoRL..3118402S |s2cid=36917564 |doi-access=free |hdl=11603/24296 |hdl-access=free }}</ref>

The collapse of the Larsen B shelf was preceded by thinning of just 1&nbsp;metre per year, while some other Antarctic ice shelves have displayed thinning of tens of metres per year.<ref name="IPCCc4" /> Further, increased ocean temperatures of 1&nbsp;°C may lead to up to 10&nbsp;metres per year of basal melting.<ref name="IPCCc4" /> Ice shelves are always stable under mean annual temperatures of −9&nbsp;°C, but never stable above −5&nbsp;°C; this places regional warming of 1.5&nbsp;°C, as preceded the collapse of Larsen B, in context.<ref name="IPCCc4" />