Editing Ice age (section)

==Feedback processes==
Each glacial period is subject to [[positive feedback]] which makes it more severe, and [[negative feedback]] which mitigates and (in all cases so far) eventually ends it.

===Positive===
An important form of feedback is provided by Earth's [[albedo]], which is how much of the sun's energy is reflected rather than absorbed by Earth. Ice and snow increase Earth's albedo, while [[Boreal forest|forests]] reduce its albedo. When the air temperature decreases, ice and snow fields grow, and they reduce forest cover. This continues until competition with a negative feedback mechanism forces the system to an equilibrium.

One theory is that when glaciers form, two things happen: the ice grinds rocks into dust, and the land becomes dry and arid. This allows winds to transport iron rich dust into the open ocean, where it acts as a fertilizer that causes massive algal blooms that pulls large amounts of {{CO2}} out of the atmosphere. This in turn makes it even colder and causes the glaciers to grow more.<ref>{{Cite web |url=https://www.smithsonianmag.com/science-nature/complicated-role-iron-ocean-health-and-climate-change-180973893/ |title=The Complicated Role of Iron in Ocean Health and Climate Change |access-date=2022-08-02 |archive-date=2022-08-02 |archive-url=https://web.archive.org/web/20220802125320/https://www.smithsonianmag.com/science-nature/complicated-role-iron-ocean-health-and-climate-change-180973893/ |url-status=live }}</ref>

In 1956, Ewing and Donn<ref>{{Cite journal|last1=Ewing|first1=M.|last2=Donn|first2=W. L.|date=1956-06-15|title=A Theory of Ice Ages|journal=Science|volume=123|issue=3207|pages=1061–1066|doi=10.1126/science.123.3207.1061|issn=0036-8075|pmid=17748617|bibcode=1956Sci...123.1061E}}</ref> hypothesized that an ice-free Arctic Ocean leads to increased snowfall at high latitudes. When low-temperature ice covers the Arctic Ocean there is little evaporation or [[Sublimation (phase transition)|sublimation]] and the polar regions are quite dry in terms of precipitation, comparable to the amount found in mid-latitude [[desert]]s. This low precipitation allows high-latitude snowfalls to melt during the summer. An ice-free Arctic Ocean absorbs solar radiation during the long summer days, and evaporates more water into the Arctic atmosphere. With higher precipitation, portions of this snow may not melt during the summer and so glacial ice can form at lower altitudes ''and'' more southerly latitudes, reducing the temperatures over land by increased albedo as noted above. Furthermore, under this hypothesis the lack of oceanic pack ice allows increased exchange of waters between the Arctic and the North Atlantic Oceans, warming the Arctic and cooling the North Atlantic. (Current projected consequences of [[global warming]] include [[Arctic sea ice decline#Ice-free summer vs. ice-free winter|a brief ice-free Arctic Ocean period by 2050]].) Additional fresh water flowing into the North Atlantic during a warming cycle may also [[shutdown of thermohaline circulation|reduce]] the [[Thermohaline circulation|global ocean water circulation]]. Such a reduction (by reducing the effects of the [[Gulf Stream]]) would have a cooling effect on northern Europe, which in turn would lead to increased low-latitude snow retention during the summer.<ref>{{cite book|last=Garrison|first=Tom|title=Oceanography: An Invitation to Marine Science|publisher=Cengage Learning|edition=7th|date=2009|pages=582|isbn=9780495391937}}</ref><ref>{{cite journal|doi=10.1038/nature04385 |author=Bryden, H.L. |author2=H.R. Longworth |author3=S.A. Cunningham |title=Slowing of the Atlantic meridional overturning circulation at 25° N|journal=Nature|issue=7068|pages=655–657|year=2005|pmid=16319889|volume=438|bibcode = 2005Natur.438..655B |s2cid=4429828 }}</ref><ref>{{cite journal|doi=10.1126/science.1109477 |author=Curry, R. |author2=C. Mauritzen|author2-link= Cecilie Mauritzen |title=Dilution of the northern North Atlantic in recent decades|journal=Science|volume=308|issue=5729 |pages=1772–1774|year=2005|bibcode = 2005Sci...308.1772C|pmid=15961666|s2cid=36017668 }}</ref> It has also been suggested{{by whom|date=November 2020}} that during an extensive glacial, glaciers may move through the [[Gulf of Saint Lawrence]], extending into the North Atlantic Ocean far enough to block the Gulf Stream.

===Negative===
Ice sheets that form during glaciations erode the land beneath them. This can reduce the land area above sea level and thus diminish the amount of space on which ice sheets can form. This mitigates the albedo feedback, as does the rise in sea level that accompanies the reduced area of ice sheets, since open ocean has a lower albedo than land.<ref>{{Cite book|last1=Huddart|first1=David|url=https://books.google.com/books?id=_64G6cYuz3AC&q=%22lowering+in+sea+level+that+accompanies+the+formation+of+ice+sheets%22&pg=PT1624|title=Earth Environments: Past, Present and Future|last2=Stott|first2=Tim A.|date=2013-04-16|publisher=John Wiley & Sons|isbn=978-1-118-68812-0|language=en}}</ref>

Another negative feedback mechanism is the increased aridity occurring with glacial maxima, which reduces the precipitation available to maintain glaciation. The glacial retreat induced by this or any other process can be amplified by similar [[inverse positive feedback]]s as for glacial advances.<ref>{{Cite book|url=https://archive.org/details/glacialgeologyic00benn_0|url-access=registration|quote=Another factor is the increased aridity occurring with glacial maxima, which reduces the precipitation available to maintain glaciation. The glacial retreat induced by this or any other process can be amplified by similar inverse positive feedbacks as for glacial advances.|title=Glacial Geology: Ice Sheets and Landforms|last1=Bennett|first1=Matthew M.|last2=Glasser|first2=Neil F.|date=2010-03-29|publisher=Wiley|isbn=978-0-470-51690-4|language=en}}</ref>

According to research published in ''[[Nature Geoscience]]'', human emissions of [[carbon dioxide|carbon dioxide (CO<sub>2</sub>)]] will defer the next glacial period. Researchers used data on Earth's orbit to find the historical warm interglacial period that looks most like the current one and from this have predicted that the next glacial period would usually begin within 1,500 years. They go on to predict that emissions have been so high that it will not.<ref>{{cite news|last=Black|first=Richard|title=Carbon emissions 'will defer Ice Age'|url=https://www.bbc.co.uk/news/science-environment-16439807|work=BBC News|access-date=10 August 2012|date=9 January 2012|archive-date=18 August 2012|archive-url=https://web.archive.org/web/20120818115004/http://www.bbc.co.uk/news/science-environment-16439807|url-status=live}}</ref>