Editing Ice core (section)

=== Visual analysis ===
[[File:GISP2 1855m ice core layers.png|thumb|19 cm long section of GISP 2 ice core from 1855&nbsp;m showing annual layer structure illuminated from below by a fibre optic source. Section contains 11 annual layers with summer layers (arrowed) sandwiched between darker winter layers.<ref>{{Cite web|url=http://www.ncdc.noaa.gov/paleo/slides/slideset/15/15_281_slide.html|title=Summer and winter core layers|last=Gow|first=Anthony|date=12 October 2001|publisher=NOAA|archive-url=https://web.archive.org/web/20100213035305/http://www.ncdc.noaa.gov/paleo/slides/slideset/15/15_281_slide.html|archive-date=13 February 2010}}</ref>|left|alt=A series of dark and light bands, with arrows identifying the lighter bands]]

Cores show visible layers, which correspond to annual snowfall at the core site.  If a pair of pits is dug in fresh snow with a thin wall between them and one of the pits is roofed over, an observer in the roofed pit will see the layers revealed by sunlight shining through.  A six-foot pit may show anything from less than a year of snow to several years of snow, depending on the location.  Poles left in the snow from year to year show the amount of accumulated snow each year, and this can be used to verify that the visible layer in a snow pit corresponds to a single year's snowfall.<ref>{{harvnb|Alley|2000}}, pp. 44–48.</ref>

In central Greenland a typical year might produce two or three feet of winter snow, plus a few inches of summer snow.  When this turns to ice, the two layers will make up no more than a foot of ice.  The layers corresponding to the summer snow will contain bigger bubbles than the winter layers, so the alternating layers remain visible, which makes it possible to count down a core and determine the age of each layer.<ref>{{harvnb|Alley|2000}}, p. 49.</ref> As the depth increases to the point where the ice structure changes to a clathrate, the bubbles are no longer visible, and the layers can no longer be seen.  Dust layers may now become visible.  Ice from Greenland cores contains dust carried by wind; the dust appears most strongly in late winter, and appears as cloudy grey layers.  These layers are stronger and easier to see at times in the past when the Earth's climate was cold, dry, and windy.<ref>{{harvnb|Alley|2000}}, pp. 50–51.</ref>

Any method of counting layers eventually runs into difficulties as the flow of the ice causes the layers to become thinner and harder to see with increasing depth.<ref>{{harvnb|Alley|2000}}, p. 56.</ref> The problem is more acute at locations where accumulation is high; low accumulation sites, such as central Antarctica, must be dated by other methods.<ref name="Jouzel-2013-4" /> For example, at Vostok, layer counting is only possible down to an age of 55,000 years.<ref name=Ruddiman>{{Cite journal|last1=Ruddiman|first1=William F.|year=2003|title=A methane-based time scale for Vostok ice|url=http://moraymo.us/wp-content/uploads/2014/04/2003_ruddimanraymo.pdf|journal=Quaternary Science Reviews|volume=22|issue=2|pages=141–155|last2=Raymo|first2=Maureen E.|doi=10.1016/S0277-3791(02)00082-3|bibcode=2003QSRv...22..141R}}</ref>

When there is summer melting, the melted snow refreezes lower in the snow and firn, and the resulting layer of ice has very few bubbles so is easy to recognise in a visual examination of a core.  Identification of these layers, both visually and by measuring density of the core against depth, allows the calculation of a melt-feature percentage (MF): an MF of 100% would mean that every year's deposit of snow showed evidence of melting.  MF calculations are averaged over multiple sites or long time periods in order to smooth the data.  Plots of MF data over time reveal variations in the climate, and have shown that since the late 20th century melting rates have been increasing.<ref>{{harvnb|Jouzel|2013}}, p. 2533.</ref><ref>{{Cite journal|last=Fisher|first=David|year=2011|title=Recent melt rates of Canadian arctic ice caps are the highest in four millennia|url=http://arctic.eas.ualberta.ca/downloads/Fisheretal2011onlineGPC.pdf|journal=Global and Planetary Climate Change|volume=84–85|pages=1–4|doi=10.1016/j.gloplacha.2011.06.005}}</ref>

In addition to manual inspection and logging of features identified in a visual inspection, cores can be optically scanned so that a digital visual record is available.  This requires the core to be cut lengthwise, so that a flat surface is created.<ref>{{harvnb|Souney et al.|2014}}, p. 25.</ref>