Explain xkcd: It's 'cause you're dumb.
Title text: Data adapted from 'The Laurentide and Innuitian ice sheets during the Last Glacial Maximum' by A.S. Dyke et. al., which was way better than the sequels 'The Laurentide and Innuitian ice sheets during the Last Glacial Maximum: The Meltdown' and 'The Laurentide and Innuitian ice sheets during the Last Glacial Maximum: Continental Drift'.
The comic shows the ice levels at major North American cities at the peak of the last ice age, 21,000 years ago. During this period, a vast amount of frozen water covered North America as well as other areas around the world. So much ice that it affected the global sea level (see Sea level rise) to lower it by more than a hundred meters.
Toronto and Montreal are both Canadian cities, while Boston and Chicago are in the United States. The skylines of each city are shown at the bottom of the ice sheet to scale. The tallest structure shown is the CN Tower in Toronto, the tallest free-standing structure in the Western Hemisphere, at a height of 553 m. The tallest ice sheet is 3.3 km tall, almost six time higher than that tower. Although over Toronto the ice was "only" 2.1 km tall
The tallest ice sheet takes up 265 pixel so each pixel is about 12.4 meters and the hight of the panels is 3.7 km with less than 200 m of the ground shown in black below the cities making the white "air" above ground reaching up to 3.5 km, leaving only 200 m of air above the highest ice sheet.
The title text references the "The Laurentide and Innuitian ice sheets during the Last Glacial Maximum (PDF)," an actual series of scientific papers about the ice sheet (see figure 4). But it also refers to the animated Ice Age film series, specifically to Ice Age: the Meltdown, and Ice Age: Continental Drift which are the second and fourth Ice Age film. Many agree that the first film/book in a series is better than its sequel(s).
Ice sheets over Boston during the last ice age was also referenced in 1379: 4.5 Degrees. The image of Boston in this comic is reused at the top of the huge chart in 1732: Earth Temperature Timeline, and had already been reused earlier in the what if? post Google's Datacenters on Punch Cards. Randall lives in that area.
Although this comic doesn't mention (modern) climate change, it does show the difference climate can have on our surroundings. And in the two later comics mentioned above, Randall makes it clear that we are now heading as far in the opposite hotter direction compared to the "normal" temperature during the rise of human civilization, as the ice age temperature was colder.
- [Caption above the four panels:]
- Thickness of the ice sheets
- at various locations
- 21,000 years ago
- compared with modern skylines.
- [In four panels the skylines of four major metropolises are superimposed against a blue ice sheet of the proper thickness for the aforementioned time period. The panels are much taller than the skyline reaching about 3500 meter up above ground level. Below the gray skylines there is a black slab indicating this ground level, above the ice the air is white. Also above the ice sheet the hight of the ice is noted in light gray text. Inside the ice sheet the name of the city is written in black. Some clear landmarks can be seen in each skyline.]
- [The first skyline shows among other a tall TV tower. The ice reaches more than halfway to the top of the panel.]
- [The second skyline shows several skyscrapers with specially one towering over the other. The ice reaches about a quarter of the way to the top. This is the lowest of the ice sheets only reaching about twice as high as the tallest building in the city.]
- [The third skyline shows only a few skyscrapers with two of them clearly taller than the rest of the skyline. The ice reaches a bit more than a third towards the top.]
- [The fourth skyline do not have many real skyscrapers but there are three buildings that are taller than the rest of the skyline. The ice reaches almost to the top, making it the tallest ice sheet. The name of the city clearly is written higher up in the ice than the other three. Even above the lowest ice sheet of the second panel.]
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The original paper  Sebastian --22.214.171.124 07:38, 14 June 2013 (UTC)
It is commonly stated that EVERY sequel is worse that the original film (exceptions are few and often disputed). And very few producents are able to stop filming sequels sooner that they produce sequel worse that all previous. If you see a series with every film better that the previous, then producent is already preparing next one ... or died. -- Hkmaly (talk) 09:20, 14 June 2013 (UTC)
- However, going to the bottom of the Wikipedia page for Ice Age shows that Rotten Tomatoes strongly agrees that the sequels were not better Odysseus654 (talk) 16:34, 14 June 2013 (UTC)
For those of us who do not live in one of these four cities, does anyone have a more comprehensive set of data for the rest of the continent? Or specifically NYC? ;)
Striations on the rocks in Central Park are evidence that a glacier did reach as far south as New York City and in the referenced article on page 21, Figure 4 shows a map of the extent of the glacier just reaching NYC and Long Island and is labeled as somewhere between 0 and 600 meters thick. This page on the City of New York Parks and Recreation site  says the glacier in NYC was about 1000 feet thick which is about 300 meters. I should add that the Freedom Tower being built on the WTC site will be 1776 feet high (counting the broadcast antenna) and the Empire State building is 1454 feet high, so some of the current buildings would have poked out of the ice.
Was just curious, is this a jab at "Global Warming" and the fact that Glaciers have always been melting and getting thinner?--126.96.36.199 13:36, 14 June 2013 (UTC)
- I don't think so, just because someone finds it amazing how deep the ice during the last glaciation was, doesn't imply anything about their opinion on the causes of changes in climate over the few centuries. By the way, the glaciers have melted and refrozen lots of times, they haven't always been melting.NHSavage (talk) 18:44, 14 June 2013 (UTC)
- Global Warming
The actual climate discussion is still not solved. But we do know very well that the ocean sea level was 130 Meters lower than today at that time. At the end of that period the sea level was growing fast, but then it did raise slower later, and that raise didn't stop until today. Randall is only showing ICE levels, not more.--Dgbrt (talk) 22:17, 14 June 2013 (UTC)
I've read and re-read both comic and explanation (and, moreover, the above comments) and I just can't agree with the current third and final paragraph of the explanation ("But the joke ... (see Sea level rise)."). It is, to my mind, merely interesting that at a time of ice-age there was far more depth of ice pressing down upon sites than there currently are famous heights of buildings above the present day, and ice-less, horizons at these particular locales, as depicted. The sole joke, to my mind, is in the title-text, with the direct swapping of the common Ice Age film series's prefix for the souce paper's title. (Which is jolly funny!) And is in line with some of Randall's other largely "informative" strips, jokes sneaking in only as captions and labels.
It is indeed also interesting to note, as a side-issue, that glaciation (and thus deglaciation) relates somewhat to sea level (but note also Post-glacial rebound as a significant effect in some areas, both reinforcing and opposing sea-level changes, depending on locale). However, there's not even any mention of relative sea-level in the images. This could perhaps have been implemented as an arrow tacked onto the side of each depth-extended cross-section, pointing base to point (or vice-versa) between the axial position of the historic Mean Sea Level (if known) and that relating to the current state of affairs, in perfect scale against the column of sky-line and ice. But right now there's no reference at all to support this thought, and thus hardly asks for any such 'explanation' or reference. (There's similarly no invocation of the "climate discussion", unsolved or otherwise, outside of our combined commentary on the comic.)
Mayhap are people being confused by the (admitedly) water-like appearance of the depiction of ice-layer? Possibly thinking that these diagrams are of submerged cities (a la representations in various /The Day After Tomorrow/, /A.I. Artificial Intelligence/ or /2012/-ish films), not ones figuratively transplanted back back into the ancient ice-mass... Or are people inadvertently trolling their personal pro/anti-Climate Change views here (esp. w.r.t. Human impact)? Perhaps subconciously reading more (potentially pro- or anti-!) into the comic than (so far as I can see) was ever intended. If you'll forgive my hopefully 'neutralist' stance on the issue (i.e. divorced of my semi-moderate stance on the issue, which is in whatever direction it is that I lean... but which I have deliberately tried to have kept unstated).
Add to this that Randall shows only Leftpondian locations (and upper-Leftpondian ones, at that, due to the specialised scope of the source paper) this also makes me wonder why Europe and Asia are mentioned in that para, (unless it's meant to say "Ice Ages didn't just happen to North American, but also to similar latitudes in a circumpolar fashion", in which case could we also add anything we know about the southern hemisphere as well?). But that's a minor niggle I have in a paragraph that (obviously, from the length I've been taking pains to discuss all this within) I just can't get a handle on in the first place. But perhaps I'm missing something so, rather than editing and excising the main article, here I ramble on about it. Perhaps to pursuade some prior contributor to re-explain their particular contributions. (We now return you to your regularly scheduled programme...) 188.8.131.52 22:32, 16 June 2013 (UTC)
- The end of the explanation
It seems to suggest that the comic shows European and Asian glaciers, which is ... clearly false. What's that bit about?
- Well maybe that's Boston, Lincolnshire or Montreal, Catalonia. Or even the respective ones in the Philippines/Jordan. But I still doubt it. ;) Anyway, as per the TL;DR; mess of my prior comment 'contribution' I still don't like that ending. But that could just be me misreading. 184.108.40.206 11:49, 17 June 2013 (UTC)
- Nope, those are definitely the North American cities. The skylines are accurate - you can clearly see Hancock Place and the Prudential Tower in the Boston skyline, and Mount Royal in the Montreal drawing.220.127.116.11 15:41, 24 June 2013 (UTC)
- I did edit this section a week ago. It should be clear now that only North American cities are shown on the comic (see below).--Dgbrt (talk) 18:21, 24 June 2013 (UTC)
- Apart from the end of the explanation being written ambiguously, as to lead to your confusion, I think it is unnecessary to even include it as there is no mention of sea levels in the comic at all. --Dangerkeith3000 (talk) 15:49, 17 June 2013 (UTC)
- I did an update to this section. My former edit was in fact ambiguous; the comic shows only four major cities from North America. But I think that sea level issue is a direct result and should be mentioned here.--Dgbrt (talk) 17:10, 17 June 2013 (UTC)
- FWIW, I like that version much better. (And addresses my point, and hopeully DK3k's, about no mention of sea levels in the comic by now relating the indicated ice with global sea levels as an off-comic exemplar.) A final remaining bit of 'picky' is that it still puts the subordinate title-text joke as primary with the meaning of the comic image as a 'by the way', but that's minor semantics in comparison. Good show. 18.104.22.168 19:16, 17 June 2013 (UTC)
- The title text is the joke here. The comic image is simply a bit of information found in a paper on glacial action in north america overlaid with another bit of information on the heights of buildings in metropolitan areas. This visual juxtaposition adds interest to both sets of data by correlating two topics that we don't often think about together that are already linked through location and displaced in time. The title text joke further strengthens reader interest in the paper by humorously comparing the act of writing scientific papers to the creation, review and marketing of movies in north american culture. By asserting that the original paper is better than the fictitious sequels we get a brilliant satire of north american entertainment culture and an appeal to readers to seek more entertainment in learning of and thinking about interesting data. As the above comment mentions this is another example of interesting data presented in a visual way with a masterfully subtle satirical joke in the title text.
- There is no need to bring any discussion of glacial melt, sea level change or any other implications of glacial action into the explanation of the comic. I propose that the third paragraph be removed from the explanation and the explanation considered complete.
- Mrarch (talk) 17:15, 2 December 2013 (UTC)
- The title text is well covered in paragraph two, but an explain showing the consequences should also explained here. Any other suggestions? If this issue is solved the incomplete tag has to be removed.--Dgbrt (talk) 21:11, 2 December 2013 (UTC)
- If it must remain it should be rephrased as part of the context regarding the ice sheet. more information on the particulars of sea levels and other effects of the ice sheet are not required here. If readers want to know more they can refer to the paper cited in the comic.Mrarch (talk) 22:47, 2 December 2013 (UTC)
- And I'm sorry, but the sea level did not raise at that times.--Dgbrt (talk) 23:43, 2 December 2013 (UTC)
- But I don't like edit wars, just a small enhancement will be done. PEACE and "live long and prosper". <- STAR TREK --Dgbrt (talk) 23:51, 2 December 2013 (UTC)
- i still don't see the reason to include any reference to sea levels as it does not help to explain the comic in any way. I dont like the edit war either so i'm fine to leave it as you wrote it though i think it could be improved by leaving out the part about how much the sea level was effected. -- Mrarch (talk) (please sign your comments with ~~~~)