Ice Sheets
Ice Sheets
[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.] 2100m Toronto
[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.] 900m Chicago
[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.] 1250m Boston
[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.] 3300m Montreal

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 times as tall as that tower. Although, over Toronto, the ice was "only" 2.1 km tall.
The tallest ice sheet takes up 265 pixels. From that, each pixel is about 12.4 meters and the height 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 films.
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.] 2100m Toronto
[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.] 900m Chicago
[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.] 1250m Boston
[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.] 3300m Montreal
