Ice through time

In North America, the ice, glacial lakes and sea level all come from Austermann’s model, so they match one another. Elsewhere, the ice and coastal change come from PaleoMIST. The two sources can disagree where they meet. The page does not include a complete history of the Great Lakes’ levels. Austermann’s grid, about 0.7°, is too coarse for narrow arms of the sea such as the Champlain Sea, which flooded the St. Lawrence and Ottawa valleys from about 13,000 to 10,500 years ago; the map doesn’t show it.

Mountain glaciers come from GLACIMONTIS, a compilation of published reconstructions. Each outline is a glacier at its largest, sometime between 57,000 and 14,000 years ago, and gives no thickness. The outlines don’t record when a glacier grew, so each is drawn from 57,000 years ago and fades out across the dates its study gives for the start of its retreat. An undated glacier stays until 14,000 years ago. Today’s glaciers, RGI 7.0’s outlines from around 2000, are drawn beneath the rest at every date from 57,000 years ago on, so a glacier of the last ice age that fades out leaves today’s in its place. Through the last ice age glaciers were larger than today’s. Through the Holocene they were mostly near today’s size, but smaller in its warm middle and larger in the Little Ice Age; the map doesn’t show those changes. No mountain glaciers are shown before 57,000 years ago: the last interglacial’s warm spells shrank them, and none of these sources says by how much. The Glacier strip shows the same for the blue dot. Where the blue dot lies on one of today’s glaciers, the strip runs on to today and the glacier’s name heads the strips. A dashed line marks where a study’s mapping stops, at a border or the edge of its study area, and the ice may have gone on past it. GLACIMONTIS doesn’t mark every such edge, so other straight edges in the outlines are likely study edges too.

In the strips, ice thickness rises upward and lake depth extends downward, each on its own scale, and global sea level has its own strip below them. Ice and lake depth are for the blue dot. Sea level is the same worldwide, from Austermann’s published estimate of ice volume. The strips share a time axis but not an elevation scale. The shading along coasts follows local relative sea level, which includes land movement: tan is exposed seafloor and blue is land flooded by the ocean. Neither is glacial lake water. Darker teal means deeper lake water (0–500+ m). The readouts come from the original lake grid, which is finer than the map’s. Lake depth is interpolated only between two dates when the spot is under a lake, so the curve breaks where a lake appears or drains. Today’s place names stay on the map for orientation.

The map shows every date the sources publish. Between dates, the maps fade from one to the next and the values at the spot are interpolated, so the frames in between are illustrative. In the strips, dashed curves bridge missing samples. Today is the 1950 baseline, and older dates are approximate years before now, rounded to 100 years.

Before 80,000 years ago, only North America has a reconstruction, so spots there go back to 118,000 years. Earlier ice ages reached farther in places, beyond these models’ range: ice covered north London about 450,000 years ago and southern Illinois about 150,000 years ago.