precipitation and cooling the surface temperatures. The oldest known ice in the current ice sheet is as much as 1,000,000 years old. The ice cores also record human impact, such as lead from the Roman Empire. The weight of the ice has depressed the central area of Greenland; the bedrock surface is near sea level over most of the interior of Greenland, but mountains occur around the periphery, confining the sheet along its margins. If the ice suddenly disappeared, Greenland would most probably appear as an archipelago, at least until isostasy lifted the land surface above sea level once again. The ice surface reaches its greatest altitude on two north–south elongated domes, or ridges. The southern dome reaches almost at latitudes 63°–65°N; the northern dome reaches about at about latitude 72°N (the fourth highest "summit" of Greenland). The crests of both domes are displaced east of the centre line of Greenland. The unconfined ice sheet does not reach the sea along a broad front anywhere in Greenland, so that no large ice shelves occur. The ice margin just reaches the sea, however, in a region of irregular topography in the area of Melville Bay southeast of Thule, as well as in Jokel Bay. Large outlet glaciers, which are restricted tongues of the ice sheet, move through bordering valleys around the periphery of Greenland to calve off into the ocean, producing the numerous icebergs that sometimes occur in North Atlantic shipping lanes. The best known of these outlet glaciers is Jakobshavn Glacier (), which, at its terminus, flows at speeds of per day. On the ice sheet, temperatures are generally substantially lower than elsewhere in Greenland, due to the local effects of ice-albedo feedback. The lowest mean annual temperatures, about , occur on the north-central part of the north dome, and temperatures at the crest of the south dome are about . On 22 December 1991, a temperature of was recorded at an automatic weather station near the topographic summit of the Greenland Ice Sheet, making it the lowest temperature ever recorded in the Northern Hemisphere. The record went unnoticed for more than 28 years and was finally recognized in 2020. The ice sheet as a record of past climates The ice sheet, consisting of layers of compressed snow from more than 100,000 years, contains in its ice today's most valuable record of past climates. In the past decades, scientists have