Subglacial sediment from ~1.4 km (0.87 mi) beneath the ice stored since 1966 indicates that Greenland was completely ice-free and vegetated at least once within the last million years. Scientists have, using those ice cores, obtained information on (proxies for) temperature, ocean volume, precipitation, chemistry and gas composition of the lower atmosphere, volcanic eruptions, solar variability, sea-surface productivity, desert extent and forest fires. In the past decades, scientists have drilled ice cores up to 4 kilometres (2.5 mi) deep. 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. See also: Greenland ice core project and Greenland Ice Sheet Project The record went unnoticed for more than 28 years and was finally recognized in 2020. On 22 December 1991, a temperature of −69.6 ☌ (−93.3 ☏) 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 lowest mean annual temperatures, about −31 ☌ (−24 ☏), occur on the north-central part of the north dome, and temperatures at the crest of the south dome are about −20 ☌ (−4 ☏). On the ice sheet, temperatures are generally substantially lower than elsewhere in Greenland. The best known of these outlet glaciers is Jakobshavn Glacier ( Greenlandic: Sermeq Kujalleq), which, at its terminus, flows at speeds of 20 to 22 metres or 66 to 72 feet per day. 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 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. The unconfined ice sheet does not reach the sea along a broad front anywhere in Greenland, so that no large ice shelves occur. The crests of both domes are displaced east of the centre line of Greenland. The southern dome reaches almost 3,000 metres (10,000 ft) at latitudes 63°– 65°N the northern dome reaches about 3,290 metres (10,800 ft) at about latitude 72°N (the fourth highest "summit" of Greenland). The ice surface reaches its greatest altitude on two north–south elongated domes, or ridges. 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 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. The ice cores also record human impact, such as lead from the Roman Empire. The oldest known ice in the current ice sheet is as much as 1,000,000 years old. Computer modelling shows that the uplift would have enabled glaciation by producing increased orographic precipitation and cooling the surface temperatures. The Western and Eastern Greenland mountains constitute passive continental margins that were uplifted in two phases, 10 and 5 million years ago, in the Miocene epoch. Ice sheet formed in connection to the uplift of the West Greenland and East Greenland uplands. There was an intensification of glaciation during the Late Pliocene. The Greenland Ice Sheet formed in the middle Miocene by coalescence of ice caps and glaciers. From about 11 million years ago to 10 million years ago, the Greenland Ice Sheet was greatly reduced in size. The presence of ice-rafted sediments in deep-sea cores recovered from northwest Greenland, in the Fram Strait, and south of Greenland indicated the more or less continuous presence of either an ice sheet or ice sheets covering significant parts of Greenland for the last 18 million years. 2.6 Twenty-first century melting events.2.4 Observation and research since 2010.2.3 General considerations on rate of change.2.2 General considerations on ice melting.2.1 The ice sheet as a record of past climates.
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