History of the Greenland Ice Sheet: paleoclimatic insights

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Abstract

Paleoclimatic records show that the Greenland Ice Sheet consistently has lost mass in response to warming, and grown in response to cooling. Such changes have occurred even at times of slow or zero sea-level change, so changing sea level cannot have been the cause of at least some of the ice-sheet changes. In contrast, there are no documented major ice-sheet changes that occurred independent of temperature changes. Moreover, snowfall has increased when the climate warmed, but the ice sheet lost mass nonetheless; increased accumulation in the ice sheet's center has not been sufficient to counteract increased melting and flow near the edges. Most documented forcings and ice-sheet responses spanned periods of several thousand years, but limited data also show rapid response to rapid forcings. In particular, regions near the ice margin have responded within decades. However, major changes of central regions of the ice sheet are thought to require centuries to millennia. The paleoclimatic record does not yet strongly constrain how rapidly a major shrinkage or nearly complete loss of the ice sheet could occur. The evidence suggests nearly total ice-sheet loss may result from warming of more than a few degrees above mean 20th century values, but this threshold is poorly defined (perhaps as little as 2 °C or more than 7 °C). Paleoclimatic records are sufficiently sketchy that the ice sheet may have grown temporarily in response to warming, or changes may have been induced by factors other than temperature, without having been recorded.

Section snippets

Overview

The Greenland Ice Sheet (Fig. 1) is approximately 1.7 million km2 in area, extending as much as 2200 km north to south. The maximum ice thickness is 3367 m, the average thickness is 1600 m (Thomas et al., 2001), and the volume is 2.9 million km3 (Bamber et al., 2001). The ice has depressed some bedrock below sea level, and a little would remain below sea level following ice removal and bedrock rebound (Bamber et al., 2001). However, most of the ice rests on bedrock above sea level and would

Paleoclimatic indicators bearing on ice-sheet history

Here, marine indicators of ice-sheet change are discussed, followed by terrestrial archives. For a broader overview, see, e.g., Cronin (1999) or Bradley (1999).

Ice-sheet onset and early fluctuations

Before 65 Ma, dinosaurs lived on a high-CO2, world that was warm to high latitudes; mean-annual temperature exceeded 14 °C at 71°N based on occurrence of crocodile-like champsosaurs (Tarduno et al., 1998; also see Markwick, 1998, Vandermark et al., 2007). The ocean surface near the North Pole warmed from ∼18 °C to a peak of ∼23 °C during the short-lived Paleocene–Eocene Thermal Maximum about 55 Ma (Sluijs et al., 2006). Such warm temperatures preclude permanent ice near sea level and, indeed, no

Discussion

Glaciers and ice sheets are controlled by many climatic factors and by internal dynamics. Attribution of a given ice-sheet change to a particular cause is generally difficult, and requires appropriate modeling and related studies.

It remains, however, that in the suite of observations as a whole, the behavior of the Greenland Ice Sheet has been more closely tied to temperature than to anything else. The Greenland Ice Sheet shrank with warming and grew with cooling. Because of the generally

Synopsis

Paleoclimatic data show that the Greenland Ice Sheet has changed greatly with time. From physical understanding, many environmental factors can force changes in the size of an ice sheet. Comparison of the histories of important forcings and of ice-sheet size implicates cooling as causing ice-sheet growth, warming as causing shrinkage, and sufficiently large warming as causing loss. The evidence for temperature control is clearest for temperatures similar to or warmer than recent values (the

Acknowledgements

RBA acknowledges partial support from the US National Science Foundation under grants 0531211 and 0424589. GHM acknowledges partial support from the US National Science Foundation under grants ARC0714074 and ATM0318479. LP acknowledges partial support from the US National Science Foundation under grants ARC0612473 and 0806999. JWCW acknowledges partial support from the US National Science Foundation under grants 0806387, 053759, and 059512. BLO acknowledges support from the US National Science

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