| Summary: | The Greenland Ice Sheet is expected to shrink or disappear with warming, a conclusion based on a survey of paleoclimatic and related information. Recent observations show that the Greenland Ice Sheet has melted more in years with warmer summers. Mass loss by melting is therefore expected to increase with warming. But whether the ice sheet shrinks or grows, and at what pace, depend also on snowfall and iceberg production. The Arctic is a complicated system. Reconstructions of past climate and ice sheet configuration (the "paleo-record") are valuable sources of information that complement process-based models. The paleo-record shows that the Greenland Ice Sheet consistently lost mass when the climate warmed and grew when the climate cooled. 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 of change, and the changes to the ice sheet themselves, 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 paleo-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 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 ...
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