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.