A Historic Perspective on the Greenland Ice Sheet and its Contribution to Global Sea Level
One of the most feared of all model-based projections of CO2-induced global warming is that temperatures will rise enough to cause a disastrous melting/destabilization of the Greenland Ice Sheet (GrIS), which would raise global sea level by several meters. But how likely is this scenario to occur? And is there any way to prove such melting is caused by human activities?
The answer to this two-part question involves some extremely complex and precise data collection and understanding of the processes involved with glacial growth and decay. Most assuredly, however, it also involves a scientifically accurate assessment of the past history of the GrIS, which is needed to provide a benchmark for evaluating its current and future state. To this end, a recent review paper by Vasskog et al. (2015) provides a fairly good summary of what is (and is not) presently known about the history of the GrIS over the previous glacial-interglacial cycle. And it yields some intriguing findings.
Probably the most relevant information is Vasskog et al.’s investigation of the GrIS during the last interglacial period (130-116 ka BP). During this period, global temperatures were 1.5-2.0°C warmer than the peak warmth of the present interglacial, or Holocene, in which we are now living. As a result of that warmth, significant portions of the GrIS melted away. Quantitatively, Vasskog et al. estimate that during this time (the prior interglacial) the GrIS was “probably between ~7 and 60% smaller than at present,” and that that melting contributed to a rise in global sea level of “between 0.5 and 4.2 m.” Thus, in comparing the present interglacial to the past interglacial, atmospheric CO2 concentrations are currently 30% higher, global temperatures are 1.5-2°C cooler, GrIS volume is from 7-67% larger, and global sea level is at least 0.5-4.2 m lower, none of which signal catastrophe for the present.
Clearly, therefore, there is nothing unusual, unnatural or unprecedented about the current interglacial, including the present state of the GrIS. Its estimated ice volume and contribution to mean global sea level reside well within their ranges of natural variability, and from the current looks of things, they are not likely to depart from those ranges any time soon.
References
Reyes, A.V., Carlson, A.E., Beard, B.L., Hatfield, R.G., Stoner, J.S., Winsor, K., Welke, B. and Ullman, D.J. 2014. South Greenland ice-sheet collapse during Marine Isotope Stage 11. Nature 510: 525–528.
Vasskog, K., Langebroek, P.M., Andrews, J.T., Nilsen, J.E.Ø. and Nesje, A. 2015. The Greenland Ice Sheet during the last glacial cycle: Current ice loss and contribution to sea-level rise from a palaeoclimatic perspective. Earth-Science Reviews 150: 45-67.