Edited by Robert E. Dickinson, The University of Texas at Austin, Austin, TX, and approved May 1, 2014 (received for review March 23, 2014)
Through an examination of shallow ice cores covering a wide area of the Greenland Ice Sheet (GIS), we show that the same mechanism drove two widespread melt events that occurred over 100 years apart, in 1889 and 2012. We found that black carbon from forest fires and rising temperatures combined to cause both of these events, and that continued climate change may result in nearly annual melting of the surface of the GIS by the year 2100. In addition, a positive feedback mechanism may be set in motion whereby melt water is retained as refrozen ice layers within the snow pack, causing lower albedo and leaving the ice sheet surface even more susceptible to future melting.
In July 2012, over 97% of the Greenland Ice Sheet experienced surface melt, the first widespread melt during the era of satellite remote sensing. Analysis of six Greenland shallow firn cores from the dry snow region confirms that the most recent prior widespread melt occurred in 1889. A firn core from the center of the ice sheet demonstrated that exceptionally warm temperatures combined with black carbon sediments from Northern Hemisphere forest fires reduced albedo below a critical threshold in the dry snow region, and caused the melting events in both 1889 and 2012. We use these data to project the frequency of widespread melt into the year 2100. Since Arctic temperatures and the frequency of forest fires are both expected to rise with climate change, our results suggest that widespread melt events on the Greenland Ice Sheet may begin to occur almost annually by the end of century. These events are likely to alter the surface mass balance of the ice sheet, leaving the surface susceptible to further melting.
Author contributions: K.M.K. designed research; K.M.K. and J.R.M. performed research; K.M.K., M.R.A., J.R.M., and I.B. analyzed data; and K.M.K., M.R.A., and I.B. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1405397111/-/DCSupplemental.
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