WUWT readers surely remember all the media hype over this story. This was REP’s last entry on WUWT:
As WUWT readers are aware, there has been a great deal of attention paid by the main stream media to the extensive melt on the Greenland icecap that occurred during July (for example, see here, here, here, here, and here). The topic was addressed here at WUWT in two postings here and here. Anthony noted in the later posting that Andrew Revkin was almost alone in taking a more nuanced and skeptical view of the unprecedented nature of the event and has taken a fair amount of heat in comments for his effort.
And then there was the paper that showed that a shift in the jet stream caused warmer than normal temperatures in July 2012. Now a new paper in PNAS nails the trigger for the “insta-melt”, finding carbon soot combined with warmer temperatures was the trigger, not just in 2012, but also in 1889.
They get one attribution wrong, confusing climate change and weather events, but the science on the black carbon (BC) and albedo seems right. Their use of Oxygen 18 isotope records show that it was unusually warm in 1889 as well. They say in the Figure 2 caption:
…widespread melt events only occurred in 1889 and 2012. In C and E, melt occurred because of the deposition of high concentrations of BC and ammonium, indicating an albedo reduction due to BC from summer forest fires.
Visual evidence of carbon soot can be found in meltwater ponds in Greenland from this and other photos by James Balog, a real eye opener:
In the winter a huge among of snow are accumulated on the Ice (2-3 meters, sometimes more) and we are not talking about 1 or 2 square-miles, it’s about 100.000′s of square miles (up to 1 million) on the Westside of the Ice cap and a similar picture on the Eastside… when the melting season starts in april-sep… the meltwater has to go somewhere, and for sure it goes downhill in huge meltwater rivers.
The black stuff on the bottom of the lakes is carbon dust and pollution in general… but not from one year, but several decades (the topographical conditions don’t change from year to year). On a flight over the Ice Cap a sky clear day, you can see hundreds of huge lakes with the black spot on the bottom.
Climate change and forest fires synergistically drive widespread melt events of the Greenland Ice Sheet
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.
Melt extent over the GIS determined from Oceansat-2 satellite scatterometer, Special Sensor Microwave Imager/Sounder, and Moderate-resolution Imaging Spectroradiometer satellite data for (A) July 8, 2012, and (B) July 12, 2012. Red areas indicate melt detected by the satellites, white areas indicate no melt, and blue represents ocean. The surface of almost the entire ice sheet, including the dry snow region, experienced melt on July 12, 2012. Figure courtesy of Dorothy Hall, NASA Goddard Space Flight Center.
(Lower) The annual average BC concentrations (ng g−1) from 1750 to 2010 of the Summit-2010 firn core and the 2012 surface section. (Upper) Sections of the BC record along with δ18O and ammonium records, plotted on a relative scale normalized to the maximum and minimum values in each record, for the time intervals (A) 1783–1788, (B) 1865–1870, (C) 1887–1892, and (D) 1905–1910, as well as (E) the normalized average value of BC and ammonium concentrations from the July 2012 surface sample, and approximate δ18O. These time intervals demonstrate extreme scenarios in the center of the GIS with (B−E) depicting the highest concentrations of BC, and (A) the warmest temperature since 1750, but widespread melt events only occurred in 1889 and 2012. In C and E, melt occurred because of the deposition of high concentrations of BC and ammonium, indicating an albedo reduction due to BC from summer forest fires. Importantly, these deposition events occurred during warm summers. In B, a high concentration of BC and presence of ammonium during a cooler summer suggest that the surface was below the energy threshold for melt. In D, the highest concentrations of BC and ammonium in the record were recorded during an average summer, suggesting that the BC was deposited at a time of the year when the available surface energy was well below the threshold for melt. The warmest temperature recorded in the core occurred in 1785, but widespread melting did not occur due to low BC concentration.