THE EXTREME MELT EVENT OF 2012 ACROSS THE GREENLAND ICE SURFACE - A SUDDEN ANOMALY SINCE THE MEDIEVAL WARM PERIOD

The Greenland ice sheet (GIS) holds a massive capacity of water equivalent of 6-7 m in sea level rise (SLR) by its complete melt. From satellite remote sensing, we present here the discovery of the 2012 extreme melt across almost the entire GIS surface. This extreme melt event was first observed from Oceansat-2 satellite scatterometer (OS2) near-real-time data, which was rapidly confirmed by Moderate-resolution Imaging Spectroradiometer (MODIS) and subsequently by Special Sensor Microwave Imager/Sounder (SSMIS) satellite measurements. In-situ temperature data and field observations provided further evidence of the melt event.

To obtain the most complete melt detectable across the ice sheet, composite melt maps are produced from the combination of OS2/MODIS/SSMIS with a satellite integration algorithm developed under the NASA Science Innovation Fund for Greenland melt research. Satellite results reveal that melt occurred at or near the surface across 97% of the entire GIS extent on 12 July 2012, including the cold polar areas at high altitudes like Summit in the dry snow facies of the ice sheet. This melt event coincided with an anomalous ridge of warm air that became stagnant over Greenland.

As seen in melt layers from the ice core record at Summit reported in the published literatures, such a melt event is rare with the last significant one occurring in 1889 and the next previous one approximately 680 years earlier in the Medieval Warm Period (a.k.a. the Medieval Climatic Anomaly). Compared to the 7-century interval between the previous two events, the 2012 extreme melt occurred only over a century from the 1889 event. In this respect, the 2012 event is considered as a sudden anomaly since the Medieval Warm Period. These significant melt events are widely sporadic in different periods of the Holocene, clearly exhibiting their non-stationary behavior.

Regarding the implication on SLR, the change in total GIS mass balance is intricately dependent on competitive changes in mass outflux (e.g., snowmelt, glacier calving) and influx (e.g., snow accumulation). Extreme events of snow accumulation can also be observed by satellite remote sensing. Thus, spatially extensive measurements from multiple satellites are critical to closely monitoring the change in GIS mass balance for an accurate estimation of its contribution to SLR.