With global sea level rising and Arctic temperatures increasing more rapidly than any other location on the globe, it is important to understand how surface melting in Greenland today compares with that in the past. A melt history (1765 – 2006 C.E.) has been reconstructed from an ice core collected in 2007 at Crawford Point (69.9°N; 47.0°W, 2022 masl) in west central Greenland. This site lies in the zone of intermittent summer melt and percolation which may result in formation of melt features, thus potentially providing a record of surface warmth. Melt features are primarily bubble free and thus appear clearer and brighter than the surrounding snow, firn, or ice.
This paper presents an efficient and relatively inexpensive technique for quantifying the annual melt percent from an ice core. The Crawford Point core was photographed and high-resolution digital images were used to quantify the extent of melt in each year using a time scale previously constructed by counting seasonal variations in δ18O, complemented by seasonal variations in the concentrations on dust, nitrate, and sulfate. Melt percent was determined using a 1x1 cm grid which was overlain on each year within the core.
Regression analysis was used to determine that melt within the Crawford Point ice core has been increasing at a rate of 0.08% per year, since 1765, and has increased to 0.11% per year after 1900. This record was compared to contemporaneous observations including other melt histories, the existing satellite record, meteorological observations, patterns of atmospheric circulation, other ice core-derived proxy records, and volcanic activity. Regression relationships were employed to examine the correlations between melt at Crawford Point and these other variables.
While melt preserved in ice cores has been previously assessed, there has been no sustained methodology to facilitate comparisons among the resulting records. The results of this study establish the need for standardization of the process used to determine and report melt histories from ice cores. Recommendations are also made regarding future work relating to the extraction of melt histories from Greenland.