Surface melt off of the Greenland ice sheet has reached record levels in 2005, 2007, 2010, and 2012. As the global temperature continues to rise, more ice will continue to melt, with severe consequences for global sea levels. However, this ice sheet’s meltwater is often refrozen in the ice sheet’s percolation zone, an area that is perennially covered by snow and partially compacted snow (firn). Here, meltwater can affect the ice sheet’s flow dynamics, sea levels, and mass balance if it melts completely and runs off, but not if it remains refozen in this percolation zone. By observing firn structure and meltwater retention of the Greenland ice sheet, Harper et al. (2012) were able to understand the flow dynamics of the meltwater and consequently predict future global impacts of the ice sheet’s shrinking size. They found that some meltwater from the Greenland ice sheet will fill pore space in the percolation zones of the ice sheet, and will thus not affect sea levels. While this pore space is not unlimited, Haper et al. suggested that water will be rerouted to these areas and will help preserve sea levels for at least 15 years.—Olivia Jacobs
Harper, J., Humphrey, N.. Pfeffer, W., Brown, J., Fettweis, X., 2012. Greeland ice-sheet
contribution to sea-level rise buffered by meltwater storage in firn. Nature 491,
To analyze the patterns of meltwater movement, Harper et al. established fifteen study sites along the Greenland ice sheet and collected data for two years between 2007 and 2009. The team collected 34 ice cores and many firn temperatures readings and identified refrozen meltwater. Thermistor strings installed in boreholes showed thermal events under the surface of the percolation zone, and revealed reheating events in the ice sheet’s pores. These thermal data showed that refreezing events occurred well below earlier years of accumulation, which suggests that meltwater in the percolation zone moves downward through the firn instead of simply moving across the percolation zone and into the ocean. This means that pores in this area of the ice sheet act as “storage” space to hold and refreeze meltwater, thus avoiding further sea level escalation.
After the team discovered that meltwater moved down through the pores in the percolation zone, they were able to calculate the mass of water per unit of area needed to fill remaining open pore space using ice core and radar data. The available space in different firns varied with changes in elevation because at lower elevations (below 2000 feet), where temperatures are warmer, some pore space has already been filled with ice, thus limiting its capacity to hold more meltwater. At elevations above 2000 meters, however, firn capacity is not reduced by previous meltwater infiltration and freezing. Accounting for this difference in storage with elevation, Harper et al. estimated a storage space between 322 and 1,289 gigatons throughout the entire percolation zone.
There are many uncertainties in this modeling, including the fluctuating rates of ice sheet melt, rainfall, firn compaction, and pore space additions from snowfall. Accounting for these uncertainties, and using the aforementioned limits of storage in the percolation zone, the most severe climate forecasting models suggest that current percolation zones will become completely filled with Greenland’s meltwater within 15 to 20 years, while more lenient models estimate that this will take about 30 years. From this data it is apparent that although the amount of meltwater from the Greenland ice sheet continues to increase as global temperatures increase, its effects on global sea levels may not become apparent for years to come. However, once the current percolation space is filled, meltwater will likely empty to the ocean and raise sea levels since porous spaces in percolation zone take decades to form. Thus current, observable rises in sea levels do not account for all dimensions of glacial melt, and sea levels may rise more rapidly in the decades ahead even if meltwater rates remain consistent.