New research shows looks specifically at glaciers ending on land
Parts of the Greenland Ice Sheet may actually be slowing down, rather than speeding up, in response to decades of climate change, but that doesn’t mean it’s time to stop worrying about sea level rise.
In a new study, glaciologists measuring ice movement on the southwest portion of the Greenland Ice Sheet found that glaciers terminating on land have slowed by an average of 12 percent across 84 percent of the study area between 2007 and 2014, compared to the years between 1985 and 1994. The study looked specifically at ice sheets terminating on land, not those flowing into the ocean.
The scientists said their findings appear to contradict conventional wisdom. Many recent studies have suggested that more surface melting will speed up ice sheet movement. The amount of meltwater draining from the ice sheet in four out of the five years between 2007 and 2012 has been the most substantial of the last 50 years.
“This suggests that further increases in melting will not cause these land-terminating margins of the ice sheet to speed up,” said lead author Andrew Tedstone, a glaciologist at the University of Edinburgh, Scotland.
“Nevertheless, it is unclear how much more slowdown we will see under the current and future melting conditions,” said co-author Noel Gourmelen, University of Edinburgh. “More research and observation are needed to determine this.”
The study was published Oct, 29 in the journal Nature. The findings suggest that, while the larger summertime meltwater volume of recent years has led to greater lubrication of the ice sheet base, speeding up its flow as expected, by the end of summer the meltwater has also established channels at the base that act as efficient drainage systems to lessen the water under the ice sheet, slowing it down by winter.
The researchers cautioned that their results shouldn’t be misinterpreted. The fact that land-terminating ice sheets may slow down, at least temporarily, doesn’t change the fact that Greenland ice sheets flowing into the sea will continue to contribute to rising sea level.
“The ongoing acceleration of both glacier surface melt volumes and the ice motion of ocean-terminating glaciers ensures that Greenland’s contribution to sea level rise will likely increase in our warming world,” said co-author Peter Nienow, University of Edinburgh.
Greenland has shed on average 303 gigatons of ice per year since 2004, and with every successive year the loss has increased by 31 gigatons. (Each gigaton equals one billion metric tons.) Recent estimates suggest that surface melting is responsible for 60 percent of Greenland’s ice sheet losses, while the remainder is caused by ice sheet discharge into the ocean.
“By analyzing velocity estimates extracted from 30 years of Landsat data, this study highlights the complex, and sometimes counterintuitive, interplay between surface meltwater and ice motion,” said Thomas Neumann, a cryospheric scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland who was not involved in the study.
The results show the importance of maintaining a long time series of remote sensing data, such as the Landsat record. NASA and the United States Geological Survey have already begun work on Landsat 9 to help continue this record.