Published on Aug 29, 2015
SAN FRANCISCO — Greenland’s disappearing ice shifted gears in the past decade, switching from shrinking glaciers to surface melting, researchers reported here last week at the American Geophysical Union’s annual meeting.
Instead of losing ice where massive glaciers meet the sea, Greenland now sends meltwater rushing into the ocean via a vast network of lakes and rivers, according to several studies. The results do not mean that glaciers have stopped their speedy flow, only that surface melting now exerts a more powerful influence on ice loss, researchers said.
“We no longer see giant icebergs calving” from glaciers, releasing ice into the sea, said Lora Koenig, a glaciologist at the National Snow and Ice Data Center, who led one of the new studies. “The majority of water is coming from surface melt.
Greenland, one of the largest ice sheets in the world, is melting. In fact, it is melting ahead of schedule as the world warms. Scientists are working hard to deepen their understanding of this ice sheet’s behavior so that we can predict how fast and how much of the ice sheet will melt in the coming decades and centuries.
It might seem obvious that in a warming world, the Greenland ice sheet will melt. But, what seems obvious and simple can be more complex when investigated more deeply. With respect to Greenland, it is expected that warmer temperatures increase melting but warmer temperatures can also mean more snowfall, as there is more moisture in warm air which can then fall as snow. So, it has been a question of which of these two competing processes would win out. Would Greenland get smaller because of melting or would it grow as more snow fell?
Over the past few years, the verdict has become clear. The Greenland ice sheet is losing mass at an increasing rate. In fact, Greenland currently contributes twice as much as the Antarctic to rising sea levels.
A new study, just published in Nature Geoscience, makes an important new contribution to our understanding of the forces at play in Greenland. Dr Samuel Doyle and an international team captured the wide-scale effects of an unusual week of warm, wet weather in late August and early September, 2011. They found that weather cyclonic led to extreme surface runoff – a combination of ice melt and rain – that overwhelmed the ice sheet’s basal drainage system. This drive a marked increase in ice flow across the entire western sector of the ice sheet that extended 140 km into the ice sheet’s interior.
The cyclonic weather system delivered heat and rain to the western edge of the Greenland ice sheet and under these warm, wet, cloudy conditions the way that the ice sheet receives energy for melt is very different to that under the more typical clear sky conditions. As we all know from a cloudy day, clouds block a certain amount of sunshine, but under certain conditions they can absorb the outgoing longwave radiation and re-radiate it back to the surface. This is why a cloudy night is often warmer than when the sky is clear. The same thing happens on the ice sheet.
In fact during the August 2011 weather event, melt continued throughout both day and night creating exceptionally high daily melt totals for this time of year. Moisture in the atmosphere also reduces the rate at which the air cools as it rises over the ice sheet, allowing warm temperatures and therefore melt and rain to attain abnormally high elevations. The heat released by condensation and by rain refreezing in the snowpack enhanced melt even further.
Even given these factors, the water runoff from melt and rain did not exceed mid-summer peak values when ice flow was relatively slow. This is because the ice sheet’s drainage system continually adapts to melt inputs: in mid-summer an efficient drainage system forms and the ice sheet can easily accommodate high water inputs. This isn’t the case in late summer though, as the drainage system rapidly freezes shut when air temperatures fall below zero.
The influence of such intense rainfall events has not, until now, been considered in assessments of the melt and flow response of any ice sheet. This is an important omission because cyclonic conditions are predicted to increase in the future, therefore likely playing an increasing role in driving ice mass loss from the Greenland ice sheet.
Since the 1980s when rainfall measurements began in the west Greenland town of Kangerlussuaq, the focus of the study, the proportion of precipitation now falling as rainfall rather than snow has both increased and extended into the late summer and autumn in line with increased circulation and moisture availability within a warmer, more energetic atmosphere.