Implications of the Firn Aquifer
By Matt Owens February 24, 2014
H₂O is unlike almost all other naturally occurring substances. One of its remarkable characteristics is that when its liquid form solidifies, the resulting ice floats on the liquid. This density transition is central to why freezing water tends to tear up roads during harsh winters; and the same process likewise cracks open rocks, thereby helping to release vital mineral nutrients to the biosphere. Floating ice also protects lakes and oceans from freezing all the way to the bottom and therefore protects the life they contain during periods of prolonged cold. Considering the remarkable properties of water and ice, it's fitting that there continue to be surprising new discoveries by glaciologists studying the two polar ice sheets.
One of the most recent discoveries is a vast aquifer of liquid water that exists year-round in the very top layers of Greenland's ice sheet. Two questions immediately come to mind: How is it possible that a huge store of liquid water could be located within the ice, let alone at the top of the ice? And what are the implications for future rates of sea level rise?
The later question is especially important because the prevailing estimates are based on calculations that treat surface meltwater as either quickly running off the surface into the ocean, or penetrating to the base of the ice sheet via existing ice tunnels and then flowing fairly quickly out to the ocean via drainage channels beneath the ice sheet. However, if the increasing amounts of meltwater are instead partially stored inside the ice sheet, that could not only change the timing of sea level rise, but it could also impact the internal temperature and therefore structure and flow rate of the ice sheet.
To help understand the discovery and get a broader context on the whole situation, I spoke with glaciologist William Colgan, a researcher at the Geological Survey of Denmark and Greenland, and a member of the International Glaciological Society's executive council. He is among a new wave of scientists examining the issue of what happens to meltwater inside ice sheets (englacial hydrology).
First, some definitions are in order so we have some idea of just what we're talking about with this ice sheet aquifer: The 'firn' layer is a thick covering of snow at the ice sheet's upper-most surface. It's roughly 50 meters thick (about 164 feet thick), and as fresh snow accumulates on the top, the added weight compresses the snow layers below until eventually compressing the snow into solid ice at about the 50 meter depth mark. As a consequence, there exists a network of very small open pores between the grains of snow in the firn. These connected pores allow both air and water to flow through this compressing firn layer. As a side note, the process of pore-closure when the firn becomes ice, is why there is a difference in the age of the ice and the age of the gas trapped in it, known as the 'gas age-ice age difference.'
The ice sheet beneath the firn is much thicker than the firn though, as much as a few thousand meters thick. See the diagram below to get an idea of what the shape of Greenland's ice sheet is like in profile.
Now, glaciologists have already known that surface meltwater penetrates down into the pore spaces of the firn. But according to Colgan, the observed surface melting seems to have so far matched the outflow discharge around the edges of the ice sheet, so it doesn't seem likely that huge amounts of meltwater are accumulating in the ice sheet annually.
Before the aquifer discovery, a 2012 research paper in Nature concluded that in the future as melt rates increase, the firn layer over the entire Greenland ice sheet might be able to store somewhere between 300 and 1,300 gigatonnes of water mass in its pore spaces.¹ That work included field surveys from 2007 to 2009 at one western section of the ice sheet near Jakobshavn Isbræ and described finding some frozen meltwater in the firn, but not liquid water.
The newly discovered - and confirmed - liquid water aquifer in the firn is estimated to total 140 metric gigatonnes; it was first noticed in 2011 and confirmed in 2013 from surveys across a section of Greenland in the southeast. So it's not entirely clear if the aquifer is a recent development or not. According to Colgan, “if the firn aquifer is a new feature in response to climate change, then eventually the pore space will fill up and whatever sea level rise buffering the aquifer is now providing will be lost. If the firn aquifer is a historical feature of the ice sheet (i.e. always been there, we just didn't know), then only our understanding of hydrological routing, not hydrological routing itself, is changing.”
Putting the ice aquifer in an even broader context, Colgan said that for Greenland, “the melt that's around the corner is really quite substantial,” and that “it's the effect of water that stays many years and warms the ice” that has significant implications. He added that the extra meltwater produced on the ice sheet surface could be in the neighborhood of 1,000 gigatonnes per year by 2100. That would entirely fill the potential firn storage volume in about one year.
Colgan described lingering water inside the ice sheet as “thermally corrosive,” which is an easier way to think of what's technically known as cryo-hydrologic warming.
And whatever portion of meltwater makes it to the base of the ice (through cracks and tunnels in the ice sheet) will also thermally corrode the ice there at the base and along the way. Previous research Colgan participated in showed that the warming from lingering englacial water can soften the ice and therefore accelerate the ice speed at both the surface and base of the sheet.²
It seems clear that surface melt will dominate the future sea level rise contribution from Greenland,³ at least for this century; other research Colgan participated in concluded that by 2025, there's a 50% chance the entire surface of the ice sheet will experience routine summer melting,⁴ as shown in the animation below. This is in sharp contrast to the situation today where only the sloped-edges of the ice sheet are experiencing regular summer melt.
The extra surface melting will initiate and accelerate a series of feedback processes, including the internal hydrological ones that Colgan calls thermally corrosive.
One key feedback processes includes crevasse formation and cryo-hydrologic warming where water transfers substantial amounts of heat to the interior of the ice sheets. As the ice warms and softens, it deforms faster and therefore flows even faster than it already is flowing - out towards the oceans. That accelerated flow will induce more cracking (crevasse formation) on the surface and at the base of the ice sheet in vulnerable places, thereby enabling more water storage and heat transfer at both the top and bottom of the sheet, among other important effects.⁵ The diagram above summarizes several key hydrological feedback processes, including cryo-hydrologic warming (image courtesy of William Colgan).
The Greenland ice sheet won't just suddenly collapse though, at least not for a while according to Colgan. In a follow-up email, he wrote that “this warming and softening leading to a "thermal collapse" is centuries out. The name of the game right now is surface mass balance and meltwater runoff.”
Adding all this up, it seems like the newly discovered Greenland firn aquifer is not itself a cause for alarm, but it is one more alarming sign that climate inertia is building up. We will have to push back against this inertia in the near future by removing carbon dioxide from the air - or accept the steep consequences of sea level rise.
For some perspective, current estimates (that mostly exclude englacial hydrology) predict about 1 meter of sea level rise (from all sources) by 2100 with business as usual emissions. And because of the immense inertia in the system, if all emissions were stopped today, those predictions still call for about half a meter of rise by 2100.
As a final note, it's worth considering the proportions we're talking about when it comes to water on Earth. If all the water in the air were to rain out at once, about 1 inch of water (about 2.5 centimeters) would pour over the surface of the Earth. But if both the polar ice sheets were to melt, and/or disintegrate and discharge their ice mass into the oceans, the global sea level would increase by about 260 feet (about 80 meters).
¹ Harper et al., 2012 - “Greenland ice-sheet contribution to sea-level rise buffered by meltwater storage in firn;” Nature; doi: 10.1038/nature11566.
² Phillips et. al., 2013 - “Evaluation of cryo-hydrologic warming as an explanation for increased ice velocities in the wet snow zone, Sermeq Avannarleq, West Greenland;” Journal of Geophysical Research Letters: Earth Surface; doi: 10.1002/jgrf.20079.
³ Enderlin et al., 2014 - “An improved mass budget for the Greenland ice sheet;” Geophysical Research Letters; doi: 10.1002/2013GL059010.
⁴ McGrath et al., 2013 - “Recent warming at Summit, Greenland: Global context and implications;” Geophysical Research Letters; doi: 10.1002/grl.50456.
⁵ Colgan et al., 2011 - “An increase in crevasse extent, West Greenland: Hydrologic implications;” Geophysical Research Letters; doi: 10.1029/2011GL048491.