CR
Cryospheric Sciences

Surface mass balance

Image of the Week – A high-resolution picture of Greenland’s surface mass balance

Image of the Week – A high-resolution picture of Greenland’s surface mass balance

The Greenland ice sheet – the world’s second largest ice mass – stores about one tenth of the Earth’s freshwater. If totally melted, this would rise global sea level by 7.4 m, affecting low-lying regions worldwide. Since the 1990s, the warmer atmosphere and ocean have increased the melt at the surface of the Greenland ice sheet, accelerating the ice loss through increased runoff of meltwater and iceberg discharge in the ocean.


Simulating the climate with a regional model

To understand the causes of the recent ice loss acceleration in Greenland, we use the Regional Atmospheric Climate Model RACMO2.3 (Noël et al. 2015) that simulates the evolution of the surface mass balance, that is the difference between mass gain from snowfall and mass loss from sublimation, drifting snow erosion and meltwater runoff. Using this data set, we identify three different regions on the ice sheet (Fig. 1):

  • the inland accumulation zone (blue) where Greenland gains mass at the surface as snowfall exceeds sublimation and runoff,

  • the ablation zone (red) at the ice sheet margins which loses mass as meltwater runoff exceeds snowfall.

  • the equilibrium line (white) that separates these two areas.

From 11 km to 1 km : downscaling RACMO2.3

To cover large areas while overcoming time-consuming computations, RACMO2.3 is run at a relatively coarse horizontal resolution of 11 km for the period 1958-2015. At this resolution, the model does not resolve small glaciated bodies (Fig. 2a), such as narrow marginal glaciers (few km wide) and small peripheral ice caps (ice masses detached from the big ice sheet). Yet, these areas contribute significantly to ongoing sea-level rise. To solve this, we developed a downscaling algorithm (Noël et al., 2016) that reprojects the original RACMO2.3 output on a 1 km ice mask and topography derived from the Greenland Ice Mapping Project (GIMP) digital elevation model (Howat et al., 2014). The downscaled product accurately reproduces the large mass loss rates in narrow ablation zones, marginal outlet glaciers, and peripheral ice caps (Fig. 2b).

Fig. 2: Surface mass balance (SMB) of central east Greenland a) modelled by RACMO2.3 at 11 km, b) downscaled to 1 km (1958-2015). The 1 km product (b) resolves the large mass loss rates over marginal outlet glaciers [Credit: Brice Noël].

 

The high-resolution data set has been successfully evaluated using in situ measurements and independent satellite records derived from ICESat/CryoSat-2 (Noël et al., 2016, 2017). Recently, the downscaling method has also been applied to the Canadian Arctic Archipelago, for which a similar product is now also available on request.

Endangered peripheral ice caps

Using the new 1 km data set (Fig. 1), we identified 1997 as a tipping point for the mass balance of Greenland’s peripheral ice caps (Noël et al., 2017). Before 1997, ablation (red) and accumulation zones (blue) were in approximate balance, and the ice caps remained stable (Fig. 3a). After 1997, the accumulation zone retreated to the highest sectors of the ice caps and the mass loss accelerated (Fig. 3b). This mass loss acceleration was already reported by ICESat/CryoSat-2 satellite measurements, but no clear explanation was provided. The 1 km surface mass balance provides a valuable tool to identify the processes that triggered this recent mass loss acceleration.

Fig. 3: Surface mass balance of Hans Tausen ice cap and surrounding small ice bodies in northern Greenland before (a) and after the tipping point in 1997 (b). Since 1997, the accumulation zone (blue) has shrunk and the ablation zone (red) has grown further inland, tripling the pre-1997 mass loss [Credit: Brice Noël].

 

Greenland ice caps are located in relatively dry regions where summer melt (ME) nominally exceeds winter snowfall (PR). To sustain the ice caps, refreezing of meltwater (RF) in the snow is therefore a key process. The snow acts as a “sponge” that buffers a large amount of meltwater which refreezes in winter. The remaining meltwater runs off to the ocean (RU) and contributes to mass loss (Fig. 4a).

Before 1997, the snow in the interior of these ice caps could compensate for additional melt by refreezing more meltwater. In 1997, following decades of increased melt, the snow became saturated with refrozen meltwater, so that any additional summer melt was forced to run off to the ocean (Fig. 4b), tripling the mass loss.

Fig. 4: Surface processes on an ice cap: the ice cap gains mass from precipitation (PR), in the form of rain and snow. a) In healthy conditions (e.g. before 1997), meltwater (ME) is partially refrozen (RF) inside the snow layer and the remainder runs off (RU) to the ocean. The mass of the ice cap is constant when the amount of precipitation equals the amount of meltwater that runs off. b) When the firn layer is saturated with refrozen meltwater, additional meltwater can no longer be refrozen, causing all meltwater to run off to the ocean. In this case, the ice cap loses mass, because the amount of precipitation is smaller than the amount of meltwater that runs off [Credit: Brice Noël].

  In 1997, following decades of increased melt, the snow became saturated with refrozen meltwater, so that any additional summer melt was forced to run off to the ocean, tripling the mass loss.

We call this a “tipping point” as it would take decades to regrow a new, healthy snow layer over these ice caps that could buffer enough summer meltwater again. In a warmer climate, rainfall will increase at the expense of snowfall, further hampering the formation of a new snow cover. In the absence of refreezing, these ice caps will undergo irreversible mass loss.

What about the Greenland ice sheet?

For now, the big Greenland ice sheet is still safe as snow in the extensive inland accumulation zone still buffers most of the summer melt (Fig. 1). At the current rate of mass loss (~300 Gt per year), it would still take 10,000 years to melt the ice sheet completely (van den Broeke et al., 2016). However, the tipping point reached for the peripheral ice caps must be regarded as an alarm-signal for the Greenland ice sheet in the near future, if temperatures continue to increase.

Data availability

The daily, 1 km Surface Mass Balance product (1958-2015) is available on request without conditions for the Greenland ice sheet, the peripheral ice caps and the Canadian Arctic Archipelago.

Further reading

Edited by Sophie Berger


Brice Noël is a PhD Student at IMAU (Institute for Marine and Atmospheric Research at Utrecht University), Netherlands. He simulates the climate of the Arctic region, including the ice masses of Greenland, Svalbard, Iceland and the Canadian Arctic, using the regional climate model RACMO2. His main focus is to identify snow/ice processes affecting the surface mass balance of these ice-covered regions. He tweets as: @BricepyNoel Contact Email: b.p.y.noel@uu.nl

An Antarctic Road Trip

An Antarctic Road Trip

Working in the Arctic and Antarctic presents its own challenges. It is perhaps easy to imagine how a station situated close to the coast is resupplied: during the summer, one or more ships will arrive bringing fuel, food and equipment, but what about stations inland? Flying in supplies by aircraft is expensive and, in the case of large quantities of fuel, unsustainable. Besides, many stations are closed during the winter season, so there is nowhere for a plane to land until the skiway has been reestablished. The answer is of course that you drive. In other words, you go on a polar road trip, and one such road trip is the traverse that starts every year from the German Neumayer III station. The route is almost 800km long and it typically takes the traverse team 10 days to make their way across the East Antarctic Ice Sheet to their goal: Kohnen station at 75 degrees S, 0 degrees W and 2.9km altitude.

This year I got the chance to join the traverse and do a bit of science along the way with my colleague Anna Winter. Read below for a riveting tale of hardships, drilling and bamboo poles!

Map of our traverse route starting at the Neumayer III station on the coast. Credit: Anna Winter.

Who is holding up the traverse?

If you were to look at the traverse from above, you would see six large “Pisten bullies” pulling several sledges, each leaving a track across the ice sheet. However, you would also see two people on a tiny vehicle; a skidoo with two small sledges. Some times the skidoo will be in front of the traverse train, but often it will be trailing behind, and you would definitely notice that the people on the skidoo are stopping frequently. The two people are Anna and myself. We had set out to investigate how much snow is falling in this part of the Antarctic, and to do this we used a range of equipment from highly advanced radar instruments to bamboo sticks and a measurement tape.

Drilling into the past

The Antarctic ice sheet has a long memory. When snow falls, the old snow is buried, so when you drill down into the surface you go back in time and can look into the past. This is how we know what the climate was like in the past. Drilling an ice core all the way to the bottom of Antarctica takes a very long time: often 3 – 5 years or more, but since we want to know something about the very recent changes, we do not have to drill very far.

Drilling a firn core requires patience, focus and sturdy gloves. Credit: Anna Winter.

Drilling a firn core requires patience, focus and sturdy gloves. Credit: Anna Winter.

On the 31st of January the traverse stopped a bit earlier than usual, and while the drivers tended to the vehicles and the cook prepared the New Years Eve dinner, We started drilling a firn core (firn is old snow that is not ice yet) with the help of Alexander and Torsten. In order to drill a firn core,  you need a drill that can capture the firn inside, a small engine for powering the drill and several extensions so you can go as deep as you like (see photo). It is not an easy process and many things can go wrong. For example, it should not be too warm when you drill. A few metres into the snow the temperature is no longer the same as the air, but instead it is the average annual temperature. Since we are drilling in the summer time this means that the firn we retrieve will be maybe 20 degrees colder than the temperature at the surface. When the drill comes up the metal gets warm and the core will get stuck inside the drill. A real nightmare! This is also the reason why we drilled during the evening even if that cut our New Years Eve celebrations short. Fortunately, we did manage to get a break and enjoyed a delicious New Years Eve meal, before finishing the drilling ten minutes before midnight. We celebrated the success of the drilling and the New Year with a whisky, before the cores were packed in boxes so they can be shipped to Germany for more analyses at the Alfred Wegener Institute.

Measuring a the height of a bamboo pole includes high-technology equipment, namely, another bamboo pole with peanut-can and a measurer tape stuck to it. Credit: Nanna B. Karlsson.

Measuring a the height of a bamboo pole includes high-technology equipment, namely, another bamboo pole with peanut-can and a measurer tape stuck to it. Credit: Nanna B. Karlsson.

The endless row of bamboos

So, how do the bamboo poles fit in the picture? The firn core tells us a lot about the snowfall in the place where it was drilled, but we also want to know what is happening along the route of the traverse, and what is happening right now. Therefore, last year, bamboo poles were set up every 1km along the first part of the traverse. Our task was to increase the number of bamboo poles to one pole every 500m. We also measured the height of the old poles, and compared it to their original height. The further we got from the coast, the taller the bamboo poles were. This is what we expected since we know that very little snow falls in these parts of Antarctica, maybe less than half a metre a year! From our measurements, we now know directly how much snow has fallen since last year. Next year, other people will measure the height of the old bamboo poles and the new ones we put up, and we will know even more about the snowfall. It is a laborious and hard process: the traverse route is almost 800km so it is almost an endless row of bamboo poles. If only they could be seen from space they would make an impressive sight.

This blog post was originally brought on the website of the Alfred Wegener Institute in German. You can see more photos and read the originals here and here.

Tea break with Kottas Mountains in the background. For once we were ahead of the rest of the traverse. Credit: Anna Winter.

Tea break with Kottas Mountains in the background. For once we were ahead of the rest of the traverse. Credit: Anna Winter.

(Edited by Sophie Berger and Emma Smith)