Glacier mass balance | Measuring mass balance | Mass balance gradients | Mass balance through time | Further reading | References | Comments |
Glacier mass balance
Glacier mass balance and atmospheric circulation. By NASA. From Wikimedia Commons.
The mass balance of a glacier is a concept critical to all theories of glacier flow and behaviour. It is simple enough, really: mass balance is simply the gain and loss of ice from the glacier system1. A glacier is the product of how much mass it receives and how much it loses by melting. It can be thought of as the ‘health of a glacier’; glaciers losing more mass than they receive will be in negative mass balance and so will recede. Glaciers gaining more mass than they lose will be in positive mass balance and will advance. Glaciers gaining and losing approximately the same amount of snow and ice are thought of as ‘in equilibrium’, and will neither advance nor recede. For clarification: when we talk about glaciers advancing, receding or being in equilibrium, we are talking about the position of their snout. Glaciers will continually flow under the force of gravity; ice is continually being moved from the upper reaches to the lower reaches, where it melts.
Unnamed Glacier, Ulu Peninsula, James Ross Island. The accumulation zone for this glacier extends from the plateau downwards.
The glacier system receives snow and ice through processes of accumulation. Surface accumulation processes include snow and ice from direct precipitation, avalanches and windblown snow. There may be minor inputs from hoar frost. The snow and ice is then transferred downslope as the glacier flows. Precipitation falling as rain is usually considered to be lost to the system. Internal accumulation may include rain and meltwater percolating through the snowpack and then refreezing. Basal accumulation may include freezing on of liquid water at the base of the glacier or ice sheet2. The snowline is often used to demarcate the equilibrium line on satellite images of glaciers.
Meltwater stream on Mendenhall Glacier, Alaska. From: Gillfoto, Wikimedia Commons
Glaciers lose mass through processes of ablation. Surface ablation processes include surface melt, surface meltwater runoff, sublimation, avalanching and windblown snow. Glaciers on steep slopes may also dry calve, dropping large chunks of ice onto unwary tourists below. Glaciers terminating in the sea or a lake will calve photogenic icebergs. Other processes of ablation include subaqueous melting, and melting within the ice and at the ice bed, which flows towards the terminus2.
Equilibrium line altitude
Accumulation usually occurs over the entire glacier, but may change with altitude. Warmer air temperatures at lower elevations may also result in more precipitation falling as rain. The zone where there is net accumulation (where there is more mass gained than lost) is the accumulation zone. The part of the glacier that has more ablation than accumulation is the ablation zone. Where ablation is equal to accumulation is the Equilibrium line altitude.
Equilibrium line altitudes in a hypothetical glacier
So what is Glacier Mass Balance?
So, glacier mass balance is the quantitative expression of a glacier’s volumetric changes through time.In the figure below, Panel A shows how temperature varies with altitude. It is colder at the top than it is at the bottom of the glacier. This is crucial, as surface air temperature strongly controls melting and accumulation (as in, how much precipitation falls as snow or ice).
Mass balance (b) is the product of accumulation (c) plus ablation (a). Mass balance (b) = c + a Mass balance is usually given in metres water equivalent (m w.e.). It varies over time and space; accumulation is greater in the higher reaches of the glacier, and ablation is greater in the lower, warmer reaches of the glacier (Panel B in the figure).
Mass balance also varies throughout the year; glaciers typically get more accumulation in the winter and more ablation in the summer (Panel C in the figure). Glacier mass balance therefore usually can therefore be expressed as a mass balance gradient curve, showing how c + a varies attitudinally across the glacier (Panel D in the figure). The balance gradient is the rate of change of net balance with altitude3. A glacier’s net mass balance is a single figure that describes volumetric change across the entire glacier across the full balance year.
Principles of glacier mass balance
Measuring Mass Balance
Jonathan Carrivick prepares to stake out Glacier IJR45 on James Ross Island.
Glacier mass balance is normally measured by staking out a glacier. A grid of ‘ablation stakes’ are laid out across a glacier and are accurately measured. They can be made of wood, plastic, or even bamboo like you’d use in your garden. These stakes provide point measurements at the glacier surface, providing rates of accumulation and ablation. These methods are time consuming, logistically challenging and arduous; the stakes will need to be visited several times through the balance year. Accumulations and ablation are generally measured by reference to stakes inserted to a known depth into the glacier, and fixed by freezing and packing in3. The location is fixed with GPS. Automatic weather stations on the glacier surface are key to understanding energy fluxes on the glacier. Probing, snowpits and crevasse stratigraphy are also used to measure mass balance on glaciers, ideally supplemented with stakes. Remote sensing of glacier mass balance is obviously a good alternative, as it allows many glaciers to be assessed using desk-based studies. It is a cheap and simple alternative to arduous fieldwork, but ground truthing of mass balance measurements will always be necessary. Researchers from Aberystwyth University use satellite measurements to track changes in the mass balance of the Greenland Ice Sheet.
Mass balance gradients
Mass balance gradients of some typical glaciers.
The mass balance gradient of a glacier is a key control in factors such as the glacier’s response time. A glacier’s mass balance gradient is critically determined by the climatic regime in which it sits; temperate glaciers at relatively low latitudes, such as Fox Glacier in New Zealand, may be sustained by very high precipitation. They will therefore have a greater mass balance gradient (more accumulation, more ablation). These wet, maritime glaciers may have a shorter response time and higher climate sensitivity than cold, polar glaciers that receive little accumulation but also have correspondingly low ablation. These cold, dry glaciers may respond more slowly to climate change.
In the figure on the left, temperate glaciers with greater mass balance gradients are represented by the shallower lines; more mass is transferred from the top to the bottom of the glacier. Cold, polar-type glaciers with smaller mass balance gradients are represented by the steeper lines.
Mass balance through time
The Cumulative mass balance is the mass of the glacier at a stated time, relative to its mass at some earlier time. Some glaciers have mass balance measurements going back decades, which means that scientists can analyse how mass balance is changing over time. These measurements give us detailed information about climate change, as glaciers are sensitive ‘barometers’ to our changing world. Usually, the net mass balance over the balance year is plotted on a graph. There are several projects monitoring glaciers all over the world, and these analyses show that glacier mass balance is generally decreasing (becoming more negative) over time.
In Europe, European Environment Agency has records of many glaciers, and makes their cumulative mass balance measurements publically available. The Glaciers (CLIM 007) analysis shows that the vast majority of European glaciers are receding, with the rate of recession accelerating since the 1980s.
The North American region shows a similar trend, with a generally declining mass balance each year.
North American glacier mass balance. Image courtesy of Mauri Pelto
Further afield, the IPCC AR4 shows cumulative specific net mass balance of glacierised regions worldwide. The differing behaviours of different regions shows the variable strength of climate change.
Cumulative mean specific mass balances (a) and cumulative total mass balances (b) of glaciers and ice caps, calculated for large regions (IPCC AR4)
How glaciers flow:
Also of interest:
1. Benn, D.I. &Evans, D.J.A. Glaciers & Glaciation. London: Hodder Education. 802 (2010).
2. Cogley, J.G., Hock, R., Rasmussen, B., Arendt, A., Bauder, A., Braithwaite, R.J., Jansson, P., Kaser, G., Moller, M., Nicholson, L., & Zemp, M. Glossary of Glacier Mass Balance and related terms. Paris: IHP-VII Technical Documents in Hydrology No. 86, IACS Contribution No. 2, UNESCO-IHP. 124 (2011).
3. Hubbard, B. &Glasser, N.F. Field Techniques in Glaciology and Geomorphology. Wiley. 412 (2005).
Two established techniques for correcting the root cause of the heart rhythm disorder atrial fibrillation show similar effects and safety outcomes, according to research presented at the American College of Cardiology's 65th Annual Scientific Session.
The study, called FIRE AND ICE, is the largest randomized trial to compare radiofrequency and cryoballoon ablation, two techniques designed to treat atrial fibrillation by disabling small portions of the heart that generate out-of-sync electrical signals. Radiofrequency ablation uses heat energy to disable the targeted heart tissue, while the cryoballoon, a newer technique, uses extreme cold to achieve the same effect. The trial revealed no differences between the two techniques for the study's primary outcomes--the recurrence of an irregular heart rhythm or the need for medication or subsequent procedures to address atrial fibrillation. It was funded in part by Medtronic, which makes the cryoballoon device.
"The FIRE AND ICE trial demonstrated that the cryoballoon, a newer, easier-to-use ablation catheter, worked as well as the established technology, which ultimately means that more patients can be treated for atrial fibrillation without having [to go to a] specialized cardiac center," said Karl-Heinz Kuck, M.D., Ph.D., head of cardiology at St. Georg Hospital in Hamburg, Germany, and the study's lead author. "In addition, there was, in general, a low risk of procedural complications in both groups, demonstrating that catheter ablation has become much safer over the years."
Atrial fibrillation, estimated to affect more than 33 million people worldwide, is an irregular heart rhythm that can cause fatigue, lightheadedness, shortness of breath, chest pain and an increased risk for stroke. Although medications and lifestyle changes can help manage the condition's risk factors and symptoms, about 30 percent of patients do not benefit from available medications or cannot take them due to side effects or other reasons. Ablation is one option for treating these patients. During ablation, a physician threads a small medical device through a vein in the groin to kill a small number of cells around the heart's pulmonary veins, preventing them from issuing electrical signals that are out of sync with the rest of the heart.
The trial, conducted in eight European countries, enrolled 769 patients needing ablation for intermittent atrial fibrillation. Patients were randomly assigned to receive either the radiofrequency or cryoballoon technique; both patients and physicians were aware of which technique was being used. The two groups were similar in terms of demographic factors, such as age and gender, as well as health status, based on parameters such as body mass index, blood pressure and various measures of heart function.
In addition to using different methods for disabling the target heart tissue, the two techniques involved different procedures to help the physician locate the target tissue. For radiofrequency procedures, physicians were guided by 3-D electroanatomical mapping to create tissue lesions in a point-by-point ablation approach. For cryoballoon procedures, physicians used a type of X-ray imaging known as fluoroscopy to create tissue lesions in a single-step ablation approach.
Outcomes were assessed through in-person patient visits conducted three months after the procedure, six months after the procedure and every six months thereafter. Each visit included an electrocardiogram test to assess heart rhythm and function, as well as the use of a Holter monitor, in which the patient wears a monitor for 24 hours to check for any abnormal heart rhythm. Patients were tracked for just over 18 months, on average.
The results revealed no significant difference in the rates of recurrence of an irregular heart rhythm or the need for medication or subsequent procedures to address atrial fibrillation, outcomes that collectively occurred in 64.1 percent of patients receiving radiofrequency ablation and 65.4 percent of cryoballoon patients within 12 months after the procedure.
There were also no significant differences in the overall safety profile of the two techniques. Safety was assessed with a composite endpoint of death, stroke and procedure-related serious adverse events; 87.2 percent of patients receiving radiofrequency ablation and 89.8 percent of cryoballoon patients had not experienced any of these safety endpoints by 12 months after the procedure.
In both groups, there was generally a low rate of procedure-related complications such as infection, dangerous heart rhythms or accumulation of fluid in the heart. However, patients receiving cryoballoon ablation were significantly more likely to experience injury to the phrenic nerve, which can affect the functioning of the diaphragm and require patients to use an artificial ventilator. Such injuries occurred in 2.7 percent of cryoballoon patients and zero patients receiving radiofrequency ablation. In all but one of these cases, functioning was restored by 12 months post-operation.
The study revealed some significant procedural differences between the two techniques. Because it involved 3-D anatomical mapping, radiofrequency ablation required about five minutes less fluoroscopy time and, thus, exposed patients and physicians to radiation for a shorter period of time, though Kuck said that the overall usage of fluoroscopy was relatively limited in both groups, at 21.7 minutes and 16.6 minutes total on average for the cryoballoon and radiofrequency procedures, respectively. Cryoablation was associated with a shorter overall procedure time by 18 minutes per procedure, on average, and a similarly reduced amount of time in which the catheter was present inside the heart's left atrium while the ablation was carried out.
"The procedure time was interesting because there are more cost pressures on the healthcare system for more efficient tools that keep procedures short and predictable," Kuck said.
Kuck said the findings could help inform future medical guidelines on the use of different catheter ablation techniques for treating atrial fibrillation. One limitation of the study is that it did not investigate ablation for treating patients with more advanced stages of atrial fibrillation. A separate trial would be needed to assess the ablation techniques' effectiveness and safety for that patient population, he said.
Materials provided by American College of Cardiology. Note: Content may be edited for style and length.
Cite This Page:
American College of Cardiology. "Two atrial fibrillation ablation techniques equal on efficacy and safety: No significant differences in outcomes from radiofrequency versus cryoballoon ablation." ScienceDaily. ScienceDaily, 4 April 2016. <www.sciencedaily.com/releases/2016/04/160404181002.htm>.
American College of Cardiology. (2016, April 4). Two atrial fibrillation ablation techniques equal on efficacy and safety: No significant differences in outcomes from radiofrequency versus cryoballoon ablation. ScienceDaily. Retrieved March 7, 2018 from www.sciencedaily.com/releases/2016/04/160404181002.htm
American College of Cardiology. "Two atrial fibrillation ablation techniques equal on efficacy and safety: No significant differences in outcomes from radiofrequency versus cryoballoon ablation." ScienceDaily. www.sciencedaily.com/releases/2016/04/160404181002.htm (accessed March 7, 2018).