What's in the news?

A news article on the Nature website posted on the 22^nd March 2011 looks at the people living alongside the Colonia River, in the Aysen region of Chilean Patagonia. Their significance is the threat they live under of sudden glacial lake outbursts from the above glacial mountains.

Whilst these lake outbursts are not uncommon globally (see this previous blog post), the region has experienced seven of these events since April 2008. Evidently, this is a much higher rate than seen anywhere else. During each of these seven outbursts, Lake Cachet 2 (of the Colonia glacier), has drained approximately 200 million m³ of water into the Colonia Lake and River in only a few hours. This has created a wave observed as far as 100 km downstream to the Pacific Ocean. The risks to the people of the Colonia River need no further explanation.

The Nature article uses the findings of a paper by Casassa et al. (2010), which has studied the floods to identify a primary cause.  The three year study ultimately found that the main cause was:

“...the repeated opening and closing of a tunnel 8 kilometres beneath the Colonia glacier, connecting Lake Cachet 2 above the glacier and Colonia Lake below it.”

Of course, this alone does not reveal the entire process of glacial lake outburst from the Colonia glacier. As a result, Casassa et al. (2010) found that climate change was the main culprit behind the frequency of these events. This happens as a result of the shrinking and thinning of the glacier in the past few decades, causing a weakening of the natural dam structure that the glacier forms. Thus the water moves between the two lakes with much greater ease. There seems little that can be done about this at present, although the paper offered some hope for the future:

“The researchers conclude that the discharges will continue until the ice has receded or thinned sufficiently to generate a permanent natural drainage channel.”

Figure 1. Ice-laden lake leading up to the Colonia glacier

Questionable impacts of glacier retreat on regional water security: dealing with uncertainties

Following on from the previous blog post, a paper by Archer et al. (2010) looks quite broadly at how the sustainability of water resources in the Indus basin might be altered by future changes in climate, as well as changes in socio-economic conditions. As was noted by the Indus water commissioner in the previous post, climate change effects on glacier shrinkage were suggested to be the primary cause of water scarcity. However, in this paper by Archer et al. it is argued that the impact of glacier retreat may be limited, especially when compared with other physical and socio-economic factors downstream of the Himalayan glaciers.

Agriculture in Pakistan is very much reliant on water which originates in the mountain sources of the upper Indus. As the authors note, these water resources are already highly stressed and are likely to get worse with projected rises in population. The paper considers the impact of climate change on these water resources in terms of three distinct hydrological regimes: a nival regime (dependent on melting winter snow), a glacial regime, and a rainfall regime. The mountainous sources of water are known to be affected by changes in temperature and in precipitation. The authors note that this is due to most of the runoff being derived from the melting of seasonal glacier snow and ice. Thus, any ablation of glaciers could quite easily affect water scarcity downstream.

Figure 1. The Indus basin

However, as the paper importantly notes, there is a great deal of uncertainty regarding how climate change might affect glaciers and river flow in the region. It cites several studies, which show regional conflicts with global patterns. Firstly, that summer temperatures (key for glacial melt) have actually fallen in the Karakoram between 1961 and 2000. Secondly, similar falls in temperature were found for the monsoon and pre-monsoon periods (April to May) in the Karakoram. Thirdly, that there have been significant increases in Upper Indus precipitation (both winter and summer) between 1961 and 1999. Fourthly, that extensive glacial mass balance records do not show shrinking glaciers. And finally, that in the late 1990s, there was widespread evidence of glacier expansion in the Karakoram.

With such conflicting evidence in the upper region of the Indus basin, it is unknown whether climate change will have a positive or negative effect on water resources in the region. The authors state that the hypothesis of reduced water resources relies on two assumptions: firstly that temperature and glacier melt are the primary impact on water resources, and secondly, that temperatures in the Upper Indus will rise in line with global climate predictions. As a result of this, they state that both of these assumptions are questionable. In particular, it is stated that river flow has been shown to not depend uniquely on glacier melt, but also rely on seasonal snowmelt and rainfall. This thus gives us the three hydrological regimes: nival, glacial and rainfall.

In the nival regime, the area of seasonal snow melt gives the largest contribution to downstream flow. This comes largely because the area of seasonal snow is much bigger than the perennial snow and ice. Of course, however, the area greatly reduces during the melt season. For this regime, winter precipitation has been shown likely to have the most significant impact on summer runoff. And unlike the glacial regime, there is a significant negative relationship between runoff and temperature on nival regimes. Archer et al. state that this:

“Can be explained by greater evaporative losses from the snow cover under higher temperatures and thus reduced runoff.”

As a result, the authors estimate that for a 2^oC rise in summer temperature, there would be an 18 percent reduction in runoff. However, the observed Karakoram decline in summer temperature would produce increased summer runoff.

In the glacial regime, the contribution to flow in the very high catchments is significant. However, the combined flow of these high catchments into the Indus represents an average of less than 30 percent. Here, there is a significant positive correlation between summer runoff and temperature. As well as this, winter precipitation doesn’t have such an influence. As such, spring and summer temperatures have the greatest impact on runoff. In this regime, runoff will rise initially with increased global temperature, but reduce sharply with declining glacier mass.

However, the findings of falling summer temperatures in the region mean that with this positive correlation between runoff and summer temperature, there is presently a downward trend in flow. The present and past behaviours of the Karakoram glaciers are noted in the paper:

“...glacier recessions were observed in almost all Karakoram glaciers for most of the 20th century until the mid-1990s. However, at lower elevations glaciers continued to decline. This seems to confirm that glacier loss is reduced in the Karakoram compared both with the neighbouring Himalaya and the Pamir mountains to the west.”

Finally, in the monsoon rainfall regime, the main influence falls over the southern plains and foothills of the Himalaya. Here, seasonal volume of runoff (as a result of rainfall) is lower than in the glacial and nival regimes. However, the monsoon rainfall produces more intense runoff and therefore highest flooding in the region. Therefore, this regime can be very important for water resources in the Indus basin. Yet as the paper states, the IPCC indicate that estimates of precipitation change hold great uncertainty, and that the impact of climate change on monsoon precipitation cannot be safely assumed.

In summary, based on the three regimes identified for the upper Indus basin, the paper shows that there appears to be little evidence for reductions in runoff and thus availability of water resources in the region. It must also be accepted, however, that much of this relies on uncertainty over glacier response to climate change in the Karakoram. Indeed, more recent past climate may not be a reliable guide to future change as well.

Whilst not the focus of this blog, it is important to mention briefly, the overall findings of the paper when physical and socio-economic conditions downstream are considered. Whilst the authors found inconclusive evidence of climate change causing reductions in runoff, they did find that several other factors had much greater influence on water resources further down in the Indus basin. Firstly, that urban growth and industrialisation will increase and demand further shares of the scarce water resources. Thus, economics plays an important role in water resource management, as large investment is needed to provide practical solutions whilst balancing the needs of security, health and education. Secondly, reservoir sedimentation means that water resources will diminish as storage is taken up by sediment. This problem will not reduce unless new reservoirs are built. Finally, the alternative of using groundwater in the spring for agriculture may soon have no practical use when the water tables fall from over pumping.

As can be seen, the issue of water scarcity in Pakistan’s Indus basin is very complex. The presence of and future changes to glaciers upstream is a very important part of the highly stressed water scarcity issues here. Only with greater knowledge of how these regional glaciers will react to global climate change, using past records and future modelling, will a truly certain answer be able to be given for these problems.

Changing glaciers and regional security

Focussing here on the human impacts of potential glacier loss, a 2009 Bloomberg article assesses the security risks associated with water scarcity, through looking at a report by the Asia Society. The article begins by stating that the water supplies of China and India are predicted to decline alongside the shrinking of Himalayan glaciers as a result of global climate change. This in turn is likely to begin or exacerbate regional conflicts.

This conflict, the author states, could come as a result of several factors. Asia contains half of the global population, yet has least water of any continent (with the superfluous exception of Antarctica). There exist problems of waterborne diseases throughout Asia, and these could certainly be amplified by water scarcity. As well as this, lack of fresh water could trigger mass migration and invigorate cross-border conflicts over water control. Evidently, there exist many potential sparks of conflict.

As well as this, the threat to agricultural production in the region is very real. Increased amplitude of dry and wet seasons caused by the influence of climate change on regional atmospheric patterns may overwhelm and destroy a proportion of crops. As well as this, any change in Asian crop yields (particularly China or India) would most certainly affect world food prices.

There is a particular problem in Pakistan, with approximately 77 percent of its water resources coming from outside of its borders. As a result of this situation, Pakistan holds a water treaty (since 1947) with India which guarantees sufficient supply. Of course, since that time, there have been several territorial and other disputes between the two countries. Most recently, Pakistan-based terrorists launched an attack on Mumbai in 2008. This further tension caused those in Pakistan to criticise the failures of India to always adhere to the treaty. Underlying all of this, they suggested, lay the long standing violent disagreement over the Kashmir region. These alleged abuses of the treaty in the past are particularly important when the population figures of 180 million presently (a tripling since 1950) and of a predicted 335 million by 2050 are seen.

The article then looks, albeit briefly, at the rapid melting of some Himalayan glaciers as a consequence of climate change and attempts to link this with the security issues. The author states that:

   

“Melting Himalayan glaciers now account for up to 70 percent of the summer flow of the Ganges River and about 55 percent of Asia’s other major river systems, according to the report. In 30 years, as the glaciers continue to retreat, the Indus and Mekong rivers could be dry during part of the year, the report said.”

Therefore, with the regional effects of climate change on temperature and a seemingly large degree of uncertainty regarding how the Himalayan glaciers will respond (unlikely uniformly), there is a great deal of potential danger associated with glacier retreat and water scarcity in the region.

In a separate article shown on the Policy Research Group website, much the same conclusions are found regarding the Pakistan example. The author, using World Bank estimates, shows that:

“... Pakistan’s Indus River will be negatively impacted by climate change, with incidents of flooding in the Indus basin expected to increase over the next 50 years. In addition, estimates suggest that there will be a 30-40% reduction in river flow over the next 100 years due to natural climatic and environmental changes.”

The article then notes that the Pakistani government, and more specifically the Indus water commissioner, feel the issue has received insufficient media coverage when compared to the potential damage that could take place. And looking at western media outlets, it would be hard to disagree with this assessment. Finally, the Indus water commissioner states belief in climate change as the primary driver of Pakistan’s water scarcity problems both now and in the future. However, as will be discussed in a following blog post, this is certainly not the belief of all scientists. Indeed, several have stated the exact opposite of this opinion.

Figure 1. View of the Indus River, fed from the glacial Himalayas

With regards to this last opinion of the Indus water commissioner; according to a weblog article by the Centre for Strategic and International Studies (CSIS) several officials in India believe that it is governmental mismanagement of water resources - and not climate change - that are responsible for water scarcity. Indeed, the article goes into slightly greater depth over the risk of conflict over water between Pakistan and India. It highlights the frustration of many of the rural poor in Pakistan regarding the observed extended dry spells, and a subsequent temptation to blame India for these droughts. Despite this desperation, there seems to be no evidence of this practice (according to the Indus water commissioner). Whilst cooperation on the water treaty has survived many of the previous disputes between the two countries, several militant Islamic groups have been seen to use this regional frustration for self gain. This threat of water cut-off by India is used by the militants to garner support for their wider agenda as well, which is in all probability aimed at destabilising the region.

It can be seen that water scarcity and regional security are very closely interlinked. Thus, with regional uncertainty of how glaciers may react to climate change, it is of wide ranging importance that greater certainty is achieved. Whilst this blog has looked more at the physical characteristics of past and future glacier change, it has been shown here that there is a inextricably linked human dimension to our knowledge of future glacier dynamics.

Black soot on the Tibetan Plateau

A paper by Xu et al. (2009) examines the impact of black carbon (soot) on the Tibetan Plateau - which contains the largest ice mass outside of the polar regions – and how this affects their survival for the future.

As has often been examined in this blog, the human as well as physical impacts of any changes to the region can be plainly seen. Most importantly, the authors offer clear explanation of the regional significance of water originating from the glaciers of the Tibetan Plateau. Firstly, the meltwaters are key in supplying the Indus, Ganges and Brahmaputra river systems, amongst others. Secondly, this freshwater source sustains over 1 billion people. Thirdly, for 25 percent of the people in western China, the meltwater from the Tibetan Plateau provides the main dry season water source (December – March). It is hard to envisage any clearer evidence of the reliance that so many people have on mountain glaciers.

Indeed, with the loss of headwater glaciers (supplying the source of the rivers), there will inevitably be a rapid decline in the availability of meltwater during the dry season. The increased melting of these glaciers is instead likely to provide spring time flooding. This then brings us onto what is causing this melting. The answer remains mostly obvious, in that glacier retreat is driven primarily by increased air temperature, on which much research exists. However, as Xu et al. (2009) note; areas of over 4000m elevation have warmed by 0.3°C per decade over the last 30 years, which is twice the rate of observed global warming. This high rate of temperature increase and subsequent rapidity of glacial retreat suggests that “additional mechanisms may be involved”. This of course then brings us onto the influence of black carbon on rates of glacier melt.

Whilst black carbon has an atmospheric impact (of warming the troposphere), the focus of this paper remained on the incorporation of black carbon onto snow and ice and the effect that this has for surface melt and albedo. When black soot forms on snow and ice surfaces, it serves to darken them, which as a result increases solar absorption and increases surface melting. As for the input pathways of black carbon, two factors are seen to bring it to the glaciers of the Tibetan Plateau. The first of these is the positioning of the plateau close to SE Asian industry. As the authors state, the region is presently and predicted to continue as the largest source of black carbon globally. Secondly, this positioning allows for the black soot to be carried upwards by winds and attached to snowflakes, prior to their deposition on top of the glacier surfaces. Thus, the glacier surface is darkened.

The study was conducted by taking ice cores from five key locations across the plateau. The aim of this was to establish temporal changes in the amount of black carbon since the 1950s. As well as this, the sources of snowfall for each part of the Plateau were established. For the N and NW parts:

“... associated mainly with the westerly jet stream, which moves southward toward the Himalayas in winter. Thus, black soot deposited on Himalayan glaciers derives primarily from two directions: west and south... so its upwind sources are principally Europe and the Middle East.”

For the S parts of the plateau:

“... [they] receive deposits from the west in winter and from the south in summer.”

Finally, the largest source of snowfall for the entire plateau is shown to be the Indian monsoon, which can move as far as 30-32°N in the summer.

Evidence of these soot and snowfall sources can be seen in the ice core records, which saw decreasing carbon in the NW and central regions of the plateau during the 1970s and 1980s. Such a reduction wasn’t seen in other areas, and reflects the successful implementation of new environmental regulations in Europe at the time. As well as this, the ice core records showed that carbon concentrations have increased in the southern Tibetan Plateau since 1990. This north/south plateau contrast shows both a difference in global attitudes to the emission of black carbon as well as showing the effect of different transport lengths (flow from Europe more easily disrupted/dissipated).

Figure 1. Black and Organic Carbon levels of the 5 ice cores. With annual and 5-year running averages.

The paper showed that the observed amount of black carbon was enough to influence the surface reflectivity of the glaciers. This comes together with the increased industrial activity, rapidly increased soot deposition from the 1990s and the accelerating rate of glacier retreat in the last 30 years. The study also showed that black carbon concentrations of 10 ng g^-1 reduce the visual albedo of thick snow to 0.01 – 0.04. This means that absorption of visible radiation is increased by 10 – 100 percent (dependent on snowflake type and the extent of soot mixing with snow). As well as this, the arrival of soot in spring allows for the melt season to begin earlier. Indeed the deposition of black carbon peaks between November and March, when the snow is at its maximum extent. More worrying, is the presence of black carbon in the accumulation zone of glaciers (where natural snow melt isn’t seen to any great extent). As well as all of these factors, the study has found that the melting process in the ablation zone can serve to increase the concentration of black carbon (and therefore intensifying its effects).

The authors also reference several studies which attempt to model the impacts of black soot on the regional climate of glaciated areas. (Hess et al., 1998) (Hansen & Nazarenko, 2004) (Flanner et al., 2007) (Shindell & Faluvegi, 2009). As Xu et al. note, the findings of these initial studies are very significant:

“These studies suggest that black soot is responsible for a substantial fraction of the regional warming of the past century, comparable to the fraction attributable to carbon dioxide. Assessment of black soot’s impact on glaciers will need to include the contribution that black soot makes to regional climate change, as well as direct effects on the glacier.”

Evidently, the influence of black soot on glacier change has been shown to be quite significant. If loss of the Tibetan Plateau as a fresh water source is to be prevented, then policy aimed at reducing SE Asian black carbon emissions is a necessity. It has been shown not only to directly alter surface albedo, but also to significantly affect regional climate as well. The authors state finally that if black soot levels are reduced and CO₂ levels are stabilised (somewhat unknowingly), then there remains a possibility that Tibetan glaciers may remain beyond this century. It is perhaps better put by stating that the future of the glaciated Tibetan Plateau remains very uncertain indeed.

Further reading: Whilst not looked at directly in this blog, this paper by Kopacz et al. (2011) takes a more technical look at the sources to the Tibetan Plateau and Himalayas, by modelling the global transport and radiative forcing of black carbon.

In the news: This topic was also covered in an article from The Guardian (2009), which looks at black soot originating from developing countries, impacting across the Tibetan Plateau and Himalayas.

What's in the news?

From looking at the changes in subglacial drainage of outlet glaciers before, this news article from August 2010 shows the efforts of French engineers to remove the subglacial lake which had formed from meltwater as a result of increased warming in recent years. The water, which lies under the Tete-Rousse glacier, threatened the safety of people in the underlying Saint Gervais valley with flooding from a sudden outburst event.

Indeed, the danger of this kind of event has been witnessed before. In 1892, 175 people were killed in the St Gervais valley by flooding from a subglacial lake. This occurred on the same glacier, and saw the collapse of the ice wall which had previously held in the subglacial lake. The origins of this original outburst can be seen in the paper by (Vincent et al. 2010). Today, several thousand tourists visit and stay within the valley each year, perhaps justifying the intervention seen here.

Figure 1. Large cavity from 19th century outburst

Therefore, to apply this pre-emptive measure, the engineers aimed to pump the water away by drilling through the ice and thus accessing the 2.3m³ ft subglacial lake. The process of draining took several months and aimed to reduce subglacial pressure by removing one third of the water present.

Figure 2. Drainage hole drilled into the glacier

The level of stress seen under the glacier was proposed to be as a result of both increased temperature causing greater amounts of meltwater, and by a brief anomalous cold period freezing the natural drainage routes for this meltwater.

Melting in the mountains...

Keeping with the topic of ice melt and global sea level rise; a study by Radic & Hock (2011) looks to determine the potential contribution of mountain glaciers to sea level rise. The authors modelled the predicted mass loss of mountain glaciers globally, and from that calculated the impact that this melting could make.

The study, which looked at 40 percent of the world mountain glacier total, was the most comprehensive to date. Unlike the previous Intergovernmental Panel on Climate Change (IPCC) study which conducted the same modelling on much fewer sample sites, Radic & Hock (2011) based their study on 120,299 mountain glacier and 1,638 ice caps. Despite these differences however, the prediction that the majority of small mountain glaciers will have disappeared by the year 2100 is shared by both this study and that done by the IPCC. The paper also shows that by 2100, the melting of mountain glaciers will contribute an equivalent amount to global sea level as the Greenland and Antarctic ice sheets. Evidently then, the concern over rising global sea levels should not just be confined to the large polar ice sheets, when the potential future impact of smaller glacier melting is just as great.

Figure 1. Regional glacier volume predictions for the 21st Century.

Using ten different global climate models to predict the volume changes of the glaciers, the study found that approximately 50 percent of the glaciers with the smallest surface area (< 5 km) will have gone. The method of future volume prediction is described below:

“To quantify future volume changes, we run the calibrated mass balance model ...with downscaled monthly twenty-first-century temperature and precipitation from ten GCMs, based on the widely used mid-range greenhouse emission scenario A1B. As glaciers lose mass owing to temperature increase, they retreat and hence their hypsometry changes. We use volume-area-length scaling to account for these changes and their feedbacks to glacier mass balance ... allowing receding glaciers to approach a new equilibrium in a warming climate.”

Evidently, as discussed many times in this blog, the potential impacts on people around the world could be huge. Particularly in developing countries, the reliance on glacial meltwater for drinking water and irrigation in agriculture can be massive. Looking beyond these regional impacts of glacier shrinking or disappearance, the ten climate models predicted that on average, the small glaciers and ice caps will themselves raise the global sea level approximately 12 cm by 2100. Across all of the models, the contribution of the complete or partially melted glaciers to global sea level was shown to be in the range of 8.7 to 16 cm. As the authors noted:

“All projections for the twenty-first century show substantial mountain glacier and ice cap volume losses.”

Thus, knowing how these mountain glaciers might impact on us globally is important! As the paper finds, these small mountain glaciers are relatively large contributors to sea level rise. Yet as the author has stated, that is important as the small glaciers only represent 1 percent of the Greenland and Antarctic ice sheets, and therefore become easy to overlook.

Additionally worrying is the conclusion by the authors; that their predictions of sea level rise from glacial melt are likely to represent cautious estimates. This comes as a result of modelling only the surface mass balance, which is particularly important for the ice caps that were looked at in this study, as the ocean influence on melting could potentially be greater than surface melting. As seen in previous blog posts, this is particularly true for polar glaciers, where many come into contact with warm ocean waters.

Of course, there are several factors that weren’t consider, quite possibly due to limitations outside of the authors’ control. However, considerations of glacier depth through global measurements could provide a better indication of the differences between individual glaciers based on their thickness. And as discussed in the previous paper by Schaefer et al. (2009), no consideration was made of glacier debris, which could certainly have an effect on glacier melting up to the year 2100.

However, the authors did produce new findings which showed that the Himalayan glaciers (which are affected by debris cover) might potentially grow slightly by 2100. This again disproves the originally contentious IPCC statement that Himalayan glaciers could have disappeared by 2035. The results of this study showed that the ten climate models predicted between a 15 percent decline or possibly net growth by 2100. This slow shrinking or even growth could come as a result of increased snowfall in the future. Of course, the predictions elsewhere are more sobering; with a 50-90 percent loss predicted for the European Alps and a 60-85 percent loss for New Zealand’s Alps.

As one of the lead authors notes: “Most of them will be gone by 2100.”

Increasing melt, decreasing flow?

In contrast to the traditional views on the relationship between ice loss and surface melting, there do exist conflicting opinions on the state of glacier acceleration in Greenland. One of the proponents of such a viewpoint are Sundal et al. (2011) who suggest that the Greenland ice sheet response to melting over time is in fact not increased acceleration, but a reduced flow into the fjords.

Figure 1. Ice velocity flows for the study area

The main premise of the paper involves the adaptation of the Greenland ice sheet’s subglacial drainage system to increased melting in warmer years. Thus the ice sheet, which could raise the global sea level by 7 metres if completely melted, could actually be seeing a reduction in the flow of ice into the fjords and oceans. Evidently, the implications for future predictions of global sea level rise are quite large. Indeed, the Greenland ice sheet has shrunk in the last decade due to rising temperatures, but the question of where the meltwater could be transported to remains unresolved.

The orthodox view of ice loss due to increased melting suggests that warmer temperatures cause ice to melt on the surface of the ice sheet. This meltwater is then transported to the base of the glacier, whereby it acts to increase the rate of ice flow across the bedrock and into the fjords. However the authors of the study, using remotely sensed imagery of 6 glaciers in SW Greenland, produced vastly different indications of how ice flow changes across years of variable melting. They instead found that the expected increases in ice sheet melting over the 21^st century (as a result of increased air temperature) may have no impact on ice loss as a product of flow into the ocean. However, it must be noted that the ice sheet isn’t threatened any less, as the studies of Straneo et al. (2011) Rignot et al. (2010) and Holland et al. (2008) revealed in the uncertainty of ocean-glacier interactions.

The study shows that the initial increase of ice flow was the same for both warmer and colder years. However, it also revealed that the expected slowdown in ice flow (as a result of glacier drainage adaptation) actually came quicker in the warmest years. Sundal et al. propose that as a result of much greater meltwater in warmer years, the internal basal drainage switches more quickly, which creates a drop in pressure and therefore slower ice movement across the bedrock. As the paper notes:

“Abundant melt-water can trigger a switch from inefficient (cavity19) to efficient (channelized20) modes of drainage and, consequently, to a reduction in subglacial water pressure and ice speed. Such events have been observed at High Arctic10 and Alaskan valley glaciers11, where summer speed-up is of shorter duration during years of high melting.”

The authors also conducted a numerical simulation of this switching between modes of drainage, which showed that above a critical meltwater flow rate of 1-2 cm per day, switching and thus glacier slowdown would occur.

Whilst displaying these findings, the authors accept that there are many conflicting studies on the topic and that whilst some studies have shown increased ice sheet acceleration from increased melting (Zwally et al. 2002), other studies have identified a long-term decrease in ice flow from the Greenland ice sheet over such a period of increased melting (van de Wal et al. 2008). The authors believe however, that both findings can be combined. Whilst the data shown in this study does identify an increase in the peak rate of flow during years of high melting; the important finding here is that the subsequent faster transition to more efficient subglacial drainage means that the duration and speed of ice flow is much lower than when compared to years of low melting. In cooler years, where the critical meltwater flow rate of 1-2 cm per day is not exceeded, the more efficient ‘channelized20’ mode of drainage does not occur.

With the predicted increase in Greenland ice sheet contribution to global sea levels over the 21^st century (Mernild et al. 2008), the impact of melt-induced ice flow acceleration must not be ignored. Thus, the findings of this study and others are important for the predictions of future climate change impacts. Particularly if the findings of this study are correct, then the observed and predicted rises in air temperature over the next century may not push the Greenland ice sheet over a suggested ‘tipping point’, as a result of melt induced glacier acceleration. However, the melting will of course still occur (increasing basal storage of meltwater) and as the authors admit, their findings do not cover how the switch to more efficient drainage might be adversely affected by more short term spikes in melting. As well as this, the previously discussed studies of warm ocean influences on the outlet glaciers suggest that a much greater melting influence occurs as a result of subtropical waters. Both ocean and melting influences on glacier acceleration remain non-definitive at present; and so it is important for further research to identify how these processes have acted in the past, so that we might better understand the potentially devastating  impacts that acceleration of the Greenland ice sheet could have in the future.