Between 1980 and 1990 glaciers of #Himalayas melted most

According to the findings of  International Centre for Integrated Mountain Development (ICIMOD) Himalayas glacier retreat rates between 1980 and 1990 were double of those observed in later decades, and it requires further investigation.
On 15 May 2014, at the closing of the second international conference on ‘Cryosphere of the Hindu Kush Himalayas: State of Knowledge’, the International Centre for Integrated Mountain Development (ICIMOD) released a publication titled ‘Status and Decadal Glacier Change from 1980s to 2010 in Nepal based on Satellite Data’. The report, primarily an inventory of glaciers in Nepal, drew significant interest from media, both local and international. However, some news reports cited inaccurate data while others suggested a link between climate change and increased avalanche activity in the Himalayas (here and here). Therefore, we would like to make three major clarifications.

First, the report on decadal glacier change in Nepal does not discuss the link between climate change and increased avalanche activity.  It presents the results of satellite-derived decadal glacier inventories (1980, 1990, 2000, and 2010) for major river basins of Nepal and examines changes in glaciers between these intervals.  The results indicated shrinkage in glacier area in all three decades, and an increase in the number of glaciers due to retreat and separation.  A widely circulated news article switched the number of glaciers in 1980 and 2010 (here) and mistakenly reported estimated ice reserves in cubic meters instead of cubic kilometers.  

Second, the terminology in the news reports that link avalanches to climate change is ambiguous.  The word ‘avalanche’ typically refers to a mass movement of snow down a sloping surface.  An avalanche requires a snowpack of sufficient depth with a weak layer, a sufficiently steep slope, and a trigger.  In contrast, the 18 April tragedy on Mount Everest was the result of a different phenomenon called serac collapse.  Seracs are large blocks of ice that are formed as a result of glacier fracture patterns and motion, and can fall or topple without warning.  The normal climbing route on Everest between Base Camp and Camp 1 is exposed to serac hazards from the Khumbu Icefall and from both the south face of Everest and the north face of Nuptse.  The risk varies from year to year depending on the state of the seracs. Changes in the frequency of either avalanches or serac falls in the Everest region have not been definitively linked to climate change. 

Finally, the reported estimates of glacier retreat of nearly 25% since 1977 should also be treated with a degree of caution. The executive summary of the report suggests that glacier retreat rates between 1980 and 1990 were double of those observed in later decades, and that this warrants further investigation.  It is likely that the misclassification of snow as glacier ice in the 1980 inventory, particularly at middle and higher elevations has resulted in the overestimation of ice loss.

#Antarctica now losing 159 billion tonnes of ice each year

A sun halo formed above the iSTAR camp at midnight
Photo credit: Anna Hogg

Three years of observations show that the Antarctic ice sheet is now losing 159 billion tonnes of ice each year – twice as much as when it was last surveyed.
A team of scientists from the UK Centre for Polar Observation and Modelling, led by researchers at the University of Leeds, have produced the first complete assessment of Antarctic ice sheet elevation change.

They used measurements collected by the European Space Agency's CryoSat-2 satellite mission, which carries an altimeter specially designed for this task.

In sharp contrast to past altimeter missions, CryoSat-2 surveys virtually all the Antarctic continent, reaching to within 215 kilometres of the South Pole and leading to a fivefold increase in the sampling of coastal regions where today's ice losses are concentrated.

Overall, the pattern of imbalance continues to be dominated by glaciers thinning in the Amundsen Sea sector of West Antarctica.

However, thanks to the improved capabilities of CryoSat-2, problem areas such as the rugged terrain of the Antarctic Peninsula can now also be surveyed.

On average West Antarctica lost 134 gigatonnes of ice, East Antarctica three gigatonnes, and the Antarctic Peninsula 23 gigatonnes in each year between 2010 and 2013 – a total loss of 159 gigatonnes each year.

The polar ice sheets are a major contributor to global sea level rise and, when combined, the Antarctic losses detected by CryoSat-2 are enough to raise global sea levels by 0.45 millimetres each year alone.

In West Antarctica, ice thinning has been detected in areas that were poorly surveyed by past satellite altimeter missions.

These newly-mapped areas contribute additional losses that bring altimeter observations closer to estimates based on other approaches.

But the average rate of ice thinning in West Antarctica has also increased, and this sector is now losing almost one third (31%) as much ice each year than it did during the five year period (2005-2010) prior to CryoSat-2's launch.

Lead author Dr Malcolm McMillan from the University of Leeds said: "We find that ice losses continue to be most pronounced along the fast-flowing ice streams of the Amundsen Sea sector, with thinning rates of between 4 and 8 metres per year near to the grounding lines of the Pine Island, Thwaites and Smith Glaciers."

This sector of Antarctica has long been identified as the most vulnerable to changes in climate and, according to recent assessments, its glaciers may have passed a point of irreversible retreat.

Launched in 2010, CryoSat carries a radar altimeter that can 'see' through clouds and in the dark, providing continuous measurements over areas like Antarctica that are prone to bad weather and long periods of darkness.

The radar can measure the surface height variation of ice in fine detail, allowing scientists to record changes in its volume with unprecedented accuracy.

Professor Andrew Shepherd, also of the University of Leeds, who led the study, said: "Thanks to its novel instrument design and to its near-polar orbit, CryoSat allows us to survey coastal and high-latitude regions of Antarctica that were beyond the capability of past altimeter missions, and it seems that these regions are crucial for determining the overall imbalance."

"Although we are fortunate to now have, in CryoSat-2, a routine capability to monitor the polar ice sheets, the increased thinning we have detected in West Antarctica is a worrying development. It adds concrete evidence that dramatic changes are underway in this part of our planet, which has enough ice to raise global sea levels by more than a metre. The challenge is to use this evidence to test and improve the predictive skill of climate models."

Professor David Vaughan of the British Antarctic Survey said: "The increasing contribution of Antarctica to sea-level rise is a global issue, and we need to use every technique available to understand where and how much ice is being lost. Through some very clever technical improvements, McMillan and his colleagues have produced the best maps of Antarctic ice-loss we have ever had. Prediction of the rate of future global sea-level rise must be begin with a thorough understanding of current changes in the ice sheets – this study puts us exactly where we need to be."

Dr Ian Joughin at the University of Washington, author of a recent study simulating future Antarctic ice sheet losses added: "This study does a nice job of revealing the strong thinning along the Amundsen Coast, which is consistent with theory and models indicating this region is in the early stages of collapse."

The findings from a team of UK researchers at the NERC Centre for Polar Observation and Modelling, are published in the journal Geophysical Research Letters. Professor Vaughan and Dr Joughin were not involved in the study.