Climate indicators: ice sheets, ice caps and glaciers

About 10% of the earth’s surface is covered by glaciers, ice caps and ice sheets, mainly in polar regions. Ice on land plays a significant role in the global climate system and is crucially involved in determining sea level. Glaciers and ice caps respond to changes in the atmosphere and oceans and give clear signals about alterations in global climate. Glaciers also contain a rich store of information about climate in ancient times. Almost all the glaciers and ice caps in the Arctic have lost mass over the last century, and in many places, ice mass loss has accelerated in the past decade. Antarctica has lost ice mass for two decades. There, however, the losses are chiefly from a restricted area.

Status and trends

About 10% of the earth’s surface is covered by glaciers, ice caps and ice sheets, mainly in polar regions. About 98% of Antarctica is covered by an ice sheet with an average thickness of at least 2.1 kilometres. This ice holds 90% of the world’s fresh water. The ice sheet on Greenland is the largest in the northern hemisphere and contains about 3 million km3 of ice. More than half of the land in Svalbard is covered by ice: a total of 2100 glaciers large and small can be found there.

Glaciers carry important information about ancient climate and function as climate archives. Ice and air bubbles trapped in ice can be analysed to reveal how air temperature and atmospheric content of greenhouse gases such as CO2 and methane have changed over long time spans. Ice cores from Antarctica can reveal temperatures 800 000 years back into the past. Ice cores from Svalbard can also be used to study climate, but these cores are trickier to interpret because summer melting is greater there than in Antarctica. The Arctic has not been continuously iced-over for as long as Antarctica, so climate archives in the north cover less time. In glaciers at the highest altitudes in Svalbard, where summer melting is smallest, it is possible to take ice cores that go 1000 years back in time.

The Arctic

Sustained changes in the expanse, volume and mass of Arctic glaciers are governed by climate and oceanographic factors that affect mass balance. A glacier’s mass balance is the yearly balance between gain of mass, mainly in the form of snowfall, and loss of mass, attributed mainly to surface melting and runoff, and calving of icebergs.

In the 5th IPCC report, the compilation of existing knowledge shows that nearly all the world’s glaciers are shrinking. Of Arctic regions, Alaska and northern Canada have seen some of the greatest losses of glacier mass over the past decade, but the Norwegian Polar Institute's long observations series from Svalbard show a similar tendency

As much as 40% of the mass is lost through calving of glaciers that flow into the ocean, at least in the areas where this has been measured.[1] From large parts of the Arctic, no measurements are available: this knowledge gap must be filled to enable future calculation of how quickly glaciers are losing mass.

A recent study calculated how the ice sheets of Greenland and Antarctica contribute to sea level rise and compared this with contributions from glaciers outside Greenland and Antarctica. Both satellite data and ground-based measurements were used. Between 2003 and 2009, meltwater from the world’s glaciers contributed about one third of the observed rise in sea level (global average). The Greenland and Antarctic ice sheets together contributed the same amount. The last third of the sea level rise is attributed increased volume of seawater (thermal expansion). The glaciers that contribute most to sea level rise are those in the Canadian Arctic, followed glaciers in Alaska and coastal Greenland. The world’s glaciers (inland ice sheets excluded) annually lose an ice mass corresponding to an approximate 0.7 mm increase in sea level.

Several parts of Greenland’s inland ice sheet have lost mass in the past two decades. This loss is spreading to new areas of the massive ice sheet, and accelerating. Surface melting and iceberg calving each cause half the loss, and both melting and calving are increasing.

Prior to 1990, Greenland’s ice sheet was in balance. Total ice mass increased with winter snowfall and decreased with summer melting and iceberg calving, and the net mass balance was essentially nil. Since then, the loss of mass has outstripped accumulation through snowfall. The inflow of fresh water to the oceans surrounding Greenland is already so great that it is increasing global sea level. In the late 1990s the Greenland ice sheet supplied 5% of global sea level rise, but estimates from 2005 show that Greenland now supplies 20%. Average sea level increased by about 0.19 m over the period 1901-2010.

Antarctica

On the other side of the globe there are also changes  – Antarctica has lost ice mass over the past two decades. Most of this loss is from the Antarctic Peninsula and the area south of the Amundsen Sea in West Antarctica.[5] The ice shelves around the Antarctic Peninsula are changing: the ice has retreated, become fissured, and large areas have collapsed on both sides of the Peninsula. Over the last 50 years, the total area of the ice shelves has decreased by an estimated 28 000 km2. The ice is now retreating about 6 000 kilometres per decade.[5]

Ice shelves retreat when the ice cracks and the cracks are filled with meltwater. Relatively warm seawater also penetrates in under the shelves. When parts of an ice  shelf disappear, the inland glaciers that feed into the shelf increase their flow rate. Collapse of some ice shelves on the Antarctic Peninsula has led to a 300-800% increase in the flow rate of glaciers in the areas where the ice shelves have disappeared.[5]

The area south of the Amundsen Sea is the part of Antarctica where the glaciers of inland ice sheet are retreating fastest. For example, the retreat of the Pine Island glacier has sped up 40% since 1970.[6] Thwaite Glacier and four other glaciers in this sector are thinning at an accelerating rate. The flow rate of Smith Glacier has increased 83% since 1992.[6] 

At present, this area is losing 50-137 gigatonnes of ice per year, which is approximately the same amount lost from the entire Greenland ice sheet. This is a significant contribution to global sea level rise.

In East Antarctica, the changes in the inland ice cap are less dramatic, with the greatest changes near the coast. The mass of the inland parts of the ice cap is growing moderately because of increased precipitation. Ice shelves differ: some are thickening slightly, others are thinning substantially. Data from passive microwave instruments suggest increased melting of ice shelves.[6]

Future scenarios

Available knowledge suggests that the world’s glaciers will continue to shrink, losing between 15 and 85% of their mass before the end of the century, depending on the representative concentration pathway.[5] The mass balance of the Greenland ice sheet is expected to continue its negative trend. In Antarctica, the rest of the century may offer a slight increase in the mass of the inland ice sheet because of increased precipitation, but this gain will be outstripped by a simultaneous loss of ice along the coast because of increased calving and melting. Thus the net effect on the inland ice sheet is expected to be negative mass balance in the future.

Prosessesc

In many parts of the Arctic, certain glaciers ”surge”. This means that they advance rapidly for a short time, then move slowly again for long spans of time. When a glacier surges, its flow rate increases quickly to 10 to 100 times its normal speed. As a consequence, the glacier develops crevasses. Surges are cyclic phenomena, repeated every 30 to 500 years. The crevassing associated with surges makes previously traversable routes unsafe or impassable. Tidal glaciers can also surge, leading to increased calving when the glacier begins to retreat after the surge. This increases the number of icebergs and has implications for shipping. Surges can also dam up lakes or rivers, leading to flooding when the glacier front retreats and the ice dam is breached.

Consequences

Melting ice will lead to a continued rise in average sea level, and the rate of increase will exceed that seen between 1971 and 2010. A sea level increase of 0.52-0.98 metres before 2100 has been projected, but the model simulations depend on warming. So far most of the sea level rise is attributed to  thermal expansion of the ocean, and this factor is expected to continue to contribute 30-50% of future sea level rise as well. Changing mass balance in the Greenland ice sheet will continue to raise sea level. The loss of ice along the coast of Antarctica (calving and melting) is expected to exceed accumulation in the inland parts of the ice sheet owing to increased precipitation; thus Antarctica will continue to contribute to sea level rise.

The rise in sea level observed between 1993 and 2003 agrees well with the observed  thermal expansion of the ocean and the loss of ice mass from the world’s glaciers.[2] Rising sea level will have serious consequences for people and communities in many parts of the world. For example, Bangladesh and the Netherlands are particularly threatened by more frequent storm surges and flooding, and some islands in the Pacific Ocean may be completely inundated.[1]

The world’s glaciers, excluding the ice sheets of Greenland and Antarctica, lose ice mass corresponding to a yearly rise in sea level of about 0.77 mm (±0.22 for the period 1991-2003).[5] The Greenland ice sheet contains so much water that if it were to melt entirely, global sea level would rise about 7 metres. This ice sheet already contributes about 20% of the global sea level rise. In the late 1990s, it contributed only 5%. Available knowledge shows that global warming above a threshold level can lead to the Greenland ice sheet melting away completely – and sea levels rising 7 metres – over the coming millennium. This temperature threshold appears to be somewhere between 1°C and 4°C above the pre-industrial average.[2] The inland ice sheet of Antarctica contains fresh water corresponding to a global sea level rise of 57 metres. It appears that on the short term, the only scenario that would yield a considerably greater sea level rise than previously estimated is collapse of those parts of the Antarctic ice sheet that are in contact with the ocean, or where the bedrock is under sea level. Current understanding shows that such a situation would lead to an increase of only a few decimetres during this century.[5]

References

  1. Arctic Monitoring and Assessment Programme (AMAP), 2012. Arctic Climate Issues 2011: Changes in Arctic Snow, Water, Ice and Permafrost. Snow, Water, Ice and Permafrost in the Arctic (SWIPA) 2011 Overview Report.
  2. Ian Joughin, Richard B. Alley 2011. Stability of the West Antarctic ice sheet in a warming world. Nature Geoscience 4, 506–513. DOI:10.1038/ngeo1194
  3. E.J.Rignot 2011. Is the Antarctic Ice Sheet melting?. The SAO/NASA Astrophysics Data System
  4. Timothy M. Lenthon et al. 2018. Tipping elements in the Earth's climate system. PNAS (Vol. 105 no. 6): 1786–1793. DOI:10.1073/pnas.0705414105 
  5. Intergovernmental Panel on Climate Change (IPCC) 2013. Fifth assessment report contribution.
  6. A.J. Turner et al. 2009. Antarctic climate change and the environment. Antarctic Science. DOI:10.1017/S0954102009990642.