The big melt

What is the cryosphere?

What’s called the cryosphere includes sea ice, ice sheets, ice shelves, glaciers, and permafrost—frozen ground. These frigid formations hold an enormous amount of frozen water. If they were all to melt away, the resulting global sea level rise would be catastrophic. The good news is that even if melting is taking place—it’s slow.  But is there any evidence that this is happening?

Take a look.

Sea ice

Sea ice is frozen seawater that floats on the ocean surface. Covering millions of square kilometres at both ends of the planet, sea ice freezes over and then melts away with the polar seasons, choreographing the seasonal rhythms of human activity and ecosystem habitats.  In the Arctic, some sea ice persists year after year, whereas in the southern ocean or Antarctic, sea ice is more seasonal—melting completely away and reforming annually.  While both Arctic and Antarctic ice are vitally important for the habitats of land and marine mammals, sea ice in the Arctic appears to play a more crucial role in influencing the climate.[1]

Sea ice thickness, its spatial extent, and the fraction of open water within the ice pack can vary rapidly in response to changing weather and climate.  Sea ice typically covers about 14 to 16 million km2 in late winter in the Arctic and 17 to 20 million km2 in the Antarctic southern ocean. On average, the seasonal decrease is much larger in the Antarctic, with only about 2 to 4 million km2 remaining at the end of summer, compared to about 7 million km2 remaining in the Arctic.

In the Arctic, the extent of sea ice has been declining dramatically.  Since 1979, winter Arctic ice extent has decreased by about 3 percent per decade [2].

In 2016, the extent of sea ice was well below average and was at record low levels for many months of the year.  The maximum extent of the ice—which usually occurs in the first few months of the year in the Northern hemisphere was the lowest ever recorded in the satellite record (which goes back almost 40 years).   The figure below shows annual levels of sea ice extent from 1979 to 2016.

             Arctic September sea ice extent 1979 2016 [3]

The trend here is unmistakeable.

While Artic sea ice was setting record lows, Antarctic sea ice was setting record highs.  In September 2014, Antarctic sea ice expanded to over 20 million km2—the highest sea ice extent in the satellite record.

Still in the Antarctic, in 2016 the sea-ice extent was close to the long-term average for the first eight months of the year, reaching a seasonal maximum of just over 18 million km2 at the end of August. This was the earliest seasonal maximum on record.  This was then followed by an exceptionally rapid spring melt, resulting in a November mean extent of 14.5 million km2—by far the lowest on record.  The reasons for the rapid collapse of the Antarctic sea ice in late 2016 are not completely understood. Clearly much more research is needed—if only because the Antarctic icesheets holds truly massive, almost incomprehensible, amounts of ice.

The figure below shows the extent of Antarctic September sea ice from 1979 to 2016.

                     Antarctic September sea-ce extent 1979 – 2016

The long term trend is very slightly positive.

However, in November 2016, sea ice extent was at record lows at both ends of the planet.  After having been 1 to 2 million km2 below the long term average for most of the year, global sea-ice extent dropped more than 4 million km2 below average in that month—an event unprecedented in the satellite record. [4].  The hydrodynamics and thermodynamics of the Antarctic cryosphere are complex, and scientists are working to understand why the Antarctic sea ice trend is slightly positive when, at the north pole, sea ice extent continues to decline.

Glaciers

While the extent of sea ice at the poles shows contrasting trends, this is not the case for the hundreds of glaciers that are found across all the continents except Australia.  Glaciers interest scientists because they are constantly on the move.

Continuous mass balance records have been kept for about 40 glaciers since the early 1960s.  These data show that in most regions of the world, glaciers are shrinking in size. From 1961 to 2005, the thickness of many small glaciers decreased  by about 12 meters or the equivalent of 9000 cubic kilometers of water.[5]

A study of observational data sets from the World Glacier Monitoring Serice (WGMS) concluded that “rates of early 21st century mass loss are without precedent on a global scale, at least for the time period observed and probably also for recorded history.

Because glaciers are shrinking so quickly, there are many really striking ‘before and after’ photos in the public record. When it comes to melting glaciers: a picture really is worth a thousand words.

The figure below shows the Glacier Bay National Park and Reserve’s White Thunder Ridge as seen on August 13, 1941 (left) and August 31, 2004 (right). Muir Glacier has retreated out of the field of view, Riggs Glacier has thinned and retreated significantly, and dense new vegetation has appeared. Muir Glacier was more than 2000 feet thick in 1941.[6]

 Glacier bay national park and Reserves White Thunder ridge then and now

Glaciers gain mass from snowfall and lose mass as ice melts from its leading edge.  If this mass balance is positive, the glacier gains mass, if the balance is negative the glacier is losing mass and retreating. The figure below  shows this balance for 41 reference glaciers tracked by scientists since 1980.  The balance every year is negative—meaning the glaciers are retreating.  The rate at which these glaciers are retreating also appears to be increasing—since the turn of the century the balance is much more strongly negative.

 

Mean annual and cumulative annual balance reported for 41 reference glaciers [7]

Ice sheets

Just like glaciers, an ice sheet forms through the accumulation of snowfall—when snowfall exceeds the annual snow melt.  Over thousands of years, the layers of snow build up, become compacted, and can form a sheet of ice several thousand metres thick, and hundreds of kilometres wide.

If the ice field covers more than 50,000 km2, it is defined as an ice sheet.  Although ice sheets covered much of the northern hemisphere during the last ice ages, the planet now has just two major ice sheets: one on Greenland and one on Antarctica. The Greenland ice sheet is the smaller of the two: covering about 1.7 million km2.  In contrast, the area of the Antactic ice sheet is enormous—almost 14 million km2.  That is more than one and a half times the area of the continental USA.

As the ice sheet gets thicker from snowfall, its weight increases–to the point where the ice sheet begins to deform and to flow slowly outwards.  Ice sheets flow outward from their centers, where they are generally thickest, and push outwards until they encounter ocean water, or where the climate is warm enough to melt the ice faster than its rate of flow.

Together the two formations on Greenland and Antarctica hold 99 percent of the world’s freshwater. If the Greenland ice sheet melted away completely, global sea levels would rise about 7 meters. If the Antarctic ice sheet were to melt, sea levels would rise at least another 50 meters [8].

Because ice sheets hold so much ice and have the potential to raise global sea levels so dramatically, measuring the mass balance of the ice sheets and tracking these changes is an area of intense scientific study.  Particularly for the Antarctic where, as noted above, the geophysics and hydrodynamics of ice sheet behaviour are not yet fully understood.

The science is complex and sophisticated: satellite radar altimetry mapping and gravimetric sensing using NASA’s GRACE satellltes have been employed together with more conventional land-based mass balance calculations.  The data, while not perfectly aligned, clearly agree on the long term trends. A study in 2012 combined satelliite altimetry, interferometry, and gravimetric data to examine ice sheet mass balances.  The study found reasonable agreement between the different satellite methods and estimated the mass balance changes between 1992 and 2011 as shown in the table below.[9]

So except for the East Antarctic ice sheet, which has gained ice, the other ice sheets are losing mass. The aggregate mass balance  for Antarctica is still strongly negative.  The Greenland ice sheet in particular is melting at an unprecedented rate. Overall, the trend is clear.

The ice is melting.

Not so permafrost

Permafrost, or permanently frozen ground, is soil, sediment, or rock that remains at 0°C for at least two years.  Despite its name, permafrost is characterized more by its instability than by its permanence.  What’s called the ‘active layer thickness’ or ALT, is the layer that thaws and freezes over the seasonal cycle; it generally increases in warmer conditions.  Permafrost has warmed over the past 2-3 decades and generally continues to warm across the circumpolar north.[10]  Field observations indicate that permafrost warmed by up to 6°C during the 20th century. Observations on Svalbard (a Norwegian island close to the Arctic circle) detected extreme permafrost warming during the winter of 2005-2006—apparently resulting from spring temperatures as much as 12°C above the 1961-1990 average.[11]

Approximately 55 percent of northern hemisphere’s land surface is covered by seasonally frozen ground—which can stay frozen for several months at high latitudes and high elevations.

When warming permafrost starts to thaw for a longer part of the year, the effects can be startling.  Trees that have collapsed from permafrost melting are sometimes referred to as ‘drunken forests’. Where large-scale thawing of ground ice has occurred, the landscape can be transformed through mudslides and the formation of flat-bottomed valleys and melt ponds, which can dramatically alter the landscape.[12]

The photograph below shows trees in a ‘drunken forest’–where the thawing  permafrost no longer support the weight of the trees.

 

 

For a deeper dive:

[1] National snow and ice data center: State of the cryosphere.  SOTC: Sea ice.  Accessed from http://nsidc.org/cryopshere/sotc/sea_ice.html. October 2017.

[2] World Meteorological Organisation :WMO statement on the state of the global climate in 2016.

[3] WMO : Statement on the state of the global climate in 2016.

[4] WMO: Statement on the state of the global climate in 2016

[5] National snow and ice data center: State of the cryosphere.  SOTC: Mountain glaciers.  Accessed from http://nsidc.org/cryopshere/sotc/glacier_balance.html. October 2017.

[6] National snow and ice data center: State of the cryosphere.  SOTC: Mountain glaciers.  http://nsidc.org/cryopshere/sotc/glacier_balance.html. 2004 USGS photo by B.F. Molnia; 1941 photo by W.O. Field. See Repeat Photography of Glaciers in the Glacier Photograph Collection.

[7] Pelto M.S., in State of the Climate 2015, Bulletin American Meteorological Society, 97 (8). Page S23.

[8] National snow and ice data center: State of the cryosphere.  SOTC: Ice sheets.  Accessed from http://nsidc.org/cryopshere/sotc/ice_sheets.html. October 2017.

[9] Shepherd et al 2012. Referenced in the National snow and ice data center roport: SOTC: ice sheets, cited above.

[10] State of the climate in 2015. Bulletin of the American Meteorological Society 97 (8).

[11] National snow and ice data center: State of the cryosphere.  SOTC: Permafrost and frozen ground.  Accessed from http://nsidc.org/cryopshere/sotc/ice_sheets.html. October 2017.

[12] The photo is from the NSIDC report : The state of the cryosphere. SOTC: Permafrost and frozen ground. Cited above.