Are Global Temperatures Rising?
In 2007, in its Fourth Assessment Report, the Intergovernmental Panel on Climate Change reported that 11 of the 12 warmest years on in the instrumental record since 1850 fell between 1995 and 2006 (IPCC 2007). The updated 100-year trend, from 1996 to 2005 of 0.74°C ± 0.18°C, was greater than the 100-year warming trend at the time of the IPCC's Third Assessment Report. The warming trend covered in the Third Assessment Report, from 1901 to 2000, was 0.6°C ± 0.2°C. Additional warm years after 2000 caused the higher warming trend reported in the Fourth Assessment Report. Temperature records supporting these findings have been assembled from thousands of land and ocean observation sites covering a large, representative portion of the Earth's surface and carefully controlled for possible biases arising from station and instrument changes.
Temperatures vary from year to year, and also from decade to decade. These variations, however, are superimposed on a longer upward trend. The range of natural variability in global temperature seems to be about ± 0.2°C, so only after the late 1970s do global mean temperatures emerge from the noise of natural variability (Karl and Trenberth 2003). The northern high latitudes have experienced greater warming than the mid-latitudes or the southern high latitudes. This is apparent in the Temperature Anomalies graph.
In some northern regions, extreme warming has been detected. Locations in Alaska and northern Eurasia, for example, have warmed by nearly 6.0°C in the winter months since 1970 (Serreze et al. 2000). The warming is not universal; some cooling has occurred in the North Atlantic and central North Pacific and is known to be a consequence of changes in the atmospheric circulation.
In its fourth assessment report, the IPCC cited atmospheric concentrations of greenhouse gases as the causative agent in warming temperatures. The panel identified fossil fuel burning and changes in land use and the primary cause of increased carbon dioxide, and agriculture as the primary causes of increased methane and nitrous oxide. Atmospheric carbon dioxide concentrations in 2005 exceeded the natural range for this gas over the past 650,000 years. The IPCC attributed a "greater than 90 percent certainty" to scientists' assertion that higher greenhouse gas concentrations have trapped more thermal radiation and consequently warmed the planet (IPCC 2007).
Is the Cryosphere Sending Signals About Climate Change?
The cryospheric regions, or regions where water is found in solid form, provide us with direct visual evidence of temperature changes. Unlike other substances found on Earth, ice and snow exist relatively close to their melting point and may frequently change phase from solid to liquid and back again. Consequently, consistent and prolonged warming trends should result in observable changes to Earth's cryosphere. Water changing from solid to liquid and back often results in dramatic visual changes across the landscape as various snow and ice masses shrink or grow.
What are some examples of these snow and ice masses, how do we monitor their conditions, and what do the results show?
In State of the Cryosphere, snow cover, glaciers, permafrost, sea ice, ice shelves, and the related parameter sea level are discussed. In all cases, scientists attempt to monitor both the areal extent and mass of these snow and ice bodies. Areal extent is easier to determine than mass. Various forms of remote sensing, from both aircraft and satellite, allow us to look down on surfaces at varying spatial scales and over time to determine if the snow or ice covered area is expanding or contracting. Long-term monitoring includes looking at the areal extent of snow cover and sea ice, as well as changes in area and mass of mountain glaciers. In all cases shown here, regardless of parameter or measurement method, the amount of snow and ice has been decreasing over the past several decades.
Northern Hemisphere Snow
We all associate snowstorms with cold weather, but snow's influence on the weather and climate continues long after the storm ends. Because snow is highly reflective, a vast amount of sunlight that hits the snow is reflected back into space instead of warming the planet. Without snow cover, the ground absorbs about four to six times more of the Sun's energy. The presence or absence of snow controls patterns of heating and cooling over Earth's land surface more than any other single land surface feature.
In many locations in recent decades, temperatures have risen while precipitation levels have remained largely the same. Satellite data have confirmed that average snow cover has decreased, especially in the spring and summer. Where snow cover is disappearing earlier in the spring, the large amounts of energy that would have melted the snow can now directly warm the soil.
Northern Hemisphere Snow Extent: What sensors on satellites are telling us about snow cover
In terms of spatial extent, seasonal snow cover is the largest single component of the cryosphere and has a mean winter maximum areal extent of 47 million square kilometers, about 98 percent of which is located in the Northern Hemisphere.
Snow cover is an important climate change variable because of its influence on energy and moisture budgets. Snow cover accounts for the large differences between summer and winter land surface albedo, both annually and inter-annually. Snow may reflect as much as 80 to 90 percent of the incoming solar energy, whereas a snow-free surface such as soil or vegetation may reflect only 10 to 20 percent. A warming trend results in decreased snow cover. With the resulting decrease in reflected energy, absorption of solar radiation increases, adding heat to the system, thereby causing even more snow to melt. This is the classic positive temperature-albedo feedback mechanism, which is a key component in climate models. Surface temperature is highly dependent on the presence or absence of snow cover, and temperature trends have been linked to changes in snow cover (Groisman et al. 1994).
During the past four decades, satellite remote sensing has provided valuable information on hemispheric-scale snow extent. Since 1966, the National Oceanic and Atmospheric Administration (NOAA) has produced weekly snow extent maps for Northern Hemisphere land surfaces using visible-band satellite imagery (Robinson and Frei 2000). Because snow has such a high albedo compared to other surfaces on Earth, snow-covered areas appear much brighter in satellite imagery than most other surface types.
Remote sensing data sets from the microwave portion of the electromagnetic spectrum can also be used to derive snow cover maps. When snow covers the ground, some of the microwave energy emitted by the underlying soil is scattered by the snow grains; therefore, when moving from snow-free to snow-covered land surfaces, a sharp decrease in emissivity indicates the presence of dry snow.
These remote sensing data sets are derived using different types of analyses and separate regions of the electromagnetic spectrum, yet their results are strikingly similar. Both visible and passive microwave data sets show similar patterns of inter-annual variability, and both consistently indicate maximum snow extent that exceeds 40 million square kilometers for the Northern Hemisphere.
The 28 year trend in snow extent derived from visible and passive microwave satellite data indicates an annual decrease of approximately 1 to 3 percent per decade with greater deceases of approximately 3 to 5 percent during spring and summer. Precipitation in regions of seasonal snow cover appears to be constant or increasing slightly in some locations over the same time period, which suggests that diminishing snow cover is the result of increasing temperatures. One region where the snow appears to be diminishing rapidly is the Western United States, especially in spring when the duration of snow cover has been decreasing by 2-3 days per decade (see blue-colored areas in the Spring Duration figure). This satllite-derived trend agrees with direct measurement of snow depth and extent on the ground (Mote et al. 2005).
Because they are so sensitive to temperature fluctuations, glaciers provide clues about the effects of global warming (Oerlemans, J. 2001). The 1991 discovery of the 5,000 year-old "ice man" preserved in a glacier in the European Alps fascinated the world, yet the discovery meant that this glacier had reached a 5,000-year minimum. With few exceptions, glaciers around the world have retreated at unprecedented rates over the last century. Some ice caps, glaciers, and even an ice shelf have disappeared altogether. Many more are retreating so rapidly that they may vanish within decades. Some scientists attribute this retreat to the Industrial Revolution; burning fossil fuels releases greenhouse gases into the atmosphere and affects our environment in ways we did not understand before.
Mountain Glacier Fluctuations: Changes in terminus location and mass balance
Over long periods, glacial response to climate change becomes obvious.
Glaciers differ from snow cover and sea ice extent in that scientists cannot use short-term changes in the areal extent of small glaciers as an index of current climatic conditions. Glaciers continually move, transporting mass from higher to lower elevations, somewhat like a conveyor belt. If the combination of climate and ice dynamics determines that the glacier is also advancing, the effect of the advance of the terminus is to increase the overall glacier area; however, because glaciers move slowly, a significant time lag occurs between the climatic conditions that caused the advance or retreat, and the actual advance or retreat. This time lag may last several years or longer, and is determined by the complicated and sometimes uncertain processes that control how fast the glacier moves.
Explosive volcanic eruptions, which contribute dust to the stratosphere and affect Earth's climate, can also affect glacier mass balance.
For glaciers outside Antarctica or Greenland—referred to here as subpolar and mountain glaciers—considerable compilation and analysis of existing mass balance measurements have occurred (Cogley and Adams 1998; Dyurgerov and Meier 1997; Dyurgerov 2002; Cogley 2002). Glaciers involved in mass balance studies are sparsely distributed over all mountain and subpolar regions, with about 70 percent of the observations coming from the mountains of Europe, North America, and the former Soviet Union.
At one time or another, researchers have measured mass balance on more than 300 glaciers since 1946, although we only have a continuous record from about 40 glaciers since the early 1960s. These results indicate that in most regions of the world, glaciers are shrinking in mass. From 1961 to 2003, the thickness of "small" glaciers decreased approximately 8 meters, or the equivalent of more than 6,000 cubic kilometers of water. The Global Glacier Mass Balance graph shows average volume change data each year from 1961 to 2003, and a plot of the cumulative change in volume, expressed in cubic kilometers of water, for this period.
Permafrost and Frozen Ground
Permafrost, or permanently frozen ground, is soil, sediment, or rock that remains at or below 0°C for at least two years. It occurs both on land and beneath offshore Arctic continental shelves, and its thickness ranges from less than 1 meter to greater than 1,000 meters. Seasonally frozen ground is near-surface soil that freezes for more than 15 days per year. Intermittently frozen ground is near-surface soil that freezes from one to 15 days per year.
Frozen ground data are critical to understanding environmental change, validating models, and building and maintaining structures in seasonal frost and permafrost regions. Climate models and observations have both pointed to likely permafrost thawing in the 21st century.
Permafrost regions occupy approximately 22.79 million square kilometers (about 24 percent of the exposed land surface) of the Northern Hemisphere (Zhang et al. 2003b). Permafrost occurs as far north as 84°N in northern Greenland, and as far south as 26°N in the Himalayas. Because reliable data on hemispheric-scale permafrost extent have only recently become available, this site provides just a snapshot of current permafrost conditions rather than time-series data.
Exactly what is permafrost? Permafrost is not defined by soil moisture content, overlying snow cover, or location; it is defined solely by temperature. Any rock or soil remaining at or below 0°C for two or more years is permafrost. It can contain over 30 percent ice, or practically no ice at all. It can be overlain by several meters of snow, or little or no snow. Understanding permafrost is not only important to civil engineering and architecture, it is also a crucial part of studying global change and protecting the environment in cold regions. In terms of area, permafrost can be characterized as continuous, discontinuous, sporadic, or isolated, but because these are descriptive terms, the boundaries separating different permafrost zones can be vague (Zhang 2005).
Determining the location and extent of permafrost is often difficult. The historical approach has been to assume that ground temperature equals the overlying air temperature, but ground and air temperatures usually differ. Even in areas where the mean annual air temperature is below freezing, permafrost may not exist. Land under glaciers, rivers, and streams is often free of permafrost, despite freezing air temperatures at the surface (Williams and Smith 1989).
Geologists and geocryologists have mapped permafrost since the mid-20th century. In 1990, the International Permafrost Association (IPA) recognized the need for a single, unified map to summarize the distribution and properties of permafrost and ground ice in the Northern Hemisphere. The IPA map shows the distribution of permafrost and ground ice for the continental land masses, areas of mountain and plateau permafrost, sub-sea and relict permafrost, relative abundance of ice wedges, massive ice bodies and pingos, and for ranges of permafrost temperature and thickness (Brown et al. 1998).
Most permafrost in the Northern Hemisphere occurs between latitudes of 60°N and 68°N. (North of 67°N, permafrost declines sharply, as the exposed land surface gives way to the Arctic Ocean.) There is also a significant amount of permafrost around 35°N, in the Qinghai-Xizang (Tibet) Plateau, and in the mountains of southwest Asia and the U.S. Rocky Mountains. About 37 percent of Northern Hemisphere permafrost occurs in western North America, mainly in Alaska and northern Canada between 165°W and 60°W. Most permafrost occurs in the Eastern Hemisphere, mainly in Siberia and the Far East of Russia, northern Mongolia, northeastern China, the Qinghai-Xizang (Tibet) Plateau, and surrounding mountains between 60°E and 180°E (Zhang et al. 1999).
Proximity to large water bodies tends to reduce temperature extremes, which affects the distribution of permafrost. Scandinavia and Iceland, for instance, have relatively little permafrost (Williams and Smith 1989). Snow cover can play a varying role in the formation or survival of permafrost. In areas of continuous permafrost, seasonal snow cover can lead to warmer ground temperatures, while in areas of discontinuous or sporadic permafrost, the absence of snow cover can contribute to permafrost formation (Zhang 2005).
Despite its name, permafrost is characterized by its instability. It is often covered by an active layer that regularly melts. Although permafrost can be thousands of years old, it is sometimes newly formed or about to thaw, and it often exists close to its melting point (Williams and Smith 1989). As permafrost thaws, it jeopardizes both man-made structures and natural features. Thawing permafrost on mountain slopes can lead to landslides (Nelson et al. 2001). Approximately 55 percent of the Northern Hemisphere's land surface is covered by seasonally frozen ground, which can last for a few weeks in the middle and lower latitudes, and for several months at high latitudes and high elevations (Zhang et al. 2003a).
[[Image:coastal_erosion.jpg|right|200px|thumb|Intact permafrost is extremely strong and resistant to erosion, and intact sea ice minimizes wave action in the ocean. When sea ice retreats and permafrost degrades, coastlines become much more vulnerable to erosion. These pictures were taken in Shishmaref, Alaska, during a storm in 2003. Only two hours separate the first photo from the second. For reference, red arrows mark the barrel. By the time the second photograph was taken, the coastline in the foreground had retreated past the barrel. Although coastal erosion was significant, this was not a particularly strong storm.
Both model projections and observations have indicated permafrost degradation. A permafrost projection published in 2005 using the Community Climate System Model, version 3 (CCSM3) suggested that by 2100, as little as 1.0 million square kilometers of near-surface permafrost might remain, increasing the freshwater discharge into the Arctic Ocean by 28 percent (Lawrence and Slater 2005). This projection applied only to the top few meters of permafrost, and depended upon climate and snow cover models. Field observations indicate that permafrost warmed by up to 6°C during the 20th century. Observations on Svalbard detected extreme permafrost warming during the winter-spring 2005-2006. The thaw apparently resulted from a temperature anomaly where January and April temperatures reached more than 12 degrees Celsius above the 1961-1990 average. These temperature anomalies were within the range of warming scenarios predicted for the late 21st century (Isaksen et al. 2007). Observations in Alaska found permafrost warming at most sites north of the Brooks Range from the Chukchi Sea to the border with Canada, coincident with statewide air-temperature warming beginning in 1976. The warming occurred primarily in the winter, with little summertime change (Osterkamp 2007).
Much of the Northern Hemisphere frozen ground is overlain by evergreen boreal forest. These boreal forests comprise both a source and a sink of carbon. In fact, the Arctic contains nearly one-third of the Earth's stored soil carbon. If the high northern latitudes were to have a significant temperature increase, the regional soils would begin to release carbon into the atmosphere, which could lead to higher temperatures, fueling the cycle of carbon release and temperature rise (Environmental News Network 1999 and Goulden et al. 1998).
Frozen ground's widespread distribution makes it a substantial component of the cryosphere (Zhang et al. 1999 and Williams and Smith 1989). Likewise, its role in the storage and release of carbon make it a major factor in future global change. Frozen ground is worth watching closely because its fate is tied to our own.
Sea ice is frozen seawater that floats on the ocean surface. Blanketing millions of square kilometers, sea ice forms and melts with the polar seasons, affecting both human activity and biological habitat. In the Arctic, some sea ice persists year after year, whereas almost all Southern Ocean or Antarctic sea ice is "seasonal ice," meaning it melts away and reforms annually. While both Arctic and Antarctic ice are of vital importance to the marine mammals and birds for which they are habitats, sea ice in the Arctic appears to play a more crucial role in regulating climate.
Global Sea Ice Extent and Concentration: What sensors on satellites are telling us about sea ice
Sea ice regulates exchanges of heat, moisture and salinity in the polar oceans. It insulates the relatively warm ocean water from the cold polar atmosphere except where cracks, or leads, in the ice allow exchange of heat and water vapor from ocean to atmosphere in winter. The number of leads determines where and how much heat and water are lost to the atmosphere, which may affect local cloud cover and precipitation.
The seasonal sea ice cycle affects both human activities and biological habitats. For example, companies shipping raw materials such as oil or coal out of the Arctic must work quickly during periods of low ice concentration, navigating their ships towards openings in the ice and away from treacherous multiyear ice that has accumulated over several years. Many arctic mammals, such as polar bears, seals, and walruses, depend on the sea ice for their habitat. These species hunt, feed, and breed on the ice. Studies of polar bear populations indicate that declining sea ice is likely to decrease polar bear numbers, perhaps substantially (Stirling and Parkinson 2006).
Ice thickness, its spatial extent, and the fraction of open water within the ice pack can vary rapidly and profoundly in response to weather and climate. Sea ice typically covers about 14 to 16 million square kilometers in late winter in the Arctic and 17 to 20 million square kilometers in the Antarctic Southern Ocean]. The seasonal decrease is much larger in the Antarctic, with only about three to four million square kilometers remaining at summer's end, compared to approximately seven to nine million square kilometers in the Arctic. These maps provide examples of late winter and late summer ice cover in the two hemispheres.
Passive microwave satellite data represent the best method to monitor sea ice because of the ability to show data through most clouds and during darkness. Passive microwave data allow scientists to monitor the inter-annual variations and trends in sea ice cover. Observations of polar oceans derived from these instruments are essential for tracking the ice edge, estimating sea ice concentrations, and classifying sea ice types. In addition to the practical use of this information for shipping and transport, these data add to the meteorological knowledge base required for better understanding climate.
Passive microwave satellite data reveal that, since 1979, winter Arctic ice extent has decreased about 3.6 percent per decade (Meier et al. 2006). Antarctic ice extent is increasing (Cavalieri et al. 2003), but the trend is small.
Satellite data from the SMMR and SSM/I instruments have been combined with earlier observations from ice charts and other sources to yield a time series of Arctic ice extent from the early 1900s onward. While the pre-satellite records are not as reliable, their trends are in good general agreement with the satellite record and indicate that Arctic sea ice extent has been declining since at least the early 1950s.
In recent years, satellite data have indicated an even more dramatic reduction in regional ice cover. In September 2002, sea ice in the Arctic reached a record minimum (Serreze et al. 2003), 4 percent lower than any previous September since 1978, and 14 percent lower than the 1979-2000 mean. In the past, a low ice year would be followed by a rebound to near-normal conditions, but 2002 was followed by two more low-ice years, both of which almost matched the 2002 record. Taking these three years into account, the September ice extent trend for 1979-2004 declined by 7.7 percent per decade (Stroeve et al. 2005). The year 2005 set a new record, dropping the estimated decline in end-of-summer Arctic sea ice to approximately 8 percent per decade. Although sea ice did not set a new record low in 2006, it did fall below normal for the fifth consecutive year. In 2007, sea ice broke all prior satellite records, reaching a record low a month before the end of melt season. Through 2007, the September decline trend is now over 10 percent per decade. (For current sea ice trends, visit NSIDC's Sea Ice Index Cryospheric Climate Indicators.)
Greenhouse gases emitted through human activities and the resulting increase in global mean temperatures are the most likely underlying cause of the sea ice decline, but the direct cause is a complicated combination of factors resulting from the warming, and from climate variability. The Arctic Oscillation (AO) is a see-saw pattern of alternating atmospheric pressure at polar and mid-latitudes. The positive phase produces a strong polar vortex, with the mid-latitude jet stream shifted northward. The negative phase produces the opposite conditions. From the 1950s to the 1980s, the AO flipped between positive and negative phases, but it entered a strong positive pattern between 1989 and 1995. So the acceleration in the sea ice decline since the mid 1990s may have been partly triggered by the strongly positive AO mode during the preceding years (Rigor et al. 2002 and Rigor and Wallace 2004) that flushed older, thicker ice out of the Arctic, but other factors also played a role.
Since the mid-1990s, the AO has largely been a neutral or negative phase, and the late 1990s and early 2000s brought a weakening of the Beaufort Gyre. However, the longevity of ice in the gyre began to change as a result of warming along the Alaskan and Siberian coasts. In the past, sea ice in this gyre could remain in the Arctic for many years, thickening over time. Beginning in the late 1990s, sea ice began melting in the southern arm of the gyre, thanks to warmer air temperatures and more extensive summer melt north of Alaska and Siberia. Moreover, ice movement out of the Arctic through Fram Strait continued at a high rate despite the change in the AO. Thus warming conditions and wind patterns have been the main drivers of the steeper decline since the late 1990s. Sea ice may not be able to recover under the current persistently warm conditions, and a tipping point may have been passed where the Arctic will eventually be ice-free during at least part of the summer (Lindsay and Zhang 2005).
Examination of the long-term satellite record dating back to 1979 and earlier records dating back to the 1950s indicate that spring melt seasons have started earlier and continued for a longer period throughout the year (Serreze et al. 2007). Even more disquieting, comparison of actual Arctic sea ice decline to IPCC AR4 projections show that observed ice loss is faster than any of the IPCC AR4 models have predicted (Stroeve et al. 2007).
Sea ice thickness has likewise shown substantial decline in recent decades (Rothrock et al. 1999). Using data from submarine cruises, Rothrock and collaborators determined that the mean ice draft at the end of the melt season in the Arctic has decreased by about 1.3 meters between the 1950s and the 1990s.
An ice shelf is a thick slab of ice, attached to a coastline an extending out over the ocean as a seaward extension of the grounded ice sheet. Ice shelves range in thickness from about 50 to 600 meters, and some shelves persist for thousands of years. They fringe the continent of Antarctica, and occupy a few fjords and bays along the Greenland and Ellesmere Island coasts. (An ice shelf occupying a fjord is sometimes called an ice tongue.) At their seaward edge, ice shelves periodically calve icebergs, some the size of a small U.S. state or European country.
Most ice shelves are fed by inland glaciers. Together, an ice shelf and the glaciers feeding it can form a stable system, with the forces of outflow and back pressure balanced. Warmer temperatures can destabilize this system by increasing glacier flow speed and—more dramatically—by disintegrating the ice shelf. Without the shelf to slow its speed, the glacier accelerates. After the 2002 Larsen B Ice Shelf disintegration, nearby glaciers in the Antarctic Peninsula accelerated up to eight times their original speed over the next 18 months. Similar losses of ice tongues in Greenland have caused speed-ups of two to three times the flow rate in just one year.
While calving or disintegrating ice shelves don't raise ocean level, the resulting glacier acceleration does, and it poses a direct threat to coastal communities. More than 100 million people currently live within 1 meter of mean sea level. Greenland contains enough ice to raise sea level by 7 meters, and Antarctica holds enough ice to raise sea level by 57 meters. While these ice sheets are unlikely to disappear anytime soon, even partial loss of the grounded ice could present a significant problem. In the early decades of the climate warming era, ice shelves and ice tongues are likely to play a prominent role in changing the rate of ice flow off the major ice sheets.
Ice Shelf Observations: Rapid response to climate change
Ice shelves fall into three categories: (1) ice shelves fed by glaciers, (2) ice shelves created by sea ice, and (3) composite ice shelves (Jeffries 2002). Most of the world's ice shelves, including the largest, are fed by glaciers and are located in Greenland and Antarctica.
One example of an ice shelf composed of compacted, thickened sea ice is the Ward Hunt Ice Shelf off the coast of Ellesmere Island in northern Canada. In the Northern Hemisphere summer of 2002, this shelf fractured and calved several large pieces into the Arctic Ocean. As a result, more than 3 billion cubic meters of fresh water drained into the Arctic Ocean. Compared to many of the world's other ice shelves, however, the Ward Hunt Ice Shelf was actually quite small.
Because they are exposed to both warming air above and warming ocean below, ice shelves and ice tongues respond more quickly than ice sheets or glaciers to rising temperatures. Antarctica has 15 major ice shelf areas, and 10 of the largest appear in this map: Ross, Ronne-Filchner, Amery, Larsen C, Riiser-Larsen, Fimbul, Shackleton, George VI, West, and Wilkins. The three largest are the Ross, the Ronne-Filchner, and the Amery.
Two kinds of events occurring on ice shelves have attracted the attention of scientists. One kind is iceberg calving, a natural event. The other kind is disintegration, a new phenomenon suggestive of climate change.
Calving of huge, tabular icebergs is unique to Antarctica, and the process can take a decade or longer. Calving can take the form of a large crack along the ice shelf edge. In the case of the Amery Ice Shelf, the calving area resembles a loose tooth. On a stable ice shelf, calving is a near-cyclical, repetitive process producing large icebergs every few decades. The icebergs drift generally westward around the continent, and as long as they remain in the cold, near-coastline water, they can survive decades or more. However, they eventually are caught up in north-drifting currents where they melt and break apart.
In Greenland, floating ice tongues downstream from large outlet glaciers are more broken up by crevasses. Calving of the ice tongues releases armadas of smaller, steep-sided icebergs that drift south sometimes reaching North Atlantic shipping lanes. Calving of the large glacier, Jacobshavn, on the east coast of Greenland is responsible for the majority of icebergs reaching Atlantic shipping and fishing areas off of Newfoundland and most likely shed the iceberg responsible for the sinking of the Titanic in 1912. These denizens of the ocean are now tracked by the National Ice Center in the United States, along with other organizations.
In recent years, calving of the largest ice tongues in Greenland (in particular, Jacobshavn, Helheim, and Kangerdlugssuaq) has accelerated probably due to warmer air and/or ocean temperatures. As the ice tongues have retreated, the reduced backpressure against the glacier has allowed these glaciers to accelerate significantly.
Large tabular iceberg calvings are natural events that occur under stable climatic conditions, so are not a good indicator of warming or changing climate. Over the past several decades, however, meteorological records have revealed atmospheric warming on the Antarctic Peninsula, and the northernmost ice shelves on the peninsula have retreated dramatically (Vaughan and Doake 1996). In fact, since 1974, seven ice shelves have retreated by a total of approximately 13,500 square kilometers.
The most pronounced ice shelf retreat has occurred on the Larsen Ice Shelf, located on the eastern side of the Antarctic Peninsula's northern tip. The shelf is divided into three regions from north to south: A, B, and C.
In January 1995, two events on the Larsen attracted public attention: the calving of a 70- by 25-kilometer iceberg from the Larsen B; and the disintegration of the remainder of the Larsen A, which began retreating in the 1980s. Although the iceberg attracted more attention, the disintegration may have been more closely related to climate change. The breakup pattern in the Larsen A, in which 2,000 square kilometers disintegrated into small icebergs, was at that time an unprecedented observation.
In 2002, satellites recorded an even larger disintegration than what occurred in 1995 (see Larsen B Ice Shelf Collapses in Antarctica). Between 31 January and 5 March 2002, approximately 3,250 square kilometers of the Larsen B shattered, releasing 720 billion tons of ice into the Weddell Sea. It was the largest single disintegration event in 30 years of ice shelf monitoring. Preliminary studies of sediment cores suggest that it may have been this ice shelf's first collapse in 12,000 years.
Building on earlier research (Weertman 1973 and Hughes 1983), Ted Scambos of NSIDC, Christina Hulbe of Portland State University, and Mark Fahnestock of the University of New Hampshire have developed a theory of how ice shelves disintegrate (see melt pond theory). Sufficiently warm summer temperatures and an impermeable surface that prevents water from being absorbed lead to melt ponds on the shelf. This meltwater can later fill small surface cracks. Depending on the amount of water and the depth of a crack, the water can deepen the crack and eventually wedge through the ice shelf (Scambos et al. 2003).
The formation of melt ponds depends most upon summer temperatures. Although a single warm summer cannot lead to collapse, a series of warm summers transforms permeable snow into impermeable ice, allowing melt ponds to form during subsequent warm summers. A glacier can also respond to summer warming. Even when the temperature of interior glacial ice remains below freezing, meltwater can percolate through the glacier to its base and decrease friction between the glacial ice and the underlying rock (Zwally et al. 2002). This is a seasonal phenomenon, and with a stable ice shelf in place, glacier acceleration ends with the warm summer temperatures. If the ice shelf shatters, however, the picture changes.
A critical feature of an ice shelf is the "grounding line," the point where the underside of the ice shelf detaches from land and floats on the ocean water. If an ice shelf retreats to the grounding line, the shelf's shape changes. More ice protrudes above the water line, and the ocean water exerts little buoyant pressure on the ice. As a result, the flow of the glacier meets very little resistance. In the 18 months following the Larsen Ice Shelf disintegration, glaciers feeding that ice shelf accelerated between three- to eight-fold (Scambos et al. 2004 and Rignot et al. 2004). Similar mechanisms are at work in the Jakobshavn Ice Stream in Greenland (Joughin et al. 2004).
The images show a tabular iceberg calving from an ice shelf. This iceberg happens to be calving from the remnant piece of the Larsen B ice shelf at the southwestern corner of the embayment. At the time these images were acquired, the Larsen B sported melt ponds. Although still intact, the Larsen C had snow firn nearly in the same state as that on Larsen B, namely dense enough to support extensive ponding.
If all the glaciers feeding the Larsen B Ice Shelf were to flow into the ocean, they would raise ocean level by only a few millimeters. Greenland's glaciers and those feeding the Ross Ice Shelf, however, would have a more significant effect.
The Ross Ice Shelf is the main outlet for several major glaciers from the West Antarctic Ice Sheet. This single ice sheet contains enough above-sea-level ice to raise global sea level by 5 meters. At present, the Ross Ice Shelf's mean annual temperature is well below freezing. Although summer temperatures in the warmest part of this shelf are currently just a few degrees too cool for the formation of melt ponds, there is no evidence of a strong warming trend on the Ross Ice Shelf at this time.
The Contribution of the Cryosphere to Changes in Sea Level
Global sea level rose by about 120 meters during the several millennia that followed the end of the last ice age (approximately 21,000 years ago), and stabilized between 3,000 and 2,000 years ago. Sea level indicators suggest that global sea level did not change significantly from then until the late 19th century when the instrumental record of sea level change shows evidence for an onset of sea level rise. Estimates for the 20th century show that global average sea level rose at a rate of about 1.7 millimeters per year. Satellite altimetry observations, available since the early 1990s, provide more accurate sea level data with nearly global coverage and indicate that since 1993 sea level has been rising at a rate of about 3 millimeters per year. Climate models based on the current rate of increase in greenhouse gases, however, indicate that sea level may rise at about 4 millimeters per year reaching 0.22 to 0.44 meters above 1990 levels by the period 2090-2099 (IPCC 2007).
Contribution from the Cryosphere
Which of the topics discussed so far in State of the Cryosphere have the potential to contribute to a rising sea level during a warming climate? Several — but some more than others.
- The seasonal snow cover melts during spring and summer and much of that water flows into rivers which eventually reach the sea. However, this is a process with a seasonal hydrologic cycle. With no net increase in seasonal snowfall over time, and no significant increase has occurred in recent decades, melting snow is not a factor that contributes to annual net sea level rise.
- Sea ice and ice shelves are already located in the ocean and thus do not have any further significant influence on sea level after they melt.
- As permafrost thaws, and the ice within the soil melts, an additional amount of liquid water becomes available but how much of this water actually reaches streams and rivers, and eventually the sea, is not well known at this time.
- The most significant contributors to sea level within the current climate are glaciers.
Current conditions: contribution from melting glaciers
Global sea level is currently rising as a result of both ocean thermal expansion and glacier melt, with each accounting for about half of the observed sea level rise, and each caused by recent increases in global mean temperature. For the period 1961-2003, the observed sea level rise due to thermal expansion was 0.42 millimeters per year and 0.69 millimeters per year due to total glacier melt (small glaciers, ice caps, ice sheets) (IPCC 2007). Between 1993 and 2003, the contribution to sea level rise increased for both sources to 1.60 millimeters per year and 1.19 millimeters per year respectively (IPCC 2007).
Antarctica and Greenland, the world's largest ice sheets, make up the vast majority of the Earth's ice. If these ice sheets melted entirely, sea level would rise by more than 70 meters. However, current estimates indicate that mass balance for the Antarctic ice sheet is in approximate equilibrium and may represent only about 10 percent of the current contribution to sea level rise coming from glaciers. However, some localized areas of the Antarctic have recently shown significant negative balance, e.g., Pine Island and Thwaites Glaciers, and glaciers on the Antarctic Peninsula. There is still much uncertainty about accumulation rates in Antarctica, especially on the East Antarctic Plateau. The Greenland Ice Sheet may be contributing about 30 percent of all glacier melt to rising sea level. Furthermore, recent observations show evidence for increased ice flow rates in some regions of the Greenland Ice Sheet, suggesting that ice dynamics may be a key factor in the response of coastal glaciers and ice sheets to climate change and their role in sea level rise.
In contrast to the polar regions, the network of lower latitude small glaciers and ice caps, although making up only about four percent of the total land ice area or about 760,000 square kilometers, may have provided as much as 60 percent of the total glacier contribution to sea level change since 1990s (Meier et al., 2007).
How glaciers' contribution to sea level is computed
Global mass balance data are transformed to sea-level equivalent by first multiplying the ice thickness (meters) lost to melting by the density of ice (about 900 kilograms per cubic meter), to obtain a water equivalent thickness, and then multiplying by the surface area of these "small" glaciers (about 760,000 square kilometers). This provides an annual average mass balance of approximately -0.273 meters for the period 1961 to 2005. When dividing the mass balance value by the surface area of the oceans (361.6 million square kilometers), the final result is 0.58 millimeters of sea level rise per year. The Glacier Contribution to Sea Level graph demonstrates how the contribution from melting glaciers began increasing at a faster rate starting in the 1990s. This is in agreement with high-latitude air temperature records. During the period 1960-1990, glaciers contributed 0.37 +/- 0.16 millimeters per year to sea level while during 1990-2004, the contribution increased to 0.77+/-0.22 millimeters per year (IPCC 2007). However, the latest predictions suggest possibly an even greater contribution by small glaciers and icecaps. Meier et al. (2007) conclude that with the current acceleration of glacier contribution to sea level rise, the total contribution from small glaciers and ice caps by the year 2100 is expected to be 240 +/- 128 millimeters, which represents an average annual increase of more than 2.0 millimeters per year.
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