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Abrupt Change in Sea Level
ABRUPT CHANGES IN THE EARTH’S CLIMATE SYSTEM
ABRUPT CHANGE IN SEA LEVEL
Abrupt change in sea level
Population densities in coastal regions and on islands are
about three times higher than the global average, with approximately 23% of the
world’s population living within 100 kilometers (km) distance of the coast and
>100 meters (m) above sea level (Nicholls et al., 2007). This allows even small sea level rise to have
significant societal and economic impacts through coastal erosion, increased
susceptibility to storm surges and resulting flooding, ground-water
contamination by salt intrusion, loss of coastal wetlands, and other issues
Figure 1.2. Portions (shown in red) of the Southeastern United States, Central America, and the
Caribbean surrounding the Gulf of Mexico that would be inundated by a 6-mete sea level rise
(from Rowley et al., 2007). Note that additional changes in the position of the coastline may occur in
response to erosion from the rising sea level.
An increase in global sea level largely reflects a contribution from water expansion from warming, and from the melting of land ice which dominates the actual addition of water to the oceans. Over the last century, the global average sea rose at a rate of ~1.7 ± 0.5 millimeters per year (mm yrˉ1). However, the rate of global sea level rise for the period 1993 to 2003 accelerated to 3.1 ±0.7 mm yrˉ1, reflecting either variability on decadal time scales or an increase in the longer term trend. Relative to the period 1961-2003, estimates of the contributions from thermal expansion and from glaciers and ice sheets indicate that increases in both of these sources contributed to the acceleration in global sea level rise that characterized the 1992-2003 period (Bindoff et al., 2007).
By the end of the 21st century and in the absence of ice-dynamical contribution, the IPCC AR4 projects sea level to rise by 0.28 ± 0.10 m to 0.42 ± 0.16 m in response to additional global warming, with the contribution from thermal expansion accounting for 70-75% of this rise (Meehl et al., 2007). Projections for contributions from ice sheets are based on models that emphasize accumulation and surface melting in controlling the amount of mass gained and lost by ice sheets (mass balance), with different relative contributions for the Greenland and Antarctic ice sheets. Because the increase in mass loss (ablation) is greater than the increase in mass gain (accumulation), the Greenland Ice Sheet is projected to contribute to a positive sea level rise and may melt entirely from future global warming (Ridley et al., 2005). In contract, the Antarctic Ice Sheet is projected to grow through increased accumulation relative to ablation and thus contribute to a negative sea level rise. The net projected effect on global sea level from these two differing ice-sheet responses to global warming over the remainder of this century is to nearly cancel each other out. Accordingly, the primary contribution to sea level rise from projected mass changes in the IPCC AR4 is associated with retreat of glaciers and ice caps (Meehl et al., 2007).
Rahmstorf (2007) used the relation between 20th century sea level rise and global mean surface temperature increase to predict a sea level rise of 0.5 to 1.4 m above the 1990 level by the end of the 21st century, considerably higher than the projections by the IPCC AR4 (Meehl et al., 2007). Insofar as the contribution to 20th century sea level rise from melting land ice is thought to have been dominated by glaciers and ice caps (Bindoff et al., 2007), the Rahmstorf (2007) projection does not include the possible contribution to sea level rise from ice sheets.
Recent observations of startling changes at the margins of the Greenland and Antarctic ice sheets indicate that dynamic responses to warming may play a much greater role in the future mass balance of ice sheets than considered in current numerical projections of sea level rise. Ice-sheet models used as the basis for the IPCC AR4 numerical projections did not include the physical processes that may be governing these dynamical responses, but if they prove to be significant to the long-term mass balance of the ice sheets, sea level projections will likely need to be revised upwards substantially. By implicitly excluding the potential contribution from ice sheets, the Rahmstor (2007) estimate will also likely need to be revised upwards if dynamical processes cause future ice-sheet mass balance to become more negative.
The Greenland Ice Sheet is losing mass and very likely on an acceleratedpath since the mid-1990s. Observations show that Greenland is thickening at high elevations, because of an increase in snowfall, but that this gain is more than offset by an accelerating mass loss at the coastal margins, with a large component from rapidly thinning andaccelerating outlet glaciers. The mass balance of the Greenland Ice Sheet during the period with good observations indicates that the loss increased from 100 gigatons per year (Gt a–1) (where 360 Gt of ice = 1 mm of sea level) in the late 1990s to more than 200 Gt a–1 for the most recent observations in 2006.
Determination of the mass budget of the Antarctic ice sheet is not as advanced as that for Greenland. The mass balance for Antarctica as a whole has experienced a net loss of about 80 Gt a-1 in the mid-1990s, increasing to almost 130 Gt a-1 in the mid-2000s. There is little surface melting in Antarctica, but substantial ice losses are occurring from West Antarctica and the Antarctic Peninsula primarily in response to changing ice dynamics.
The record of past changes provides important insight to the behavior of large ice sheets during warming. At the last glacial maximum about 21,000 years ago, ice volume and area were about 2.5 times modern. Deglaciation was forced by warming from changes in the Earth’s orbital parameters, increasing greenhouse gas concentrations, and attendant feedbacks. Deglacial sea level rise averaged 10mm a-1, but with variations including two extraordinary episodes at 19,000 years ago (ka) and 14.5 ka when peak rates potentially exceeded 50mm a-1 (Fairbanks, 1989; Yokoyama et al., 2000). Each of these "meltwater pulses” added the equivalent of 1.5 to 3 Greenland ice sheets (~10-20 m) to the oceans over a one- to five- century period, clearly demonstrating the potential for ice sheets to cause rapid and large sea level changes.
The primary factor that raises concerns about the potential of future abrupt changes in sealevel is that large areas of modern ice sheets are currently grounded below sea level. Where it exists, it is this condition that lends itself to many of the processes that can lead to rapid ice-sheet changes, especially with regard to atmosphere-ocean-ice interactions that may affect ice shelves and calving fronts of glaciers terminating in water (tidewater glaciers). An important aspect of these marine-based ice sheets is that the beds of ice sheets grounded below sea level tend to deepen inland. The grounding line is the critical juncture that separates ice that is thick enough to remain grounded from either an ice shelf or a calving front. In the absence of stabilizing factors, this configuration indicates that marine ice sheets are inherently unstable, whereby small changes in climate could trigger irreversible retreat of the grounding line.
The amount of retreat clearly depends on how far inland glaciers remain below sea level. Of greatest concern is the West Antarctic Ice Sheet, with 5 to 6m sea level equivalent, where much of the base of the ice sheet is grounded well below sea level, with deeper trenches lying well inland of their grounding lines. A similar situation applies to the entire Wilkes Land sector of East Antarctica. In Greenland a number of outlet glaciers remain below sea level, indicating that glacier retreat by this process will continue for some time. A notable example is Greenland’s largest outlet glacier, Jakobshavn Isbrae, which appears to tap into the central region of Greenland that is below sea level. Accelerated ice discharge is possible through such outlet glaciers, but we consider the potential for destabilization of the Greenland Ice sheet by this mechanism to be very unlikely.
The key requirement for stabilizing grounding lines of marine-based ice sheets appears to be the presence of an extension of floating ice beyond the grounding line, referred to as an ice shelf. A thinning ice shelf results in ice-sheet ungrounding, which is the main cause of the ice acceleration because it has a large effect on the force balance near the ice front. Recent rapid hanges in marginal regions of both ice sheets are characterized mainly by acceleration and thinning, with some glacier velocities increasing more than twofold. Many of these glacier accelerations closely followed reduction or loss of ice shelves. If glacier acceleration caused by thinning ice shelves can be sustained over many centuries, sea level will rise more rapidly than currently estimated.
Such behavior was predicted almost 30 years ago by Mercer (1978) but was discounted as recently as the IPCC Third Assessment Report (Church et al., 2001) by most of the glaciologicalcommunity based largely on results from prevailin model simulations. Considerable effort is now underway to improve the models, but it is far from complete, leaving us unable to make reliable predictions of ice-sheet responses to a warming climate if such glacier accelerations were to increase in size and frequency.
A nonlinear response of ice-shelf melting to increasing ocean temperatures is a central tenet in the scenario for abrupt sea-level rise arising from ocean-ice-shelf interactions. Significant changes in ice-shelf thickness are most readily caused by changes in basal melting. The susceptibility of ice shelves to high melt rates and to collapse is a function of the presence of warm waters entering the cavities beneath ice shelves. Future changes in ocean circulation and ocean temperatures will produce changes in basal melting, but the magnitude of these changes is currently neither modeled nor predicated.
Another mechanism that can potentiallyincrease the sensitivity of ice sheets to climate change involves enhanced flow of the ice over its bed due to the presence of pressurized water, a process known as sliding. Where such basal flow is enabled, total ice flow rates may increase by 1 to 10 orders of magnitude, significantly decreasing the response time of an ice sheet to a climate or ice-marginal perturbation.
Recent data from Greenland show a high correlation between periods of heavy surface melting and an increase in glacier velocity (Zwally et al.,2002). A possible cause for this relation is rapiddrainage of surface meltwater to the glacier bed, where it enhances lubrication and basal sliding. There has been a significant increase in meltwater runoff from the Greenland Ice Sheet for the 1998–2007 period compared to the previous three decades (Fig. 1.3). Total melt area is continuing to increase during the melt season and has already reached up to 50% of the Greenland Ice Sheet; further increase in Arctic temperatures will very likely continue this process and will add additional runoff. Because water represents such an important control on glacier flow, an increase in meltwater production in a warmer climate will likely have major consequences on flow rate and mass loss. Because sites of global deep water formation occur immediately adjacent to the Greenland and Antarctic ice sheets, any significant increase in freshwater fluxes from these ice sheets may induce changes in ocean heat transport and thus climate. This topic is addressed inChapter 4 of this report.
Figure 1.3. The graph shows the total melt area 1979 to 2007 for the Greenland ice sheet derived from passive
microwave satellite data. Error bars represent the 95% confidence interval. The map inserts display the area of melt
for 1996, 1998, and the record year 2007 (from K. Steffen, CIRES, University of Colorado).
The Greenland and Antarctic Ice Sheets are losing mass, likely at an accelerating rate. Much of the loss from Greenland is by increased summer melting as temperatures rise, but an increasing proportion of the combined mass loss is caused by increasing ice discharge from the ice-sheet margins, indicating that dynamical responses to warming may play a much greater role in the future mass balance of ice sheets than previously considered. The interaction of warm waters with the periphery of the ice sheets is very likely one of the most significant mechanisms to trigger an abrupt rise in global sea level. The potentially sensitive regions for rapid changes in ice volume are thus likely those ice masses grounded below sea level such as the West Antarctic Ice sheet or large glaciers in Greenland like the Jakobshavn Isbrae with an over-deepened channel (channel below sea level, Fig.2.10) reaching far inland. Ice-sheet models currently do not include the physical processes that may be governing these dynamical responses, so quantitative assessment of their possible contribution to sea level rise is not yet possible. If these processes prove to be significant to the long-term mass balance of the ice sheets, however, current sea level projections based on present-generation numerical models will likely need to be revised substantially upwards.
Figure 2.10. Bedrock topography for Greenland; areas
below sea level are shown in blue. Note the three channels
in the north (1: Humboldt Glacier; 2: Petermann Glacier;
3: 79-North Glacier or Nioghalvfjerdsfjorden Glacier) and
at the west coast (4: Jakobshavn Isbrae) connecting the
region below sea level with the ocean (Russell Huff and
Konrad Steffen, CIRES, University of Colorado at Boulder).
NOTE: The term "forcing” is used throughout this report to indicate any mechanism that causes the climate system to change, or respond. Examples of forcings discussed in this report include freshwater forcing of ocean circulation and changes in sea-surface temperatures and radiative forcing as a forcing of drought. As defined by the IPCC Third Assessment Report (Church et al., 2001), radiative forcing refers to a change in the net radiation at the top of the troposphere caused by a change in the solar radiation, the infrared radiation, or other changes that affect the radiation energy absorbed by the surface (e.g., changes in surface reflection properties), resulting in a radiation imbalance. A positive radiative forcing tends to warm the surface on average, whereas a negative radiative forcing tends to cool it. Changes in GHG concentrations represent a radiative forcing through their absorption and emission of infrared radiation.
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