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Sea-level rise is one of the projected changes that will accompany global warming. In this unit we examine the process of sea-level rise - what causes sea-level rises, how it is measured, and possible causes of changes in sea level other than anthropogenic global warming. Then we consider the contribution due to increased greenhouse warming and some of the impacts of the resulting changes.

Recent measurements show a sea-level rise of about 1 to 2 millimeters per year or 100 millimeters in the last century. Sources of error in these measurements come from intra-annual variations due to changing meteorological conditions, such as persistence of wind from a particular direction or at a particular speed. A stronger than normal wind from the west will cause an apparent sea level rise on the west coast of continents and lower sea level on the east coasts. A similar artificial change in sea level could arise from persistently anomalous ocean circulation. By use of historical records and numerical models, the contribution from these sources can be removed to more accurately reveal actual sea-level rise. We need significant amounts of data to do this, however, and there may be a historical geographical bias that favors North America, Northern Europe and Japan. Southern Hemisphere records are much less complete.

Relative sea level may change in response to vertical land movement or changes in the level of the ocean surface as shown in the accompanying image.

Causes of relative sea-level change. Earthquest, UCAR.

Vertical movements of the land may give the appearance of a rising sea level. For instance, sediment carried by a major river such as the Mississippi River is deposited in its delta area where it empties into the ocean. The sediment and nutrients discharged by slowly moving waters as the river meanders over the low-lying flood plain have built up marshes at a rate that has matched or exceeded the natural subsidence of the land due to settling and horizontal expansion of the saturated silt that comprises the delta subsurface. Flood control structures and drainage practices of the last 50 years have prevented the marshes from receiving replacement soil, leaving them in a state of slow and irreversible subsidence. Coastal land in Louisiana presently is being lost at a rate of 50 square miles per year, which will mean loss of land equivalent to the size of the state of Rhode Island in 21 years.

Extraction of groundwater or oil in coastal areas also can lead to settling or outward movement of the land surface. Natural vertical movements in coastal regions may be due to interaction of continental plates on geological time scales. Another process operating on time scales of 10,000 years is the process of isostatic rebound. During the last ice age, the whole northern North American Continent was covered with ice several kilometers thick. This represents an enormous amount of weight on the continent. Melting of this ice over a few thousand years (which may be considered abrupt on geological time scales) leads to an upward rebound of the continental plate due to its elastic characteristics. This rebound is similar to the rebound of a bed mattress when you jump out of bed. Vertical motion due to elastic rebound can be calculated and eliminated from sea-level measurements on the basis of known elastic properties of earth materials and knowledge of the mass of ice previously located over the continent.

Sea level is known to be rising, but there is no convincing evidence that the rate of rise has increased during the 20th century: the rate of sea level rise seems to be constant and has not measurably increased due to global warming of the last 90 years. There is weak evidence for an acceleration over the last two to three centuries that could be due to the warming since the last ice age.

If sea level were to change due to global warming, the contributing factors would be (1) thermal expansion of the oceans, (2) melting of glaciers and small ice masses in mountainous areas or high latitudes, (3) melting of the Greenland ice sheet, or (4) break-off of the West Antarctic ice sheet, and (5) change in the mass balance of the Antarctic ice sheet. Thermal expansion refers to the expansion of ocean water in a fixed size basin that results in a rise in sea level in coastal regions.

Measurements over the last 100 years shown in the accompanying graph show a change of about 12 centimeters for 100 years, or about 1 to 2 millimeters per year. The two different data sets shown seem to agree reasonably well.

Global mean sea-level rise over the last century. IPCC, 1990.

Observations of the terminus (furthest extent down a mountain valley) of a glacier provide evidence of changes in the total ice volume of the glacier. Although not a quantitative measurement of total ice volume, glacier termini offer, though old photographs, paintings, and historical accounts, a long record of changes in ice volume. The accompanying graph shows the changes in terminus locations for 6 different glaciers in Norway, Iceland, France, Switzerland and Austria. Records from certain parts of Europe go back to the 1600s.

Variations of some selected glaciers as measured by their length. IPCC, 1990

An example of the change in a glacier terminus is shown in the next two photographs taken in the Quelccaya Ice Cap in the tropical Andes in Peru. The first photo was taken in 1978 and shows large ice masses in the near vicinity of a boulder. The same site photographed in 1995 shows an ice free rock field surrounding the boulder.

First Photo Second Photo

Characteristics of glacier ice on earth are given in the next figure. The ice mass of the Antarctic continent contains about 90% of glacier ice and most of the remaining ice is on Greenland, with only a small amount in the remaining continental glaciers. Melting the Greenland ice mass would raise sea level by 7 meters and melting the Antarctic glacier would cause a rise of 65 meters. A glacier can lose mass by ablation (evaporation and melting/runoff) and calving (breaking off of icebergs at the outer margin of ice masses extending offshore). If the glacier is in equilibrium (not gaining or losing mass) the accumulation rate must equal the loss rate, defined as the sum of ablation and calving. Note that the mass loss due to ablation is inconsequential on Antarctica but comparable with calving on Greenland. The mass turnover time given in the last line is the total mass of ice in the sheet divided by the accumulation rate. This essentially is an estimate of the length of time it takes to accumulate the total mass of the sheet.

Physical characteristics of glacier ice on earth. Adapted from IPCC, 1990.

Both accumulation and loss depend on temperature, but in quite different ways as is shown in the next image. Ablation refers to loss of mass by evaporation and melting, and accumulation occurs by snowfall (and possibly some rain or freezing rain on ice fields). The accompanying graph gives the dependence of ablation and accumulation (expressed in amount of mass in arbitrary units per year) on temperature. At very low temperatures, ice loss is essentially negligible because melting is non-existent and evaporation is extremely low. Accumulation is proportional to temperature. As the annual temperature rises but remains well below 0 degrees C, accumulation rises slowly with temperature because warmer air permits more water to exist in the vapor phase and hence allow for more snowfall. As annual temperature rises further, evaporation begins to become somewhat more important, and there will be some times during the year at which melting will occur, leading to an increase in ablation. As the annual temperature approaches 0 degrees C, evaporation and, especially, melting increase rapidly. Meanwhile accumulation increased gradually with increased water vapor being available for snowfall. Even when the annual temperature is above freezing, some accumulation will occur at some times of year, but loss will be far larger.

Dependence of ablation and accumulations on annual surface temperature. IPCC, 1990.

The lower figure (from the image above) shows the combined effects of these individual processes. For very low temperatures, accumulation dominates (curve has values larger than zero). Eventually as temperature rises, the curve peaks and plummets downward passing zero and rapidly becoming negative to a point somewhat less than 0 degrees C where the ice mass is completely gone (ice may be present during winter season, but it no longer persists through all seasons). The ice sheet gains mass for temperatures represented by the dark shading and losses mass for temperatures indicated by the light shading. Also, for temperatures in the range denoted by the letter "b", an increase in temperature cause a loss of ice mass. By contrast, for temperatures in the range indicated by "a", a temperature increase will actually increase the mass of ice.

To determine whether a particular ice sheet (e.g., Greenland or Antarctica) will gain or lose mass under global warming, we need to know where its mean annual temperature is located on the previous graph. It turns out that the Greenland mean annual temperature is in region b and Antarctica's is in region a. From this we will conclude that, for a small amount of warming, Antarctica will gain ice mass (causing sea level to drop) and Greenland will lose mass (leading to sea-level rise).

The next image reveals an interesting factor relating to the change in a continental ice sheet. The top figure is a schematic representation of how an ice mass responds to changes in temperature. In certain temperature ranges, depending on the characteristics specific to each ice mass, irreversible changes can occur. An ice mass experiencing warming will at some temperature not be able to sustain ice continuously over the annual cycle of warming and cooling. Once this happens, the ice mass cannot become re-established by a modest cooling. As indicated by the line at zero ice volume, the temperature must lower substantially (the amount depending on local conditions) for the ice mass to survive the annual cycle to become re-established. This is due to the altitude - mass balance feedback as is shown in the bottom sketch.

Generalized mass balance field in which the ice sheets of Antarctica and Greenland are projected.

The bottom diagram gives symbolic representations of ice masses on Antarctica and Greenland. The closed curves represent cross-sections of the continents from sea level to the peaks of the continents. These symbolic continents are placed on a mass-balance diagram that has regions of mass accumulation (positive numbers) and regions of mass loss (negative numbers). Note that accumulation increases with altitude to a maximum and then decreases at higher altitude (lower temperatures) because of the lower water-vapor content of cold air. If the glacier did not slide down the continent toward sea level, the glacier would not exist below the zero line on the sketch. But because of glacier movement, the region of accumulation feeds ice to the region of net mass loss.

Global warming causes the pattern of horizontal lines to move upward on this diagram. As the zero line moves upward toward the highest elevation on the continent, the accumulation zone becomes very small (see Greenland). Eventually the glacier disappears. To re-establish the glacier, the zero line must move well below the top of the continent (i.e., the temperature must cool well below the value that led to demise of the glacier). This is the altitude - mass balance feedback that gives the lower critical point on the upper sketch.

Note that Antarctica is not near a critical point and, from the lower graph, this continent will experience increased mass accumulation with initial warming, but more intense warming (far greater than is likely due to present projections) also will send it to a critical point.

Because the mass accumulation region of Antarctica extends to down to sea level, there is very little loss of mass by melting and runoff, as shown in the next image. The primary mass-loss mechanism is calving, as illustrated for Antarctica in the lower of these two sketches. The West Antarctic ice sheet extends out over the water well beyond the point of attachment to the continent, known as the pinning point. The glacier slowly moves out over the ice and at some point creates enough stress to break off. The next image gives another illustration of his process.

Antarctic Ice. Earthquest, UCAR Processes affecting the stability of the West Antarctic Ice Sheet.

There is some speculation that, because it extends so far out from its pinning point, the whole West Antarctic ice sheet may collapse. If that happens it is estimated that global sea level would rise approximately 7 meters. Such a rise in sea level would inundate the southern third of Florida. Recent estimates suggest collapse is not imminent although it is not inconceivable.

In February of 1995 we received information over the Internet that a major segment of ice broke off the Ross Ice Shelf, which caused an enormous amount of excitement among glaciologists that were in the Antarctic Region, as was shown in their e-mail exchange. Glaciologists study processes that happen at glacial speeds: a movement of 1 cm per year may be considered "lightning speed" for some glacial processes. During this period they were witnessing an event that occurs only once in a lifetime, so they were ecstatic to observe these events. The "before" and "after" satellite photos show that an iceberg the size of Rhode Island had broken from the ice bridge connecting James Ross Island to the main Antarctic continent. It now is possible to navigate a ship completely around the island, something never before possible. A student in the class at the time calculated that the estimated mass of the ice block would cause global sea level to rise about a millimeter, which is the equivalent of the rise over an entire year due to natural warming at the rate of the last 100 years.

Icebergs that break free from the continent are subjected to persistent off-shore winds that create regions of open water between the continent and the seaward-drifting ice mass. Strong and persistent off-shore winds are created due to continuous cooling over the Antarctic continent that leads to downslope flow from the high latitudes on the continental interior. Although the upper surface of the ice may cool to very low temperatures, the water between icebergs, known as polynyas, will remain near 4 degrees C since further cooling will increase its density and cause it to sink. These regions of open water represent relative heat sources for the atmosphere, since they may be several degrees warmer than the ice surfaces of the surrounding icebergs. Water near the freezing point is not normally considered to be a source of heat, but compared to ice at -20 C, it is comparatively "hot". This relatively warm water encourages evaporation, leaving behind water whose density is rapidly increasing due to both cooling and increased salt concentration during evaporation. This process creates downwelling plumes of cold, highly saline water that leads to production of deep water as discussed in the lecture on ocean structure and circulation and is shown in the accompanying image.

Latent heat and open ocean polynya. Large scale meridional circulation of the ocean.

The next image shows the mass balance for Greenland. Note for this ice sheet that calving and melting/runoff make nearly equal contributions to the loss of mass and their sum equals the accumulation rate to give a steady state to the total glacier mass. The following image gives results of earlier estimates of the Greenland ice mass balance showing some discrepancy in estimates among different observers. Data given on the previous diagram should be considered more recent and reliable estimates.
	Predicted sea-level rise according to various IPCC emissions scenarios.
	Annual mass balance of Greenland Ice Sheet.

Estimates have been made of how the warming as projected by climate models will change the mass balance lines for the Greenland and Antarctic ice sheets of previous graphs. The next plot shows temperature scenarios from climate models and the resulting changes in sea level due to thermal expansion of the oceans, melting of mountain glaciers, melting on Greenland, increased calving of the West Antarctic ice sheet, and the change in the mass balance of the rest of the Antarctic continent. Note that the increased accumulation by Antarctica due to global warming will suppress sea-level rise. More recent calculations of sea level rise for different policy scenarios (discussed in precious lectures) are shown in the accompanying plot.

Temperature scenario used in present estimate of sea-level changes. Global sea-level rise for scenarios B, C, and D and for "business-as-usual". IPCC, 1990.

The final image gives a sketch of how beach erosion proceeds under rising sea levels. Also given here is a map of major river delta areas of the world that are at risk to flooding under sea-level rise. Historically, locations where navigable rivers (i.e., ones entering from flat continental areas) entered major oceans were prime areas for the development of cities. These cities, now some of the major cities of the world, are home to millions of people that potentially could be at risk under rising sea levels due to global warming.

Major delta areas vulnerable to sea-level rise.

Transcription by Theresa M. Nichols