Variations In Sea Level

There are several factors that can cause variations in sea-level rise, such as the following:

• the rebound (or rising up) of Earth's crust after the melting of ice from the last ice age

• plate tectonics and volcanism raising the height of the land's surface

• local subsidence of Earth's crust from groundwater extraction

• ground subsidence from sediment compaction

• changes in atmospheric wind patterns and ocean currents

One factor that must be taken into account when calculating sea-level rise is the mass balance. This is the difference between ice input and output in the glacial-ocean system. In other words, the amount of snowfall received in Antarctica and Greenland must be taken into account and calculated against the amount of ice that is melting into the ocean. If there is more melting of ice sheets than there is accumulation of snow on ice caps, then sea levels will rise. If the balance is the opposite, sea level will drop. Just as when all the ice cubes in a glass of lemonade melt, the level of the drink does not rise up and spill out of the glass, floating ice does not affect sea level when it melts, because it was already displacing the water at that location. Because of this, melting of the northern polar ice cap—which is floating pack ice—would not make sea levels rise. But just because it will not affect sea level does not mean melting the ice will not matter. Other, related melting ice in the area will affect sea level.

Although there are several mechanisms that cause short-term changes in sea level on the order of hours to weeks to months—such as storm surges, changes in water density and currents, El Niño events, and seasonal variations—it is the longer-term changes that affect global sea level that are receiving attention as they pertain to global warming. Along with water locked up in snow and ice, thereby affecting global sea level, geologic influence is another long-term factor that influences sea level. During an ice age, when the bulk of water on Earth is sequestered in ice, sea levels are much lower. This has also held true when Earth's continents have been nearer to the poles during past plate-tectonic periods. When the landmasses were tectoni-cally situated nearer to the equator, and the majority of water was not frozen, sea levels were much higher than they are today. Over the past several million years during glacial-interglacial periods, sea level has varied by hundreds of feet. Long-term changes that climatologists must keep in mind when studying global warming and assessing sea level changes are the following:

• Isostasy, or changes in the uplift or subsidence of Earth's crust after or during glaciations. As ice melts and the land's surface is relieved of the tremendous weight of the ice, the land will slowly rise as it returns to its previous elevation before the weight and stress of the ice was added. This can offset sea-level rise because the land is also rising in elevation.

• Change in the quantity of ocean water. Changes in sea level occur as precipitation (usually in the form of snow) is added to the Greenland and Antarctic Ice Sheets. As the temperature of the atmosphere increases, additional melting of ice caps and glaciers will occur, affecting the mass balance, adding additional water to the ocean, and causing sea levels to rise.

• Change in the configuration of the ocean basins. As plate tectonics carries on its continuous movements, changing the shape and elevation of the ocean floor and sides, this affects the shape of the basin, which in turn affects sea level. Tectonic uplift can also play a part and change sea levels.

According to an article that appeared in the New York Times on January 8, 2008, Greenland's ice is melting enough each year to become unstable and is now a concern to scientists for its role in potential sea-level rise. During the spring and summer months, the meltwater lakes and rivulets on the ice sheet absorb up to four times more energy from the Sun as unmelted snow. They form natural drainpipes, called "moulins," that penetrate straight through to the bedrock below. This process serves to lubricate the sheet of ice and helps move it faster toward the ocean. As a result, huge chunks of glaciers have been breaking off into the ocean, especially along the west coast.

According to Ted Scambos of the National Snow and Ice Data Center in Boulder, Colorado, scientists are concerned that if this process speeds as global warming increases, that sea levels will continue to rise, negatively impacting coastal areas worldwide. But, not all climate scientists share the viewpoint that the ice is unstable enough at this point to be of immediate concern. Richard Alley, from Pennsylvania State University, believes that the Greenland Ice Sheet has fluctuated throughout history and its current reaction to warming is simply part of a natural cycle.

Most important, however, is that no matter what experts' short-term views are, they seem to agree on the long term: If current greenhouse-gas emissions continue at the level they are today, the resulting global warming and loss of ice at Earth's polar regions will negatively impact the world's coastlines for centuries. Because of this, Eric Rignot of NASA's Jet Propulsion Laboratory stresses the importance of both government policymakers and the general public becoming aware of the issues and cutting back emissions from fossil fuels before it is too late. According to Rignot, if global warming is not slowed, sea levels could rise three feet (1 m) just from water flowing off Greenland, another three feet (1 m) from Antarctica, and 18 inches (0.5 m) from alpine glaciers. As Rignot says, "Things are definitely far more serious than anyone would have thought five years ago."

According to a report in USA Today, scientists are focusing on the West Antarctic Ice Sheet because water melted off of it could force global sea levels to rise an extra 20 feet (6 m). Because the ice sheet's bottom is mostly below sea level, the West Antarctic sheet is considered more likely to collapse than the East Antarctic sheet. The west sheet's ice moves onto the Ross and Ronne Ice Shelves, which are floating on the ocean. The Ross Ice Shelf is huge; it measures 450 by 600 miles (724 by 966 km) and up to 4,000 feet (1,219 m) in thickness. If global warming continues to warm the atmosphere and oceans, it could melt the shelves.

Scientists monitor the flow of ice from the West Antarctic sheet to the shelves as they travel in channels called ice streams. According to Robert Bindschadler of NASA's Goddard Space Flight Center in Green-belt, Maryland, it is important to monitor the ice streams to get an idea of where the most unstable areas are. The scientists there place GPS receivers on the ice rivers in order to receive location and elevation data from the satellite, precisely describing ice movement.

Currently, scientific consensus, based on the Goddard study, is that the ice at Antarctica is close to being "in balance" and not a threat at this point to sea-level rise. According to Michael Oppenheimer of the Environmental Defense Fund, he believes it would take several hundred years for Antarctica to melt, if global warming were to reach that point.

Scientists are concerned, however, that global warming could make the Ross Ice Shelf become thinner over time. A worst-case scenario would be for the shelf to disappear, leaving the Antarctic Ice Sheet vulnerable to collapse and melting of ice, which could hypothetically raise sea levels up to 20 feet (6 m) in 250 to 400 years. Oppenheimer believes the most likely scenario is that the Ross Ice Shelf will gradually melt over the next century and be gone within the next 200 years. During this melting period, the Antarctic will add up to seven inches (18 cm) per century to global sea-level rise.

Because of the important role that the Antarctic Ice Sheet plays in global warming and potential sea-level rise and the fact that the mass balance (whether it is shrinking or growing) is not well understood, understanding the ice's dynamics has been given a very high priority by the Polar Research Board of the National Research Council, the Scientific Committee on Antarctic Research (SCAR), and the NSF's Office of Polar Programs. The illustration on page 105 shows the various ice shelves currently being studied.

While the ice sheets represent a large potential source of sea-level rise, another source is from the long-term melting of the world's glaciers. According to the USGS and IPCC, the following tables depict the estimated potential maximum sea-level rise from the total melting of today's ice, as well as documented melting that has occurred from 1961 to 2003.

Estimated Maximum Sea-Level Rise from the Total melting of Present-Day Glaciers

LOCATION

VOLUME (CUBIC MILES/CUBIC KILOMETERS)

POTENTIAL SEA-LEVEL RISE (FEET/METERS)

East Antarctic Ice Sheet

6,247,136/26,039,200

213/64.80

West Antarctic Ice Sheet

782,595/3,262,000

26/8.06

Antarctic Peninsula

54,484/227,100

1.5/.46

Greenland

628,571/2,620,000

21/6.55

All other ice caps, ice fields, and valley glaciers

43,184/180,000

1.5/0.45

Total

7,755,972/32,328,300

263.5/80.32

Source: USGS

Antarctica has many ice shelves that could thin, melt, and cause a disruption to the delicate ecological balance of the polar region.

Sea-Level Rise (inches/centimeters per year)

source

1961-2003

1993-2003

Thermal expansion

0.02/0.05

0.02/0.05

Glaciers and ice caps

0.02/0.05

0.03/0.08

Greenland Ice Sheet

0.002/0.005

0.008/0.020

Antarctic Ice Sheet

0.006/0.015

0.008/0.020

Sum

0.04/0.10

0.1/0.25

Observed

0.07/0.18

0.1/0.25

Source: IPCC, 2007

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