Effects of Climate Change on Global Ice Coverage

Atmospheric Chemistry and Physics 52:163

University of Iowa

Written By:

Ben Nothwehr

Matt Petersen

Matt Wanat

December 16, 1998

Contents

Abstract - Next Section - Contents

The intention of this report is to provide some background information on how global warming affects global ice reserves. These reserves are found mostly in the polar ice caps and the Greenland Ice Sheet. Glaciers are included due to their importance in the role of sea level rise. Along with the current status of the global ice coverage we have utilized global climate model data in order to speculate sea level effects and the fate of the polar ice caps. As expected the situation will worsen before it gets better. Climate and ecological impacts have also been discussed to illustrate the importance of the ice caps in the global biosphere.

Current Status of the Global Ice Coverage - Next Section - Contents

The approximate global sea ice coverage is 9.6 million square miles on a yearly average. The arctic polar ice cap fluctuates from 10 million square miles in the winter to 6 million square miles in the summer. The Antarctic varies more dramatically than its northern counterpart with a wintertime maximum coverage of 12 million square miles and summertime low of 2 million square miles. The larger variability is due to the fact that the Arctic polar ice cap is surrounded by land, which acts as a heat sink in summer and keeps the water at a stable temperature conducive to sea ice formation. Figure 1 illustrates the large variability of ice extent in both poles.

Fig 1. Yearly Variation of Sea Ice extent for Arctic and Antarctic regions

Within the past 20 years researches have monitored a gradual reduction in the total amount of sea ice formed each year. The Arctic has experienced a 2.5 percent decrease in land area coverage from 1978 to 1987 and a 4.3 percent decrease from 1987 to 1994. Decrease of area covered along with several specific ice shelf break-ups has recently highlighted this trend. An example of a break-up is the large iceberg, approximately the size of Luxembourg, breaking away from the Larsen Ice Shelf of the Antarctic Peninsula. Figure’s 2 and 3 below are before and after satellite images of the ice shelf. The images are just six weeks apart. Note the large iceberg just off of the shelf in the Weddell Sea.

Fig 2. Jan 5, 1995 image of Northern Antarctic Peninsula

Fig 3. Feb 27, 1995 image of Northern Antarctic Peninsula

The actual breakaway of the ice shelf alone does not suggest increasing Antarctic temperature. When it is coupled with the fact that the total amount of ice discharged from these recent events is three to four times the annual continental snow accumulation it does however suggest that the mass of ice is decreasing, with increasing temperatures being the cause.

Polar ice caps play large roles in the determination of the global climate. They affect mean surface temperature through various channels such as Earth albedo, topographic, and ocean circulation. Some of these topics will be discussed in detail later within the report.

Future Coverage based on Climate Change Projections - Next Section - Contents

The future of the world ice reserves is in jeopardy. Some scientists believe that the West Antarctic Ice Sheet could slide into the oceans after a sustained warming. The vulnerability of this ice sheet is poorly understood. It contains enough ice to raise sea level 6 meters (20 feet), and coastal scientists generally agree that sea level was 20 feet higher than today during the last interglacial period, which was only slightly warmer than today.

Fig 4. Map of The Western Antarctica Ice Shelf. Hatched line is edge of continental Shelf

Various temperature scenarios suggested that ice retreat around the Antarctic Peninsula and the east Antarctic coast would be pronounced while the Ross and Ronne- Filchner Ice Shelves might experience only a small retreat. In the same year, NASA scientists speculated that fast-flowing ice streams on the Antarctic Ross Ice shelf may indicate ice loss from the West Antarctic Ice Sheet. A report of a January 1990 workshop on the problem concluded that "the presence of five active ice streams feeding ice from the interior of West Antarctica into the Ronne Ice Shelf and separate peripheral ice shelves on the northern boundary of the ice sheet, may be manifestations of collapse already underway."

Recent measurements at Pine Island Glacier increase fears of ice sheet collapse. Pine Island Glacier is located in western Antarctica and flows into the Amundesen Sea from Pine Island Bay. There is no substantial ice shelf in Pine Island Bay therefore most of the ice is land based and does not contribute to sea level rise (this will be explained in greater detail in the next section). These glaciers are likely to be the weakest points in the potentially unstable ice sheet because they flow directly into the sea. A recent study shows that the base of Pine Island Glacier is melting at a rate in excess of ten meters per year, more than an order of magnitude higher than has been estimated for the larger Antarctic ice shelves. Oceanographic measurements show relatively warm salty water near the sea floor and fresher outflows nearer the surface, which may be caused by meltwater from the glacier. This suggest that the West Antarctic Ice Sheet could be especially vulnerable if global warming increases deep ocean temperatures and accelerates ice cap melting.

Effects of Ice Coverage on Sea Level - Next Section - Contents

Increasing global mean surface temperature is likely to raise sea level by expanding ocean water, and melting glaciers and portions of the Greenland Ice Sheet. Warmer polar ocean temperatures could also melt portions of the Larsen and other Antarctic ice shelves, which might increase the rate at which Antarctic ice streams convey ice into the oceans.

Melting ice shelves do not directly correlate to sea level rise. Once an iceberg becomes a free floating entity it has already made its contribution to sea level rise. Ice shelves are considered to have past a grounding line between grounded and floating ice, therefore large iceberg breakaways are not considered to have a large affect on sea level. This is also a reason that the arctic ice cap is not a major contributing factor to sea level rise.

Fig 5. Movement of ice from a land based sheet to an ice shelf

Glaciers in temperate regions such as Patagonia and Alaska will react much faster and have a larger contribution to sea level rise than the polar ice caps. This is compounded by that fact that they do not already have an impact on the sea level, as ice shelves do. The conclusion can be made that the mass of ice present in the particular system (e.g. temperate glacier, ice cap) will have an overwhelming dominance on the rate of its response to a climate change.

Another factor leading to sea level rise is thermal expansion of the oceans. Although this does not lie within the scope of this project it should be noted that thermal expansion does have a large contribution to the rising sea level. According to the EPA, thermal expansion could account for 40% to 60% of the total sea level rise. The amount of Antarctic precipitation also has an impact on sea level rise. Increasing precipitation is a direct result of higher temperatures at the poles. Larger precipitation levels will lessen the effect of Antarctic ice cap melting on sea level rise assuming that the ice cap mass stays relatively constant.

Projections of future sea level rise has been determined using results of the Image 2 global climate model developed by the National Institute of Public Health and the Environment in the Netherlands. Image 2 is an integrated global climate model used to determine future climate based on population and economic growth scenarios. The model is built using three separate subsystems, the Energy - Industry System, Terrestrial Environmental System, and Atmosphere - Ocean System. Economic and demographic input is used by the three systems to calculate greenhouse gas emissions and related climate change. Climate and land use data is used as feedback for successive iterations of the model.

Estimates show that average sea level is currently rising anywhere from 1.0 to 3.0 mm/yr. Using the Image 2 software described above, sea level can also be examined as a function of climate change scenario. Table 1 summarizes the results of four scenarios effects on sea level.

Impacts of the rise in sea level will be two fold. First, low lying coastal lands will be flooded and be forced to deal with the effects. South Florida, for example, is very low in elevation. One third of Everglades National Park has an elevation of less than one foot. Delicate coastal ecosystems will be destroyed due to salt water intrusion and higher frequencies of storm surges. The number of people at risk to storm surges globally would also double. Secondly, increasing the surface area of the ocean will also have a feeding affect to greenhouse effect at the poles by providing a larger heat sink from the atmosphere and adding to sea ice reduction. Further global impacts of reduced ice coverage are discussed in the next section.

Effects of Ice Coverage on Climate Change - Next Section - Contents

The most direct effect of sea ice loss on climate change is the reduction of the deep ocean currents. When sea ice is formed salt is rejected to the surrounding water, making it denser. That denser water is displaced downward as warmer water from the tropics flows towards the poles. Density differences in water mass account for about 90% of the total circulation driving force. The other 10% is due to wind driven circulation. Temperature and salinity are the primary causes for differences in water mass densities. Density driven currents are also referred to as thermohaline circulation. As sea ice is reduced the amount of salt displaced by the freezing process is also reduced. The result is ocean water that is less dense than under the previous conditions and a slower ocean current. Higher temperatures at the poles also add to this problem.

Fig 6. Atlantic Ocean Currents

As can be seen in the circulation diagram above there are convergent zones in the Arctic and Antarctic. These zones more or less act as a turnover point in the overall ocean circulation. Ocean currents directly affect global warming as a CO₂ sink. Carbon Dioxide is absorbed into the ocean and transported to intermediate and deep ocean water. As ocean currents decrease, the amount of CO₂ removed from the atmosphere also decreases. Lessening the impact of the ocean carbon sink as a positive force in the global warming framework is one reason why the Arctic will experience much more warming than temperate regions of the globe. Other related factors that are leading to this conclusion are warmer waters being transported to the poles from surface currents passing through temperate and equatorial regions, along with the increased water surface area at the polar regions. This will increase the heat capacity of the topography and allow more energy to be stored at the surface.

Effects of Ice Coverage on Biological Resources - Contents

Reduction in the amount of global ice mass has many consequences on ecological systems. These effects range from loss of coastal wetland habitat from increasing sea level to lowering the fertility of ocean water under decreased ocean currents. The direct effects that we focused primarily dealt with ecological systems in either the Arctic or Antarctic regions.

American researchers have blamed the contracting winter sea-ice around the Antarctic Peninsula for a dramatic decline in Adelie penguin populations. According to William Fraser from Montana State University, studies of penguins on Torgerson Island show declines in Adelie numbers and the establishment of a colony of Chinstraps (decreasing Adelie numbers usually mean increased Chinstrap penguin numbers because Chinstraps feed in the ice-free seas). He believes that the presence of Chinstraps this far south is a further indication of the warming trend. Over the past four years of increasing warm weather the number of both species have severely declined falling by 35 and 40 percent for Chinstrap and Adelie penguins respectively. According to Dr. Wayne Trivelpiece, also of Montana State University, this can be linked to falling Antarctic krill populations. Antarctic krill is the primary food source for both of the birds. Stomach samples of the penguins reveal that significant size classes of krill are missing from stomach contents. This suggests that the krill are not surviving or being breed during the warmer winters. To investigate further we move one step down the food chain to find that the krill's main food supply is algae (phytoplankton such as diatoms).

Fig 7. Diatoms found in the Southern Ocean

It has been put forward that the decrease in sea ice has led to less algae being produced. Algae thrives under extensive ice coverage because the ice traps rock flour, which inhibits algae growth. Rock flour are very fine particles of sediment that result from glacial scouring and grinding of rock material as the glacier moves. Rock flour is carried at the base of glaciers and also by meltwater. Lakes in glaciated areas often have a milky turquoise color because of the suspended rock flour in the water. It can be assumed that this will have severe implications as the krill population decline affect travels up the food chain.

Looking at this issue from a different perspective yields increasing native flora in the Antarctic region. Antarctic pearlwort and Antarctic hair grass are the continent's only two indigenous flowering plants. Thinning glaciers and ice caps, and shrinking ice fields are exposing new land in which these plants can grow. Arctic hair grass has seen a 25 fold increase in coverage on Galindez Island.

On-Line References:

U.S. Global Change Research Program

The Intergovernmental Panel on Climate Change (IPCC)

Information Unit for Conventions (IUC)

GLACIER

Antarctic Warming - Early Signs of Global Climate Change

U.S. Global Change Research Information Office

Climate Change Research Center at the University of New Hampshire

World Data Center-A for Glaciology

Book References:

Climate Change, Class Handout, October 1997

Alcamo, Joseph, Image 2.0 Integrated Modeling of Global Climate Change, Kluwer Academic Publishers, 1994

Alcamo, Joseph, et al., Global Environmental Change: Human Policy and Dimensions, Elsevier Science Ltd., 1996

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