Ice Sheets and Sea Level -- Concerns at the Coast (Teachers Guide) Roughly 153 million Americans (~53% of the US population) live in coastal counties. World wide some 3 billion people live within 200 km of the ocean (world population in 2007 6.6 Billion). If the Greenland Ice sheet (equal to about 6 meters of global sea level) and the East and West Antarctic Ice Sheets (equal to about 64 meters of global sea level) were to melt completely, coastal areas would be flooded up to 70 meters (230 ft) above present. Geologists have determined that in the geologic past, these ice sheets have been known to melt back dramatically and sometimes even disappear during intervals when Earth s climate was warmer than it is now. With green house gases now driving us into a warmer world, our concern is to estimate how high sea level might get over the next few decades to centuries. Scientists from a number of disciplines are now focused on this issue in order to provide realistic predictions of sea level rise and inform policy makers about the need for wise and sensible coastal management in the US and throughout the world. Glaciologists, in particular, are studying physical processes happening at the base of the ice sheets and how rates of glacial flow into the sea are changing. On the other hand, ice shelves are floating ice bodies and their collapse does not contribute to sea level rise; however, they are known to buttress outlet glaciers which drain the large ice sheets into the sea. The loss of these ice sheets would have a significant impact on sea level. Moreover, atmospheric scientists are trying to narrow uncertainties in the rates of global warming, especially in the polar regions because this directly influences glacier mass balance. Changes in global sea level as projected into the future are not entirely driven by melting ice sheets, but also involves the thermal expansion of the world s surface ocean due to increases in surface water temperature (i.e., an increase in surface water volume as water expands). Even more complex is the fact that the position of the land relative to the sea is also not fixed in many parts of the world. For example due to the annual deposition of tons of sediment at the mouth of the Mississippi River, the coast of Louisianna is isostatically sinking due the shear weight of the accumulating sediment pile making up the Mississippi River delta complex. This sinking further increases the rise of sea level along the Gulf coast. Other deltaic parts of the word share the same fate (e.g., Amsterdam, Nile Delta, Bangladesh etc.). Ultimately, we would like students to think about the following questions during this exercise: What controls the elevation of the sea against the shore? What causes it to go up or down over thousands of years? What causes it to go up or down over days, months? How much did sea level rise at the end of the last glaciation? At what rate did it rise when the last great ice sheets disappeared? What factors contribute to sea level rise today? Are glaciers and ice sheets the only cause? How do city planners and coastal managers observe and measure sea level rise? Does sea level rise at the same rate everywhere? What types of coastal areas are more vulnerable than others? How stable are the largest ice sheets on our planet? What knowledge is needed to decrease uncertainties in future projections? What career possibilities are there to study and mitigate coastal issues? How much more information is needed before we act? How do we best manage our coastlines as sea level rises? What choices may be necessary? Objectives Students will construct graphs showing the relationship between ice sheet melting and sea level rise during the last deglaciation. Students will interpret the graphs to determine the rate at which sea level rose during the last deglaciation. Students will use a graph of future projections of sea level to trace the sea level displacement on 3 types of coastlines (this will help to assess societal vulnerability). A ED Program at the University of Massachusetts, funded by the National Science Foundation and supported by the
Students can research, discuss and debate the implications and policy issues facing cities and countries with gradual sea level rise. Equipment Student guides Rulers Graph paper Pencils Alley et al. 2005, Science, 21 Oct issue. Rahmsdorf, 2007, Science, 19 January issue Methods: 1. Start with introducing the concept of global warming and the questions listed in Before we Begin on the student sheet. Also be sure the students are familiar with the concepts of the Green House Effect and why/how sea level might change over time. Even cycles of the moon, of course, influence the tides which are significant in some parts of the world. 2. Allow the students to plot the data in Table 1 on the student sheet (sea level rise of the past 20ka). Their data will start to look like the one shown below. This can be done either by hand, or by using a program such as excel to plot all the data. From this plot (and fig. 4 below) they can consider the rate of sea level rise in meters/thousand years, centimeters per hundred years and millimeters/year. Then they can consider how fast sea level has risen over the past few decades. You could skip the plotting part and just have the students use these graphs to pick off information to determine the rates of sea level rise. This figure shows sea level rise since the end of the last glacial episode based on data from Fleming et al. 1998, Fleming 2000, & Milne et al. 2005. (Wikipedia Commons) A ED Program at the University of Massachusetts, funded by the National Science Foundation and supported by the
Standard Sea level curve from Fairbanks, R.G., 1989, A 17,000-year glacio-eustatic sea level record: influence of glacial melting rates on the Younger Dryas event and deep-ocean circulation: Nature, v. 342, p. 637-642. 07 Dec. A ED Program at the University of Massachusetts, funded by the National Science Foundation and supported by the
3. Next ask the students to color in on the three provided maps where the shore line would be for 1 m (~3 ft) rise of sea level as suggested by some for 2100. In a different color they can show the additional areas covered by the sea if sea level rose by 6 m (which would take thousands of years). How much shoreline retreat can they predict on average for each of the three map areas and what controls might there be on the rate of retreat. The analysis questions can be completed individually, in small groups or as a whole class. More advanced ideas: 4. More advanced groups might consider researching the types of structures used by engineers to slow down or prevent coastal erosion. 5. Students could also consider the thermal expansion of the ocean mixed layer as sea surface temperatures increase. Add in a plot of the density of water vs. temperature, the volume of the ocean mixed layer and how that layer would change with a hypothetical change in 1 C. Sea Level Estimates from Rahmsdorf, 2007. If this relationship between global mean surface temperature and global mean sea-level rise is applied to the IPCC temperature projections for the year 2100, it yields a sea-level rise of 0.5 to 1.4 m -- substantially higher than estimates reported by the IPCC (2007). But current sea level rise is tracking on the upper portion of this curve. Important Links http://www.realclimate.org http://tidesandcurrents.noaa.gov/sltrends/sltrends.shtml http://coastalmanagement.noaa.gov/czm/czma_vision.html http://www.ipcc.ch/ipccreports/ar4-wg1.htm (especially chapter 5) http://www.pik-potsdam.de/~stefan/publications/nature/rahmstorf_science_2007.pdf A ED Program at the University of Massachusetts, funded by the National Science Foundation and supported by the
Vocabulary Glacier Mass Balance The balance between accumulation on a glacier (by snowfall) and ablation (melting, sublimation, calving). If a glacier has a positive mass balance, it will advance and if its mass balance is negative, it will retreat. Green house effect Greenhouse gasses (including CO 2 and methane) are transparent to incoming short wave radiation, but absorb the long wave radiation that is reflected by the Earth. This traps radiation inside the Earth s atmosphere and leads to a general warming. Ice sheet Land-based ice that covers the landscape across large areas. Examples include the Greenland ice sheet and the East and West Antarctic ice sheets. Melting of ice sheets would cause sea level to rise. Ice Shelves A thick platform of ice that is floating on the ocean, but attached to land (grounded). Since this ice is floating (much like ice cubes in a glass of water), melting will not increase sea level but could allow glaciers that flow onto the ice shelves to calve and melt rapidly. Outlet Glacier In this sense, a glacier that flows from an ice sheet out onto an ice sheet or directly into the ocean. Storm surge Winds circulating around a low pressure storm system will literally push water against the shore line as the storm advances. This mound of water can be devastating especially when it occurs during a high spring tide. Thermal expansion of water As water (warmer than ~4 C) increases in temperature, its density will decrease. This decrease in density will essentially push some water molecules further apart from each other leading to a larger volume occupied by the same number of molecules. Isostatic Sinking (isostasy) The tendency for continental crust to sink into the asthenosphere when weight is added. This extra mass can be due to things like glacier growth or sediment deposition in deltas. http://www.seafriends.org.nz/oceano/beachdam.htm A ED Program at the University of Massachusetts, funded by the National Science Foundation and supported by the