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Sea level Rise and its Effect on Delaware

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Earth’s climate is warming because of an increase in the “greenhouse gases,” especially CO2, in the atmosphere. (See “Increasing Atmospheric Carbon Dioxide” and “Climate Change” in this series.) This warming is resulting in a melting of large planetary ice masses, such as glaciers, sea ice, and the polar ice caps.

This document describes the effects of climate change on sea level rise and how it will affect Delaware.

Basic principles of sea level rise

As the global average air temperature warms due to climate change, it increases the net melting of large ice masses on the planet. The effects are complex, vary regionally, cycle seasonally, have feedback effects, and shift the balance between accumulation and ablation (melting and evaporation), but the basic principle is the same as what we see of ice and snow in our neighborhoods as the weather warms. When ice floating on water melts, such as the Arctic sea ice and parts of the Antarctic ice shelves, the height of the water does not change (you can verify this with a glass water, ice cubes, and a crayon). When we melt ice that is sitting on land, such as glaciers, Greenland’s ice cap, and most of the Antarctic, the meltwater runs into the ocean and raises sea level.

To date, another cause of sea level rise has been the thermal expansion of the oceans—warm water expands. In Delaware, there is also an effect from “subsidence”, that is, the land is still dropping slightly due to after-effects of the last Ice Age. Thermal expansion and local subsidence are both significant effects to date. However, as the earth warms, the melting of ice will become the dominant effect in sea level rise, much larger than thermal expansion and subsidence combined.

How do we know this is happening?

The shrinking of glaciers is shown in the movie “An Inconvenient Truth” as evidence of overall warming. However, glaciers are not good indicators of recent global warming, as their melting and growing can lag significantly behind air temperatures. A more reliable indicator measure is sea ice, which is frozen seawater floating on the surface of the ocean. The largest single mass of sea ice is the Arctic (North Polar) Ice Cap.

Figure 1 is taken from three years of satellite pictures, 1979 through 1981, of the polar ice cap. The photos were taken in September, the time of minimum ice, and the three years’ images are averaged by the NASA/Goddard Space Flight Center Scientific Visualization Studio. A multi-year average is used because the yearly summer minimum fluctuates year-to-year. Figure 2 is the same, but about twenty years later, for 2000-2003. Animations of these and other arctic changes can be found at http://svs.gsfc.nasa.gov (search for “arctic”).

Sea Level Fig 1

Figure 1: Summer minimum ice, average of 1979-1981. From NASA/Goddard Space Flight Center Scientific Visualization Studio.

Sea Level Fig 2

Figure 2: Summer minimum ice, average of 2000-2003. From NASA/Goddard Space Flight Center Scientific Visualization Studio.

The change can be seen by comparing the two figures. The Arctic ice is shrinking (You may be able to flip back and forth on a computer screen, or for the paper images, put one over the other and hold up to the light.) This is not a model or simulation, it is data of what has already happened. How long will it take to melt all the ice? Up until December 2006, it was generally expected to take until about the end of the century. However, a new study using computer models of climate and ice loss predicts that summer Arctic sea ice could be gone by 2040 (Holland, Bitz and Tremblay 2006). Whichever study is more accurate, continuing on our current course is likely to eliminate the Arctic ice by the end of this century. Images of this prediction can be found at http://www.ucar.edu/news/releases/2006/arctic.shtml also http://www.ucar.edu/news/releases/2006/arcticvisuals.shtml.

This loss of ice can be seen from space. This has a direct effect--the lack of ice will make it difficult or impossible for walruses, some subspecies of polar bear, and other Arctic animals to survive (Learmonth et al 2006). An additional effect is that, as more ice and snow in the Arctic region melts, it changes the reflectance, shifting the regions from white to darker colors associated with land and sea surfaces. As you can see in the melting snow of white sidewalk adjacent to blacktop, a darker surface will absorb heat and further speed up the warming of the Arctic. All this will also have direct effects on Delaware, but to understand that, we have to first look at Greenland and the Antarctic,

Greenland is melting

The large ice island in the right of Figures 1 and 2 is Greenland. Greenland is the largest island in the world, a sparsely populated self-governing division of Denmark. Figure 3 shows a cross-section of Greenland, with part of the ice cut away (exaggerated vertical scale). Unlike the Arctic sea ice, this ice is sitting on rock, so when it melts, it raises the level of the ocean. Greenland is 2,650 km long (1,650 mi) and the ice is an average of about 2.5 kilometers thick (1.5 mi).

Sea Level Fig 3

Figure 3: Ice mass on Greenland, with exaggerated vertical scale.

Greenland is melting. Figure 4 is a picture of a melt water stream during the Greenland summer. The people in the picture provide perspective regarding the size of the glacier. Ice accumulates in the middle area of Greenland and melts at the edges. During the 1990s the two were roughly in balance, but in recent years the shift is strongly to greater melting. The current estimate is 155 GT ice loss per year, less 54 GT accumulation, for a net loss of 101 GT/year. (Luthcke et al 2006). (GT stands for gigaton—a gigaton is 1,000,000,000 metric tons, or 1,000,000,000,000 kilograms.) This shift to a net melt is of course at just the beginning and the rate can be expected to increase as the planet warms.

Sea Level Fig 4

Figure 4: from ice melt on Greenland, from Science Direct, Oct 20, 2006 (photo courtesy of Roger Braithwaite and Jay Zwally.).

To get a sense of 101 GT, Lake Erie contains 484 GT of water. Thus, at the current rate of melting, Greenland adds the volume of Lake Erie to the Atlantic Ocean every 4.8 years. So far, this is relatively small. 100 GT of melt is about ¼ mm of sea level rise, so at this point Greenland is contributing only about one 40th of the measured rate of sea level rise.

Large masses of ice like Greenland melt slowly, so even with melt accelerating because of continuing climate change, it could take several hundred years, or a thousand years—we do not have a solid reference for the amount of time. The more relevant question may be: “How long until we heat up the Earth so much that we will necessarily melt Greenland.” This question has been modeled, under twenty-some different assumptions, as shown in Figure 5. This figure and the results are found in Gregory et al. (2004).

Sea Level Fig 5

Figure 5: Time to reach commitment to melting Greenland’s ice under differing emissions projections (from Gregory et al 2004). Each colored line reflects differing emissions and the black dashed line is the amount of local temperature rise implying commitment to melting Greenland.

In Figure 5, the colored lines are different models of increasing CO2 from year 2000 through 2350. The black dashed line between 2 and 3 degrees C is the average temperature rise above the year 2000 temperature at which we have committed to melting Greenland. By this, we mean that if the global temperatures rise that much, then models predict that the melt rate in the center of Greenland will be high enough that the eventual melting of most of the ice becomes inevitable. Actually melting all the ice could take another 1000 years after that point (plus or minus a large unknown factor). The bulk of colored lines (different CO2 reduction levels) cross the dashed line between 2035 and 2100. One line doesn’t cross for a long time—that lowest line assumes that the level of CO2 in the atmospheres never passes 450 parts per million, which would require more aggressive attempts to limit CO2 than is currently envisoned by the Kyoto Protocol or by most national targets.

If the model in Figure 5 is correct, we would need to reduce CO2 more drastically and more rapidly than is now being planned if we do not want to commit to totally melting Greenland.

The West Antarctic Ice Sheet

Earth’s largest quantity of ice sitting on rock is Antarctica. The upper reaches of Antarctic are at high elevation and the mass of ice is so large, that it is much less subject to air temperature. However, on the edges of the West Antarctic Ice Sheet are sea ice and ice shelves. As shown in Figure 6, large sections of Larsen B ice shelf are starting to break off into the ocean. When the ice shelf goes (again, floating ice does not raise sea level), we are left with the very large West Antarctic ice sheet, which consists of ice sitting on sloping rock (the “pinched off” area on the left side of the inset area map).

Think about a heavy pack of snow on your roof, as the weather starts to warm up. Like your roof, water underneath the West Antarctic ice sheet lubricates the ice, allowing the snow and ice to slide off. If the entire West Antarctic Ice Sheet melts, sea level will rise 8.06 meters (26 feet – referenced shortly). Since this sheet is unstable and could slide much more quickly than melting, sometimes only the unstable portion of the West Antarctic Ice Sheet is discussed. Considering only the portion that might slide off before melting, that part would raise global sea level by 4 to 6 meters (O’Neill and Oppenheimer 2002).

Sea Level Fig 6

Figure 6: Satellite photos from National Snow and Ice Data Center, http://nsidc.org/iceshelves/larsenb2002/, composition by Brian Hanson.

How much sea level rise?

The amount of water contained in the world’s major ice masses is relatively easy to calculate. The following table gives the size of each major ice mass, the volume of water in it, and then, by dividing by the area of the worlds’ oceans, it gives the amount of sea level rise that would be caused by each ice mass (Williams & Ferrigno, eds. 1999).

Table: Amounts of water in those planetary ice masses that would raise sea level. Table from Williams & Ferrigno, 1999)

Ice Masses Area
(1000 sq km)
Volume
(1000 cubic km)
Potential Sea Level Rise (m)
Mountain Glaciers and Small Ice Caps
680
180
0.45
Greenland
1785
2620
6.55
Antarctica (total)

     East Antarctica
     West Antarctica
     Antarctic Peninsula
     Ice Shelves
13586

10153
1918
447
1068
30110

26039
3262
227
582
73.44

64.80
8.06
0.46
0.12
Total
16051
32910
80.44

We see that committing to melting Greenland commits us to 6.55 m (21 ft) sea level rise, and committing to melting the West Antarctic commits us to an additional 8.06 m (26 ft). Other ice masses, such as mountain glaciers and small ice caps (0.45 m) and the Antarctic Peninsula (shown in Figure 6, adding 0.46 m) will melt more rapidly but have less effect than the major ice masses (these two added together are 0.91 m or 3 feet of sea level rise). (Nevertheless, even 0.91 m would still be a major problem for many coastal cities and towns.)

What does sea level rise mean for Delaware?

It is easier to understand the impact of a change if it can be related to familiar places. From a digital map of the topography of Delaware, we have created a map to visually show the impact of the melting of these ice masses. Figure 7 shows Delaware and nearby sections of New Jersey and Maryland, with colors showing the areas affected by sea level rise. Red is the mid range of pre-2007 estimates of sea level rise by the end of this century, about 0.61 m (2 feet). This amount is due to sea level rise due to the melting of small portions of the ice masses in the table, combined with effects of land subsidence and thermal expansion of the oceans (Cooper, Beevers and Oppenheimer, 2005). The pre-2007 data was also used in the 2007 IPCC report, which also focused on a specific decade (2090-2099) rather than the longer term effects (IPCC 2007). An early 2007 study suggests that this is a low end estimate, within a range of 0.5 to 1.4 meters SLR global from expansion and melt (Rahmstorf 2007), plus addition from local subsidence. We use the smaller, pre-2007 estimate of the IPCC in the map; the choice of estimate in 2099 is much less important than the time frame being examined.

Sea Level Fig 7

Figure 7: Areas of Delaware inundated by expected sea level rise in this century (red), by Greenland (orange), and by the West Antarctic (yellow). Light green are the areas that would be inundated by additional loss of the entire Antarctic, but the probability and time constant of this are very poorly estimated.

Sea Level Fig 8

Figure 8: Impact of sea level rise on Delaware, due to ice melt from Greenland and the West Antarctic combined. The remaining land areas are shown in green.

The loss of the lands in red would be painful to Delaware—they include coastal wetlands that are important habitat and that have been protected from industrial development with substantial political effort by Delaware citizens and leaders. But this is only a small part of the eventual impact of melting ice.

By this time, at the end of this century, assuming continued large CO2 emissions and assuming the time lines in Figure 5 are correct, we will be committed to melting Greenland and possibly the West Antarctic ice sheet. As these melt over the next few centuries, the majority of the sea level rise will be caused by ice melt rather than thermal expansion. The orange is an additional 6.5 m (21 ft) due to the melting of Greenland, and the yellow is a total of 15 m (41 feet) due to the prior two, plus melting the West Antarctic ice sheet.

The dark green in Figure 5, the small strip along the border with Pennsylvania, is what would remain of Delaware if the all the ice masses in the table were melted, that is, including the entire Antarctic. However, it is possible that even a large increase in CO2 would not melt the entire Antarctic, so our discussion does not concentrate on this high level.

Figure 8 shows the effect of this century’s sea level rise, plus the commitments we make in this century to melting Greenland and the West Antarctic. All the lands that would be submerged by our actions in the century are shown the same color as the ocean.

We conclude from these data that to prevent these amounts of sea level rise, we must reduce humanity’s CO2 emissions within a few decades, not a century or more.

Delaware is not the only place affected, and sea level rise is not the only serious effect of climate change. Other documents in this series address a few of the other effects of CO2.

Document information

This document is based on peer-reviewed literature but attempts to be readable by the general public. It is scientifically grounded, provides references, and has been reviewed by colleagues. It is intended to review and provide perspective on existing science, not advance science, and is not expected to be submitted for publication in the scientific literature. Copyright © 2007, University of Delaware. Permission granted for use and redistribution of this information as long as the original source is cited. This document, and some of the separate images are available at http://CO2.cms.udel.edu.

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References Cited

Matthew J.P. Cooper , Michael D. Beevers and Michael Oppenheimer, 2005, Future Sea Level Rise And The New Jersey Coast: Assessing Potential Impacts and Opportunities. Report, available from Science, Technology and Environmental Policy Program, Woodrow Wilson School of Public and International Affairs, Princeton University. November 2005.

Jonathan M. Gregory, Philippe Huybrechts, Sarah C. B. Rapers, 2004 “Threatened Loss of Greenland Ice Sheet” Nature, April 8, 2004.

Marika M. Holland, Cecilia M. Bitz, and Bruno Tremblay, 2006 “Future Abrupt Reductions in the Summer Arctic Sea Ice” Geophysical Research Letters, December 12, 2006.

Intergovernmental Panel on Climate Change (Richard Alley et al), 2007, “Climate Change 2007: The Physical Science Basis.” Geneva, Switzerland: IPCC Secretariat.

Willett Kempton, James S. Boster and Jennifer A. Hartley, 1995, Environmental Values in American Culture. Cambridge, MA: MIT Press.

J.A. Learmonth, C.D. Macleod, M.B, Santos, G.J. Pierce, H.Q.P Crick, R.A Robinson, 2006, “Potential Effects of Climate Change on Marine Mammals” Oceanography and Marine Biology: An Annual Review 2006.

Brian C. O'Neill and Michael Oppenheimer, 2002, Climate Change: Dangerous Climate Impacts and the Kyoto Protocol. Science. (Policy Forum) 14 June 2002:
Vol. 296. no. 5575, pp. 1971 - 1972
DOI: 10.1126/science.1071238

Jonathan T. Overpeck, Bette L. Otto-Bliesner, Gifford H. Miller, Daniel R. Muhs, Richard B. Alley, Jerrrery T. Kiehl, 2006, “Paleoclimate Evidence for Future Ice-Sheet Instability and Rapid Sea Level Rise” Science, March 24, 2006.

Stefan Rahmstorf, 2007, “A Semi-Empirical Approach to Projecting Future Sea-Level Rise” Science, vol 315, pp 368-370. 19 January 2007.

Richard S. Williams & Jane G. Ferrigno (eds), 1999, USGS Professional Paper 1386A. (More by these authors at pubs.usgs.gov/imap/2600/)


Prepared by Willett Kempton and Amardeep Dhanju. College of Marine and Earth Studies, University of Delaware, Newark, DE 19716-3501. Copyright © 2007. Revised 03 February 2007.

       


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