How Much Will Sea Levels Rise

Scientists believe global warming is very unlikely to cause an ice age. Another dangerous scenario that is often discussed, however, is a massive rise in sea level. In the documentary film An Inconvenient Truth, for example, Nobel Peace Prize winner and former U.S. vice-president Al Gore argues that melting ice sheets could cause a sea-level rise of 20 feet (or about 6m) "in the near future."24 Gore presents a graphic showing inundated coastlines, and contends that this sea-level rise would cause massive dislocations and suffering.

Some scientists agree with Gore that there could be short-term, dangerous increases in sea levels, especially if Greenland's ice sheet were to melt more rapidly than predicted. Jonathan Overpeck, a scientist at the University of Arizona, has used computer models to estimate what would happen if the sea rose by various levels. The models show that a 20 foot (6m) rise would submerge swaths of coastal areas, including, for example, much of Florida. Overpeck noted in a 2004 article, "The consequences would be catastrophic."25

In contrast, some scientists argue that global warming may not cause any significant increase in sea levels at all. According to Christopher Essex, a professor of mathematics at the University of Western Ontario, and Ross McKitrick, a professor of economics at the University of Guelph, the effects of thermal expansion of the oceans may be offset by increased evaporation. Essex and McKitrick also argue in their book Taken by Storm that reports of actual sea-level variations are inconclusive. One benchmark in Australia, for example, suggests that sea level near Tasmania has actually fallen by some 30cm (about a foot) since 1841. Essex and McKitrick note that "we may know the full explanation behind" the apparent drop in sea level near Tasmania—it might be caused by mistaken measurements, for example. Either way, however, Essex and McKitrick argue that "Even if the sea level does rise, the worst-case forecasts are for something on the order of 50cm [1.5 feet] over . . . 170 years."26

"A commission of 20 international experts" estimates a sea-level rise of 1.5m to 3.5m in sea level by 2200... such an increase "would spell the end of many of our coastal cities."

The mainstream of scientific opinion falls between Al Gore's scenario and the relatively safe future envisaged by Essex and McKitrick. The Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report from 2007 estimates that sea levels will rise 18cm to 59cm (7 inches to 23 inches) in the next century. According to a January 2009 report from the U.S.

Following page: Former U.S. vice-president Al Gore used the documentary film An Inconvenient Truth to warn about rising sea level and other effects of climate change. Valery Hache/AFP/Getty Images.

Climate Change Science Program, however, "Recent observations suggest that sea-level rise rates may already be approaching the higher end of the IPCC estimates. . . . This is because potentially important meltwater contributions from Greenland and Antarctica were excluded due to limited data and an inability at that time to model ice flow processes adequately. The report concluded that "a global sea-level rise of 1m is plausible within this century if increased melting of ice sheets in Greenland and Antarctica is added to the factors included in the IPCC estimates. Therefore, thoughtful precaution suggests that a global sea-level rise of 1m to the year 2100 should be considered for future planning and policy discussions."27

A meter is not the 20 feet (6m) that Al Gore predicted. Still, over a century, it would be a significant increase in sea levels. Moreover, the sea-level rise is expected to continue in future centuries. Stefan Rahmstorf, a climate scientist and oceanographer at the Potsdam Institute for Climate Impact Research, notes that "A commission of 20 international experts" estimates a sea-level rise of 1.5m to 3.5m by 2200. Rahmstorf notes that such an increase "would spell the end of many of our coastal cities.28

The disagreements among scientists indicate a frustrating fact about sea levels—they are very difficult to predict. Essex and McKitrick argue that even "Mean sea level is rather tricky to measure." Not only do sea levels vary, but the land itself rises and falls; northern Europe, for example, is rising slightly, while southern Europe is sinking slightly. Computer models attempt to adjust for land shifts, but "there is a lot of uncertainty about how much to adjust the sea level data to account for land movements."29

Nor is land movement the only variable. For example, one study led by Julie Loisel of Lehigh University found that peat moss bogs in some regions may be expanding as more water from glaciers reaches them. This change might help offset sea-level rise, as the expanded peat absorbs more water. "When people think of sea level rise, they just look at how much ice is lost and think it all goes right into the ocean. . . . What we want to stress here is that,

RISE IN SEA LEVELS ON THE ATLANTIC COAST ^

Sea-Level Rise (mm per year)

Time Span of Record

Annapolis, MD

3.53 ± 0.13

1928-1999

Atlantic City, NJ

3.98 ± 0.11

1911-1999

Baltimore, MD

3.12 ± 0.16

1902-1999

Boston, MA

2.65 ± 0.1

1921-1999

Charleston, SC

3.28 ± 0.14

1921-1999

Eastport, ME

2.12 ± 0.13

1929-1999

Fernandina Beach, FL

2.04 ± 0.12

1897-1999

Fort Pulaski, GA

3.05 ± 0.20

1935-1999

Hampton Roads, VA

4.42 ± 0.16

1927-1999

Key West, FL

2.27 ± 0.09

1913-1999

Lewes, DE

3.16 ± 0.16

1919-1999

Mayport, FL

2.43 ± 0.18

1928-1999

Miami, FL

2.39 ± 0.22

1931-1999

Montauk, NY

2.58 ± 0.19

1947-1999

New London, CT

2.13 ± 0.15

1938-1999

Newport, RI

2.57 ± 0.11

1930-1999

Philadelphia, PA

2.75 ± 0.12

1900-1999

Portland, ME

1.91 ± 0.09

1912-1999

Portsmouth, VA

3.76 ± 0.23

1935-1999

Providence, RI

1.88 ± 0.17

1938-1999

Sandy Hook, NJ

3.88 ± 0.15

1932-1999

Seavey Island, ME

1.75 ± 0.17

1926-1999

Solomons Island, MD

3.29 ± 0.17

1937-1999

The Battery, NY

2.77 ± 0.05

1905-1999

Washington, DC

3.13 ± 0.21

1931-1999

Willets Point, NY

2.41 ± 0.15

1931-1999

Wilmington, NC

2.22 ± 0.25

1935-1999

Woods Hole, MA

2.59 ± 0.12

1932-1999

Source: James G. Ritus et al., Coastal Sensitivity to Sea-Level Rise: A Focus on the Mid-Atlantic Region, U.S. Climate Change Science Program, January 2009, p. 19. www.climatescience.gov.

wait, there's another place it can go," said Loisel.30 There might be many factors, such as peat bogs, that may reduce or increase sea-level rise. Some may matter only a little; others, such as those that affect the speed of ice melt in Greenland, might have a significant impact.

Notes

18. Quoted in John Roach, "Global Warming May Alter Atlantic Current, Study Says," National Geographic, June 27, 2005. http://news.nationalgeographic.com.

19. David Stipp, "The Pentagon's Weather Nightmare: The Climate Could Change Radically and Fast. That Would Be the Mother of All National Security Issues," Fortune, February 9, 2004.

20. John Roach, "Global Warming May Alter Atlantic Current, Study Says."

21. Richard Seager, "The Source of Europe's Mild Climate," American Scientist, August

2006. www.americanscientist.org.

22. Weaver, Keeping Our Cool, p. 147.

23. Quoted in Kent State University, "Study Sheds Light on Earth's CO2 Cycles, Possible Impacts of Climate Change," May 2007. www.kent.edu.

24. Quoted in Roger Highfield, "An Inconvenient Truth Exaggerated Sea Level Rise," Daily Telegraph, September 4, 2008. www.telegraph.co.uk.

25. Quoted in Stefan Lovgren, "Warming to Cause Catastrophic Rise in Sea Level?" National Geographic, April 26, 2004. http://news.nationalgeographic.com.

26. Christopher Essex and Ross McKitrick, Taken by Storm: The Troubled Science, Policy, and Politics of Global Warming, revised edition. Toronto, Ontario: Key Porter Books,

27. James G. Titus et al., Coastal Sensitivity to Sea-Level Rise: A Focus on the Mid-Atlantic Region, U.S. Climate Change Science Program, January 2009, pp. 19-20. www.global change.gov.

28. Stefan Rahmstorf, "We Must Shake Off This Inertia to Keep Sea Level Rises to a Minimum," Guardian, March 3, 2009. www.guardian.co.uk.

29. Essex and McKitrick, Taken by Storm, p. 286.

30. Quoted in Michael Reilly, "Sponge-Like Peat Bogs Could Offset Sea Level Rise," Discovery News, May 28, 2009. http://dsc.discovery.com.

EFFECTS

Despite disagreements about the exact amount, scientists in general do believe that there will be a rise in sea levels due to global

warming over the next hundred years. Such a rise will have important effects on coastal areas in the United States.

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    Can rising sea levels affect hydroelectricity?
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