Sea level rise: what has happened so far

By: Dr. Jeff Masters , 14:05 GMT le 10 juin 2009

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Sea level has been rising globally since the late 1700s. This rise has accelerated in recent decades, thanks to increased melting of glaciers and ice sheets due to a warmer climate, plus the fact that warmer oceans are less dense and expand, further increasing sea level. Though sea level rise appears to have slowed over the past five years, it will significantly accelerate if the climate warms the 2 - 3°C it is expected to this century. If these forecasts of a warmer world prove accurate, higher sea levels will be a formidable challenge for millions of people world-wide during the last half of this century. Sea level rise represents one of my personal top two climate change concerns (drought is the other). I'll present a series of blog posts over the coming months focusing on at-risk areas in the U.S., Caribbean, and world-wide. Today, I focus on the observed sea level rise since the Ice Age.

What's at stake
Higher sea levels mean increased storm surge inundation, coastal erosion, loss of low-lying land areas, and salt water contamination of underground drinking water supplies. About 44% of the Earth's 6.7 billion people live within 150 km (93 miles) of the coast, and 600 million people live at an elevation less than ten meters (33 feet). Eight of the ten largest cities in the world are sited on the ocean coast. In the U.S., the coastal population has doubled over the past 50 years. Fourteen of the twenty largest urban centers are located within 100 km of the coast, and are less than ten meters above sea level (McGranahan et al., 2007). The population of many vulnerable coastal regions are expected to double by 2050, according to the U.S. Census Bureau.

Sea level rise since the Ice Age
Before the most recent Ice Age, sea level was about 4 - 6 meters (13 - 20 feet) higher than at present. Then, during the Ice Age, sea level dropped 120 meters (395 ft) as water evaporated from the oceans precipitated out onto the great land-based ice sheets. The former ocean water remained frozen in those ice sheets during the Ice Age, but began being released 12,000 - 15,000 years ago as the Ice Age ended and the climate warmed. Sea level increased about 115 meters over a several thousand year period, rising 40 mm/year (1.6"/yr) during one 500-year pulse of melting 14,600 years ago. The rate of sea level rise slowed to 11 mm/year (0.43"/yr) during the period 7,000 - 14,000 years ago (Bard et al., 1996), then further slowed to 0.5 mm/yr 6,000 - 3,000 years ago. About 2,000 - 3,000 years ago, the sea level stopped rising, and remained fairly steady until the late 1700s (IPCC 2007). One exception to this occurred during the Medieval Warm Period of 1100 - 1200 A.D., when warm conditions similar to today's climate caused the sea level to rise 5 - 8" (12 - 21 cm) higher than present (Grinsted et al., 2008). This was probably the highest the sea has been since the beginning of the Ice Age, 110,000 years ago. There is a fair bit of uncertainty in all these estimates, since we don't have direct measurements of the sea level.


Figure 1. Global sea level from 200 A.D. to 2000, as reconstructed from proxy records of sea level by Moberg et al. 2005. The thick black line is reconstructed sea level using tide gauges (Jevrejeva, 2006). The lightest gray shading shows the 5 - 95% uncertainty in the estimates, and the medium gray shading denotes the one standard deviation error estimate. The highest global sea level of the past 110,000 years likely occurred during the Medieval Warm Period of 1100 - 1200 A.D., when warm conditions similar to today's climate caused the sea level to rise 5 - 8" (12 - 21 cm) higher than present. Image credit: Grinsted, A., J.C. Moore, and S. Jevrejeva, 2009, "Reconstructing sea level from paleo and projected temperatures 200 to 2100 AD", Climate Dynamics, DOI 10.1007/s00382-008-0507-2, 06 January 2009.

Sea level rise over the past 300 years
Direct measurements of sea level using tide gauges began in Amsterdam in 1700. Additional tide gauges began recording data in Liverpool, England in 1768 and in Stockholm, Sweden in 1774. These gauges suggest that a steady acceleration of sea rise of 0.01 mm per year squared began in the late 1700s, resulting in a rise in sea level of 2.4" (6 cm, 0.6 mm/yr) during the 19th century and 7.5" (19 cm, 1.9 mm/yr) during the 20th century (Jevrejeva et al., 2008). There is considerable uncertainty in just how much sea level rise has occurred over the past few centuries, though. Measuring global average sea level rise is a very tricky business. For starters, one must account for the tides, which depend on the positions of the Earth and Moon on a cycle that repeats itself once every 18.6 years. Tide gauges are scattered, with varying lengths of record. The data must be corrected since land is sinking in some regions, due to pumping of ground water, oil and gas extraction, and natural compaction of sediments. Also, the land is rising in other regions, such as Northern Europe, where it is rebounding from the lost weight of the melted glaciers that covered the region during the last Ice Age. Ocean currents, precipitation, and evaporation can cause a 20 inch (50 cm) difference in sea level in different portions of the ocean. As a result of all this uncertainty, the 1996 Intergovernmental Panel on Climate Change (IPCC) report gave a range of 4 - 10" (10 - 25 cm) for the observed sea level rise of the 20th century. The 2007 IPCC report narrowed this range a bit, to 5 - 9" (12 - 22 cm), or 1.2 - 2.2 mm/year. Rates of sea level rise are much higher in many regions. In the U.S., the highest rates of sea-level rise are along the Mississippi Delta region--over 10 mm/yr, or 1 inch/2.5 years (USGS, 2006). This large relative rise is due, in large part, to the fact that the land is sinking.


Figure 2. Absolute sea level rise between 1955 and 2003 as computed from tide gauges and satellite imagery data. The data has been corrected for the rising or sinking of land due to crustal motions or subsidence of the land, so the relative sea level rise along the coast will be different than this. The total rise (in inches) for the 48-year period is given in the top scale, and the rate in mm/year is given in the bottom scale. The regional sea level variations shown here resulted not only from the input of additional water from melting of glaciers and ice caps, but also from changes in ocean temperature and density, as well as changes in precipitation, ocean currents, and river discharge. Image credit: IPCC, 2007

Sea level rise over the past 15 years
According to the Intergovernmental Panel on Climate Change (IPCC) 2007 report, sea level accelerated from the 1.2 - 2.2 mm/yr observed during the 20th century to 3.1 mm/year during the period 1993 - 2003. These estimates come from high resolution measurements from satellite radar altimeters, which began in 1992. Tide gauges showed a similar level of sea level rise during that ten-year period. The IPCC attributed more than half of this rise (1.6 mm/yr) to the fact that the ocean expanded in size due to increased temperatures. Another 1.2 mm/yr rise came from melting of Greenland, West Antarctica, and other land-based ice, and about 10% of the rise was unaccounted for. However, during the period 2003 - 2008, sea level rise slowed to 2.5 mm/year, according to measurements of Earth's gravity from the GRACE satellites (Cazenave et al., 2008). This reduction in sea level rise probably occurred because ocean sea surface temperatures have not warmed since 2003 (Figure 3). The authors concluded that sea level rise due to ocean warming decreased more than a factor of five from 2003 - 2008, compared to 1993 - 2003, contributing only 0.3 mm/yr vs. the 1.6 mm/yr previously.


Figure 3. Global average sea surface temperatures (SSTs) from 1990-2008. SSTs have not increased in the past seven years. Image credit: NASA/GISS.

For more information
The best source of information I found while compiling my sea level pages was the Coastal Sensitivity to Sea-Level Rise: A Focus on the Mid-Atlantic Region report by the U.S. Climate Science Program. It has a huge number of references to all the latest science being done on sea level rise.

References
Bard, E., et al., 1996, "Sea level record from Tahiti corals and the timing of deglacial meltwater discharge", Nature 382, pp241-244, doi:10.1038/382241a0.

Cazenave et al., 2008, "Sea level budget over 2003-2008: A reevaluation from satellite altimetry and Argo", Global and Planetary Change, 2008; DOI:10.1016/j.gloplacha.2008.10.004

Grinsted, A., J.C. Moore, and S. Jevrejeva, 2009, "Reconstructing sea level from paleo and projected temperatures 200 to 2100 AD", Climate Dynamics, DOI 10.1007/s00382-008-0507-2, 06 January 2009.

IPCC (Intergovernmental Panel on Climate Change), 2007: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor, and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, UK, and New York, 996 pp.

Jevrejeva, S., J.C. Moore, A. Grinsted,, and P.L. Woodworth, 2008, "Recent global sea level acceleration started over 200 years ago?", Geophysical Research Letters, 35, L08715, doi:10.1029/2008GL033611, 2008.

McGranahan, G., D. Balk, and B. Anderson, 2007, "The rising tide: assessing the risks of climate change and human settlements in low elevation coastal zones", Environment & Urbanization, 19(1), 17-37.

Moberg, A., et al., 2005, "Highly variable northern hemisphere temperature reconstructed from low- and high-resolution proxy data", Nature 433, pp613-617, doi:10.1038/nature03265.

United States Geological Survey (USGS), 2006, National Assessment of Coastal Vulnerability to Sea-Level Rise: Preliminary Results for the U.S. Gulf of Mexico Coast, U.S. Geological Survey Open-File Report 00-179.

Tropical update
The tropical Atlantic is quiet, and the only region worth watching is the Western Caribbean, which could see formation of a tropical disturbance with heavy thunderstorm activity this weekend.

Jeff Masters

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Nice troughing...

Member Since: 19 juillet 2008 Posts: 43 Comments: 4051
1321. futuremet,

yea I've already blamed the upper forcing for the pattern in convection.
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Is there a little rotation going on just south of Andros off the NE coast of Cuba? Or just some back spin off the trailing edge of the front?
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Hey Draky, what is your take on this so far?
Member Since: 19 juillet 2008 Posts: 43 Comments: 4051
Quoting WeatherStudent:


OK, that's a massive huge sigh of relief, THANK GOD, thanks, Levi. Hey there, Drakster.


Act as though you just dodged a Cat 5 hurricane
Member Since: 2 août 2007 Posts: 19 Comments: 19234
Quoting Acemmett90:

Very True And we all know WPB is Due


Not really in my opinion, they got Wilma, Frances, and Jeanne. I think more or less Miami or northern Florida (like Jacksonville or Orlando) is due.
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1322. Drakoen
Quoting StSimonsIslandGAGuy:
Drak is back!


Drak attack!
Member Since: 28 octobre 2006 Posts: 57 Comments: 30620
Quoting Weather456:
The area has been persistent outside the diurnal cycle. Interesting.


It is primary due to the upper level influence caused by the trough. A dry line directly to the west is also contributing to the increase convective activity. The whole synoptic area of convective active, not just the blob south of Haiti has remain constant today. However the trough is also causing strong shear over the area--not favorable for tropical cyclogenesis.

Divergence map

Notice the strong divergent gradient

Member Since: 19 juillet 2008 Posts: 43 Comments: 4051
In my opinion, South Florida is always at threat during hurricane season.

Florida gets hit by the most tropical cyclones out of all the USA so while the caribbean disturbance may not hit SFL, more than likely there will be a tropical entity that will affect the area later on in the season. It's never out of the question for an Andrew.
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Quoting WeatherStudent:


OK, that's a massive huge sigh of relief, THANK GOD, thanks, Levi. Hey there, Drakster.
no its not, we want a good soaker, we badly need the rain!!! imo
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Quoting Weather456:
The area has been persistent outside the diurnal cycle. Interesting.
yes, i say INVEST! INVEST! INVEST!!!! LOL.
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Quoting Weather456:
The area has been persistent outside the diurnal cycle. Interesting.


Has a little bit of -80C cloud tops in it too.
Member Since: 8 juillet 2005 Posts: 259 Comments: 24512
Quoting Levi32:


No they indicate the west and southern Gulf of Mexico. You're talking about the GFS and only the GFS. Or maybe the NAM....not quite as bad as the GFS but bad...lol.
lol, ok
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Quoting Drakoen:


There is some mid level rotation there.
ooh i thought it was lower levels, thanks for informing me.
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The area has been persistent outside the diurnal cycle. Interesting.
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1307. Levi32
Quoting canesrule1:
i am just following those models and they indicate SF.


No they indicate the west and southern Gulf of Mexico. You're talking about the GFS and only the GFS. Or maybe the NAM....not quite as bad as the GFS but bad...lol.
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1306. Drakoen
Quoting canesrule1:
am i going crazy or is there a spin at 76 and 15?????


There is some mid level rotation there.
Member Since: 28 octobre 2006 Posts: 57 Comments: 30620
Quoting Drakoen:


Don't get his hopes down...
i am just following those models and they indicate SF.
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am i going crazy or is there a spin at 76 and 15?????
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1303. Levi32
Quoting canesrule1:
why?


The system is going to be weak and near the Yucatan with the upper high over the western Gulf of Mexico building east in response to the TUTT exiting to the east. The disturbance, if there is one, will get pulled to the west or NW by the high. I don't see it affecting Florida at all.
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1302. Drakoen
Quoting Levi32:


I greatly doubt it.


Don't get his hopes down...
Member Since: 28 octobre 2006 Posts: 57 Comments: 30620
Quoting Levi32:


I greatly doubt it.
why?
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1300. Levi32
Quoting WeatherStudent:



That's quite alright, I meant, a threat to South Florida?


I greatly doubt it.
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Quoting canesrule1:
i know its dead.
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Quoting Drakoen:
LOL!?!?!?!?
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1297. Drakoen
LOL!?!?!?!?
Member Since: 28 octobre 2006 Posts: 57 Comments: 30620
Quoting WeatherStudent:


I will.
never know where these storms could track.

Battle of the AOI's!!!
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1295. IKE
Member Since: 9 juin 2005 Posts: 23 Comments: 37860
Quoting CybrTeddy:


Oh yes, 50%>, RIP.
the models are killing it convection is down big time and winds are down from 30 to 25 mph, what do you have to say now, and btw i dont give if it says 50%> because that to me is utterly pointless.
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They should change the twc to the cwc. first c for commercial.
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Quoting hydrus:
wind sustained 210 kts gusts 240 kts..higher than normal tides...
yes, landfall in Miami!!! mua ha ha ha ha!!!!
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Quoting hurricane23:
epac disturbance poofed.


Oh yes, 50%>, RIP.
Member Since: 8 juillet 2005 Posts: 259 Comments: 24512
Quoting CaneWarning:


I thought everyone knew I was HOG.


Wait, CaneWarning is HOG?

The plot thickens...LOL
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1287. hydrus
Quoting canesrule1:
yea im on the same boat. lol!!!
wind sustained 210 kts gusts 240 kts..higher than normal tides...
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Quoting Michfan:


Depends on how long you define a dime to be.
3 hours i would say.
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1284. IKE
Quoting TropicTraveler:
Ike - Spector is a scary guy. A general question for anyone who knows - Why did Jim Cantore start doing the morning weathercast? Saw him the first day when he got irritated at being identified as "new to the show." Just wondered.


I never watch TWC anymore. Probably ratings are down and they need a boost.
Member Since: 9 juin 2005 Posts: 23 Comments: 37860
Quoting winter123:
i predict cat 5 at 11pm
yea im on the same boat. lol!!!
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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.

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