ESP 24

24. Sea Level

Washington, The Mall 2150 with gondoliers who have emigrated from Venice

24.1 WHAT IS ‘SEA LEVEL’ ?

Defining ‘Sea Level’ ? Most books will tell you that it is the level midway between low and high tides; at many places ‘mean low tide’ is considered the reference level. But the tides are different each day. This is because the moon, which, with the sun, exerts the gravitational pull responsible for the tides, is orbiting the Earth. However, the moon's orbit is tilted 5 degrees to the ecliptic (the plane of Earth’s orbit around the sun) and precesses in a cycle of 18.6 years. During this time the tilt of the moon's orbit relative to the earth's axis varies from 18 to 28 degrees. This variation has an important effect on the tides, so that their pattern repeats only after 18.6 years!

It is determined from the hourly height readings of a tide gage. The tide gauge is protected from the effects of ordinary waves, but may be affected by changes in the level of the water surface due to changes in atmospheric pressure. Hopefully those get averaged out over a 19-year period. Relatively few places that have an hourly record at least 19 years long. And the records are all different. So when someone says sea level is rising, someone else may say that it is not changing, and someone else may say it is really falling. Why? Mean sea level is now defined as: ‘the average height of the surface of the sea at a particular location for all stages of the tide over a 19-year period.’

24.2 WHY DOES SEA-LEVEL CHANGE DIFFERENTLY IN DIFFERENT PLACES?

1)the volume of water in the ocean 2) the motions of the solid Earth’s surface 3) the gravitational attraction of ice sheets 4) the speed of rotation of the Earth, 5) the location of atmospheric high and low pressure systems 6) the evaporation/precipitation balance 7) changes in the volume of the ocean basins. The first six can be related to climate change: The last is related to plate tectonics. SEVEN FACTORS DETERMINING THE POSITION OF SEA LEVEL

24.3 CHANGING THE VOLUME OF WATER IN THE OCEAN

CHANGING THE VOLUME OF WATER IN THE OCEAN This is the only factor which changes global sea level on a time scale of centuries or longer. There are three ways in which the volume of water in the ocean can change: 1) by changing the amount of salt dissolved in the water, i.e., the salinity of the ocean 2) by changing the temperature of the ocean. 3) by changing the mass of water (i.e., the number of molecules of water)

S ‰ T ⁰C SPECIFIC VOLUME OF SURFACE WATERS AT DIFFERENT SALINITIES AND TEMPERATURES

24.4 CHANGING THE MASS OF H 2 O IN THE OCEAN ReservoirMass (10 15 kg)Moles (10 15 ) Sea level equivalent (m) Oceans1,371, ,122, Groundwater58,613.03,253, Ice24,110.01,39, Rivers & Lakes , Atmosphere Biosphere

24.5 CHANGES IN GROUNDWATER RESERVOIRS

GROUNDWATER The amount of water in groundwater reservoirs on land is almost equal to the amount of ice in Ice sheets, but much of it is deeply buried. However, the shallow groundwater reservoirs may contain water equivalent to 10 meters of sea level change. These shallow groundwater reservoirs may slowly fill and empty with changing climates.

Exploiting groundwater reservoirs ‘Mining’ of groundwater occurs when the groundwater is extracted, mostly for irrigation purposes, faster than it can be replaced. In the western and southwestern US, groundwater that accumulated during the last glacial is being extensively mined, at a rate of about 35 km 3 per year.

Irrigation in California

Kansas

The Aral Sea Catastrophe In the former USSR water was taken from the Aral Sea region to the extent that the Sea almost disappeared. The water was taken both from the lake itself and from the groundwaters that surrounded it. During the period of most rapid extraction, around 1960, it was being mined at a rate of about 60 km 3 per year. These large extractions could account for a sea-level rise of about 0.3 mm/yr during the latter half of the 20th century.

24.6 CONVERSION OF ICE TO OCEAN WATER

Most of the ice today is in Antarctica, equivalent to 56+ meters of sea level rise. Second is Greenland, equivalent to 7+ meters of sea level rise. Mountain glaciers and ice caps account for less than 1 meter of sea level rise, but are melting rapidly. Arctic sea-ice is disappearing rapidly, but does not change sea level Melting Ice

Region Area (10 6 km 2 ) Ice thickness (m) Volume (Ice) (10 6 km 2 ) Volume (Water) (10 6 km 2 ) Sea level equivalent Antarctica Glacial Arctic Ocean Glacial Greenland Glacial Laurentide00000 Glacial CordilleranNegligible Glacial ScandinavianNegligible Glacial Other Glacial

24.7 STORAGE IN LAKES

Natural lakes have three main causes 1) Plate Tectonic Rifting 2) Excavation of depressions by glaciers 3) Blocking river flows by sediment bars or landslides Lakes

Lake Tanganyika An African Rift Valley Lake

Lake Baikal

The Dead Sea

Lakes in basins excavated by glaciers

Vierwaldstättersee Switzerland

Man Made Lakes Offsetting the potential sea-level rise due to mining of groundwater is the storage of water in man-made reservoirs and lakes. Most of these reservoirs were built in the latter half of the 20th century. Before the levels of many of these lakes began to fall in the 21st century, they stored 8,400 km3 of water. Filling these reservoirs may have reduced sea level by 0.48 mm/yr. Although all of these figures have large uncertainties, it is thought that the net effect of mining groundwater and building reservoirs on global sea level was close to zero.

Lake Mead

Change in Lake Mead Water Level

The Three Gorges Dam - China

The Yangtze River before and after construction of the Three Gorges Dam

24.8 EFFECT OF CHANGING THE SALINITY OF THE OCEAN

It is very difficult to change the salinity of the ocean as whole, but the extraction of the huge volumes of fresh water as ice during the last glacial did result in an increase in the salinity of seawater. Salinity rose from its present average of about 34.5 parts per thousand to about 36 parts per thousand. Removing the fresh water that made the ice sheets that existed at the Last Glacial Maximum should have lowered sea level by almost 139 meters. However the increased salinity of the water left behind made it occupy a slightly larger volume than it would have had if the salinity had remained constant, equivalent to about 4 meters of sea-level. This is probably the reason that the sea-level lowstand indicated by corals is about 135 m rather than 139 meters. Salinity

Remember these terms

DENSITY OF OCEAN SURFACE WATERS AT DIFFERENT SALINITIES AND TEMPERATURES S ‰ T ⁰C Range of Surface Water Salinities

S ‰ T ⁰C SPECIFIC VOLUME OF SURFACE WATERS AT DIFFERENT SALINITIES AND TEMPERATURES Range of Surface Water Temperatures

Relation between Temperature, Salinity, Depth, and the Density of Sea Water at Depth FP MD = Surface (0kPa) = 4000kPa (~ 4000m)= 2000kPa (~ 2000m)

24.9 THE EFFECT OF WARMING OR COOLING THE OCEAN

The effect of warming the ocean has a significant effect on sea level. If the warm surface layer of the tropical ocean, 100 meters thick, were to warm by 1ºC, sea level would rise by about 27.8 mm. A temperature increase of 1ºC or more may well have taken place in the equatorial region. Some of the sea-level rise observed on tropical Pacific and Indian Ocean islands may be due simply to the warming of the water. Things would become very different if the whole ocean were to warm. An increase of 1ºC in the 3700 meters of deep water would cause it the water column to expand by about 377 mm, over 1/3 of a meter. If the deep ocean were to take on the temperature of 15ºC, it has had during older times when the poles were ice free, the expansion would raise sea level by about 10 meters. Warming the Ocean

RECENT WARMING OF THE OCEAN There is a detailed record of temperatures in the upper part of the ocean that goes back to 1961, representing millions of measurements made by oceanographic ships, buoys, and probes. It indicates that between 1961 and 2003 the upper 700 meters of the ocean warmed by 0.1ºC. This corresponds to an average expansion raising the sea surface of 0.4 mm/yr.

Heat is being transferred into the ocean depths

24.10 MOTION OF THE EARTH’S SOLID SURFACE Sea Floor Spreading and Plate Tectonics

The long-term history of sea-level change

Cause of the long term sea-level change

Isostatic adjustment to loading and unloading by ice

On the 10,00 to 100,00 year time scale, isostatic adjustments to ice loading and unloading are important

24.11 GRAVITATIONAL ATTRACTION OF ICE SHEETS

Gravitational Attraction Of Ice Sheets The ice sheets of the Last Glacial Maximum were huge, massive objects on the Earth’s surface. They gravitationally attracted the surrounding ocean water. You might think that the effect couldn’t be very big. But it was, being in the order of tens of meters. If a wedge of water had somehow been able to penetrate to the center of Hudson Bay while the Laurentide Ice Sheet was still largely intact it would have been about 85 meters higher than the average global sea level due to the gravitational attraction of the ice.

Gravitational Attraction of the Greenland Ice Sheet More realistically, it was originally thought that for some reason Greenland had risen much more rapidly than North America as the deglaciation proceeded. This was before the gravitational attraction of the ice was taken into account. On the margin of Greenland, the sea-level fall due to the declining gravitational attraction of the ice was about 27 meters. The bulge of water around the ice extended out to a distance of about 1000 km, and the water came from the rest of the ocean, lowering sea level there by about 0.26 meters.

Sea level change must take the gravitational attraction of the ice sheets into account Today there are two bodies of ice large enough to attract the ocean’s waters, Antarctica and Greenland. One of the ways in which the total amount of ice on each of the areas is being tracked is by remote sensing of the sea surface elevation around them. As the ice melts, the amount of water held close around the ice sheets by gravitational attraction decreases; the sea surface relaxes and goes down. Because of this water attracted to the ice sheet, the total global sea-level rise due to both melting of the ice and relaxation of the sea surface would be equal to about 109% of that due to melting of the ice alone.

24.12 CHANGING EARTH’S ROTATION AXIS AND SPEED

As the lopsidedly-distributed Northern Hemisphere ice sheets melted during the deglaciation, the water was spread over the entire ocean. The redistribution of mass caused the Earth axis of rotation to move, and the planet to rotate more slowly. As isostatic adjustment to unloading proceeds slowly, the rotation speed of the planet will gradually increase, and the axis will move. These changes have affected sea levels. Changing Earth’s Rotation Axis and Speed

Dick Peltier contributed an important review paper to this volume

Sea level change during the deglaciation Red line is computed sea level change taking rotational changes into account; black line is older calculation w/o this correction. Data points shown as ‘+’.

Predicted SL history at Barbados (ICE5G-VM2) ‘Ice-equivalent’ history (ICE5G) Global Eustatic SL curve Walbroeck et al, 2002 Present depth corrected for 0.34 mm/yr tectonic uplift Possible depth range Acropora palmata (5m) Montastrea annularis (20m) Barbados Tahiti Huon Sunda Shelf Bonaparte Gulf LGM Data points of Lambeck & Chappell, 2001

Global Eustatic SL curve Walbroeck et al, 2002 ‘Ice-equivalent’ history (ICE5G) Predicted SL history at Barbados (ICE5G-VM2) ‘Ice + Ocean Area equivalent’ history (ICE5G) The effect of taking into account the change in area of the ocean with sea- level fall and rise

Red line is computed sea level change taking rotational changes into account; black line is older calculation w/o this correction. Data points shown as ‘+’.

24.13 EFFECT OF WINDS AND CURRENTS

Ocean topography forced by winds and currents

24.14 EFFECT OF EVAPORATION- PRECIPITATION BALANCE Changes sea surface topography moderating ocean currents Affects marginal seas causing them to have –Estuarine (+ FWB) flow patterns –Lagoonal (- FWB) flow patterns The sea level differences associated with these flow patters are of the order of a few centimeters

24.14 EFFECT OF EVAPORATION - PRECIPITATION BALANCE

THE BLACK SEA MODEL Lower Higher

The opposite of the Black Sea model is the Mediterranean model LowerHigher

24.15 SEA LEVEL CHANGE DURING THE DEGLACIATION

Catastrophic sea-level rises recorded in composite sections from the Caribbean

MP 1A MP 1B CRE 1CRE 2CRE 3

24.16 SEA LEVEL DURING THE HOLOCENE

Rapid sea-level fluctuations during the last (Eemian) Interglacial (MIS 5e) Rohling et al., 2007 Fig. 2c Holocene Sea-level

24.17 WHAT HAS HAPPENED TO SEA LEVEL IN THE PAST FEW CENTURIES

Global sea level change

24.18 SUMMARY Recent contributions to sea-level rise (mm/yr) Source Thermal Expansion0.42 ± ± 0.5 Greenland Ice Sheet0.05 ± ± 0.07 Antarctic Ice Sheet0.14 ± ± 0.35 Other Glaciers and Ice Caps 0.50 ± ± 0.22 Total =1.1 ± ± 0.7 Observed1.8 ± ± 0.7

Sea level rise and displaced population Sea level rise (m) Inundated area (10 3 km 2 ) Current population, Millions, Global Inundated area (10 3 km 2 ) Southeast US Current population, Millions, Southeast US Numbers are cumulative

Thank you for your attention