Coastal Zones and Marine Ecosystem

5.1.2.c

Sea Level Change Around the World

“It is very likely that the 20th century warming has contributed significantly to the observed rise in global average sea level and increase in ocean-heat content.” – Intergovernmental Panel on Climate Change Synthesis Report p. 50

Tide gauge records, after correcting for land movements, show that the global average annual sea level rise was between 1 and 2 mm during the twentieth century. You might suspect that the cause of this recent sea level rise is primarily due to ice melting.. However, this is not the case. When a substance warms up, the added heat energy makes the particles move faster and farther apart. This increases the volume of the substance, no matter what state it is in, solid, liquid, or gas. This applies to warming oceans. Warming has in fact been the greatest contributor to recent sea-level rise. Figure 1 illustrates that indeed, the oceans have been warming in recent years. In 1998, the upper 300 m of the world’s oceans contained more heat than it did in the mid-1950s, a warming of about 0.3oC.

Figure 1. Time-series for 1948 to 1998 of ocean heat content anomalies in the upper 300 m for the two hemispheres and the global ocean. Vertical lines through each yearly estimate are ± one standard error (Source: IPCC, TAR: Scientific Basis. Section 2.2.2.5)

Sea level Rise – the measurements

Tide Gauge measurements

The tide gauge measurement records the level of the sea surface compared to the land on which the gauge is located. Records vary. Some sites in Europe have nearly continuous records of sea level over the past 300 years. (See Figure 2 Relative Sea Level Over the Last 300 Years). Scientists consider land movement as well as sea level rise when they analyse the tide gauge data. The land the tide gauge is sitting on can also be moving. There are two types of land movements: active plate tectonics and glacial rebound. Tectonics have to do with movement of plates of the Earth’s crust. Glacial rebound is a slow uplifting of land after the glaciers melted and the enormous weight was removed. This uplifting is still happening.

Figure 2 Relative Sea Level Over the Last 300 Years (Source: IPCC, TAR: Scientific Basis. Section 11.3.1) IPCC Synthesis Report Climate Change 2001 p.52

Satellite altimeter observations

Satellites can accurately measure sea levels and provide global coverage of the Earth’s oceans. They can also measure sea level change in a particular region. Satellite altimeters also measure sea level with respect to the centre of the earth. Satellite altimetry avoids the tectonic and glacial rebound factors that affect tide gauge measurements.

The TOPEX/POSEIDON satellite-altimeter mission covers the Earth from 66oN to 66oS (almost all of the ice-free oceans). The satellites have been measuring sea level from late 1992.. The TOPEX/POSEIDON data allows global average sea level to be estimated to a precision of several millimetres every ten days.

The most recent estimates of global average sea level rise from the six years of TOPEX/POSEIDON data (using corrections from tide gauge comparisons) range from 2.1 to 3.1 mm/yr. (See Figure 1. Global Mean Sea Level Variations). Some scientists say that El Niño-Southern Oscillation (ENSO) events cause a rise and fall in global averaged sea level of about 20 mm. You can see the effect of the major 1997/98 El Niño-Southern Oscillation event in the graph below. (See El Niño-Southern Oscillation (ENSO.) Scientists try to separate long-term trends from climatic variability like an El Niño event. The TOPEX/POSEIDON data suggest that the rate of sea-level rise during the 1990s was greater than the average rate of rise for much of the twentieth century.

Figure 1.
Grey line: Global mean sea level variations computed from the TOPEX/POSEIDON satellite altimeter data
Black line: Global averaged sea surface temperature variations for 1993 to 1998.

Observations of thermal expansion are showing a higher rate of sea level rise over recent decades off the North American east coast than the higher latitude sub-polar areas of the Atlantic Ocean. In China, relative sea level is rising at about 2 mm/yr in the south but less than 0.5 mm/yr in the north, with an estimated average of the whole coastline of 1.6 mm/yr. The two longest records from Australia (over 80 years) are from Sydney and Fremantle, on opposite sides of the continent. They show observed rates of relative sea level rise of 1.26 mm/yr and 1.73 mm/yr.

Impact on Coastal Systems

Coastal zones have many different ecosystems such as deltas and intertidal zones. Coastal wetlands provide important habitat and nourishment for a large number of birds and fish. Coastal zones also support a great number of activities such as fisheries and recreation. Coastal human populations in many countries have been growing at double the national rate of population growth. About half of the global population lives in coastal zones, although there is large variation among countries.

Changes in climate will affect coastal systems through sea-level rise, an increase in storm-surge hazards, and possible changes in the frequency and intensity of extreme events like hurricanes.

Coastal wetlands generally have been able to keep pace with the historic rate of sea-level rise. Because the current rate is greater than the rate prevailing over the past several thousand year, some areas are already experiencing large losses of coastal wetlands.