Measuring Sea Level

The earliest gauges used to measure the tides were simply measuring sticks attached to piers. By the mid-1800s, tide gauges using floats began to be used. Much of the difficulty in gathering data to calculate sea level was centered on the fact that the geographic distribution was weak; there were simply not enough gauges distributed around the world to enable a reliable global model to be built. Most of the reliable records came from the United States and Europe. In addition to widespread geographic distribution, time interval is also an important factor. The longer the time interval that records are collected at gauging sites, the better, because long-term observations can be used to infer more accurately significant trends.

Tide gauge records depict not only the general global trend but also geographical and temporal variations specific to the gauge's location. These local fluctuations can be the result of interdecadal fluctuations of ocean density and circulation, isostatic adjustment of the land from the previous ice age (when the weight of the ice is removed from the land, the land relaxes again and rises up because the pressure weighing it down has been removed), or subsidence, which can happen when underground fluids (groundwater, oil, and so on) have been extracted, making the land settle. According to NOAA, an area that these characteristics have successfully been applied to is in the middle Atlantic region of the U.S. east coast.

Nowadays, sea-level measurements can be obtained globally from satellite data. In 1992, through the joint effort of NASA and France's National Center for Space Research (CNES), TOPEX/Poseidon was launched and successfully collected data until October 2005. On December 7, 2001, the Jason-1 satellite was launched by NASA, continuing the data-gathering mission started by the TOPEX/Poseidon. The satellite space program at NASA for gathering data to calculate global sea level is called Ocean Surface Topography. According to Jorge Vazquez at NASA's Jet Propulsion Laboratory, satellite oceanography and altim-etry is a new, evolving science that has revolutionized and changed the way scientists obtain global sea levels.

An altimeter is used to collect the data. It is a microwave radar pulse that is transmitted from an orbiting satellite in space. When the pulse hits its target on Earth, the signal is bounced back to the radar. The round-trip travel time of the radar pulse is recorded. This is similar to the radars that the highway patrol uses to catch speeding cars. It is also the methodology that Doppler radar uses to detect where rainstorm activity is currently occurring. The further away an object is, the longer it takes for the return pulse to reach the sensor and be recorded.

According to Vazquez, the use of this concept for the oceano-graphic sciences has revolutionized the way scientific knowledge is collected today. The function of the altimeter is to measure the height of the ocean. Although the ocean may seem level in its immense, vast state, it is far from it. Because there are physical structures, such as seamounts and trenches that influence the gravitational pull around them, it affects sea level. The Moon and the Sun also affect the ocean's surface level by pulling on it. Other factors also come into play, such as winds, ocean currents, thermal expansion from the Sun's heat, and atmospheric storms. Each of these factors changes sea level by a specific amount.

The TOPEX/Poseidon satellite sent out a radar signal to the ocean below, received the signal back, determined the height, and transmitted the reading to a real-time GPS.

The TOPEX/Poseidon was considered a technological breakthrough for NASA, because for the first time, it allowed scientists to determine by what amounts all of these factors influenced sea level. It was also from this data that scientists have been able to gain much of their understanding of the El Niño phenomenon. This satellite enabled scientists at NASA to observe how the ocean changes from one year to the next and study climate change and global warming. In 1998, a global map was built denoting sea-level change in the entire Pacific based on data from the satellite. TOPEX/Poseidon was able to detect changes of only one inch (2 cm), which was a remarkable improvement over a previous satellite, Seasat, which was used in 1978 and could only detect changes on the order of several yards. The TOPEX/Poseidons accuracy was attributed to the quality of its laser beams in combination with a global positioning system (GPS).

Oceanographers and climatologists at NOAA are now able to study and determine the relationships between sea-level rise and ocean sur-

Topex ■ Jason -60-day smoothing

Inverted barometer applied

Topex ■ Jason -60-day smoothing

Inverted barometer applied

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Global mean sea-level rise as measured from the TOPEX/Poseidon and Jason-1 satellite. These systems allow scientists to study climate change due to global warming. (Modeled after University of Colorado)

face temperature. This allows researchers to better understand phenomenon such as ENSO and the Pacific Decadal Oscillation (PDO). Scientists, both at NOAA and NASA, are currently looking at detecting changes in sea level due to changes in global climate, specifically on how the average global sea level rises during a given year.

Maps have been prepared using data from both TOPEX/Poseidon and Jason-1. Both NOAA and NASA scientists have detected complex patterns of sea-level rise using these satellites. Besides seeing patterns supporting the PDO, they have also detected a pattern in the North Atlantic caused by a slowdown in the circulation of the subpolar gyre (a clockwise circular ocean current). This has experts at NASA concerned because it can lead to a decrease of the northward heat transport of the ocean in the great conveyor belt. In addition, they have also detected a trend in the Indian Ocean of a decrease in the northward flow of the upper ocean, which is linked to a long-term warming of the upper Indian Ocean. Data from these satellites supply scientists at NOAA and NASA with information on not only sea surface height (topography) but the transport of heat, ocean circulation, wave height, ocean tides, wind speeds, and climatic events. This data can then be used in models designed to detect global climate change.

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