By Than Htun (Myanmar Geosciences Society)
Viewed from space, Earth appears as a blue marble, as approximately 70% of Earth’s surface is covered by ocean water. The vast ocean holds roughly 97% of the planet’s water and represents 99% of the living space on Earth. Below the surface, the ocean is teeming with life; in fact, most of our planet’s lifeforms live in the ocean, from tiny microscopic organisms to the enormous blue whale-the largest known animal on Earth.
In addition to its diverse spectrum of life, the ocean is well known for its salty seas. Processes that took place throughout Earth’s history, such as the weathering of rocks, evaporation of ocean water, and the formation of sea ice, have altered the ocean’s chemical properties, making it salty. At the surface, ocean water contains about 3.5% salt.
The ocean also holds a tremendous amount of heat. The main source of heat is energy from the sun. The tropics receive more energy from the sun than the polar regions; in addition, the presence of ice in the polar regions reflects a large amount of incoming solar radiation. As a result, water near the equator is much warmer than ocean water near the poles. This equator-to-pole solar heating imbalance is the primary mechanism that drives atmospheric and oceanic circulation. In the upper ocean, surface winds drive currents. Below the surface, global density gradients caused by differences in temperature (thermo) and salinity (haline) drive the ocean’s thermohaline circulation. Currents, waves, and tides help transport water, heat, and nutrients throughout the seven ocean basins-the North Pacific, South Pacific, North Atlantic, South Atlantic, Indian, Southern, and Artic. The ocean’s physical (e.g., temperature) chemical (e.g., nutrients, salinity), and biological (e.g., living organisms) components are in a constant state of flux and interact with one another in different ways. Such interactions structure marine ecosystems and influence Earth’s weather and climate; they also have an impact on the global carbon cycle. Without Earth’s ocean, our planet would be uninhabitable.
The world is getting warmer. Most this warming has occurred since the 1970s, with the 20 warmest years having occurred since 1981 and with all 10 of the warmest years occurring in the past 12 years. The ocean has absorbed much of this increased heat, with the top 700 meters (about 2,300 feet) of ocean showing a warming of 0.302 degrees Fahrenheit since 1969. With global average sea surface temperatures on the rise, scientists have also noticed a decline in phytoplankton in many ocean regions. At Earth’s poles, both the extent and thickness of Arctic sea ice has declined rapidly over the last several decades, and the Greenland and Antarctic ice sheet have decreased in mass. Global sea level rose about 17 centimetres (6.7 inches) in the last century.
Since the beginning of Industrial Evolution, the global surface ocean has experienced a 30 per cent increase in acidity. This increase is the result of the ocean absorbing atmospheric carbon dioxide (CO2). In addition to higher amounts of CO2 being absorbed by the upper ocean, historical data show that North Atlantic sub-tropical surface waters have become saltier in the last 40 years, while sub-polar North Atlantic deeper waters have become less salty. Such changes in salinity appear to be related to changes in evaporation, precipitation, and ocean circulation. Melting of polar ice caps and glaciers also impacts salinity at high latitudes. Changes in salinity patterns can have can have adverse effects on global circulation and on the ability of the ocean to absorb CO2.
Observing the Ocean
Scientists can measure ocean properties directly, through in situ sensors and by taking water samples, or indirectly, using remote sensing techniques (e.g., from Earth-observing satellites). Remote sensing techniques measure the characteristics of light, or radiance, coming from the Earth’s surface. Unlike ship-based measurements, which can only sample small portions of the ocean at a time, Earth-observing satellite can provide continuous, global coverage over long timescales. Even so, ship-based sampling remains critical for validating remotely sensed measurements.
NASA has been observing Earth’s ocean from space for more than 38 years, beginning with the launch of the first civilian oceanographic satellite, Seasat, on January 28, 1978. The mission was designed to demonstrate the possibility of global satellite monitoring of oceanographic phenomena and to help determine the requirements for an operational ocean remote sensing satellite system. Seasat operated in Earth orbit for 105 days, measuring sea surface winds and temperatures, wave heights, atmospheric liquid water content, sea ice features, and ocean topography. October of that same year saw the launch of the first ocean colour mission, the Coastal Zone Colour Scanner Experiment (CZCS), which lasted until December 1986. The CZCS was designed as a proof-of-concept to determine if satellite remote sensing of colour could be used to identify and quantify material suspended or dissolved in ocean waters. These satellite missions laid the groundwork for future ocean missions.
Today, there are several ocean-observing satellite and airborne missions that measure a variety of parameters including ocean surface topography, currents, waves, winds, phytoplankton content, dissolved and suspended organic matter, sea-ice extent, rainfall, sunlight reaching the sea, and sea surface temperature and salinity. NASA works with its domestic and international partners to support several of these missions, and plans to extend existing as well as new measurements into the future.
Combining Ocean Measurements to Observe Interactions
Like the human body, Earth’s systems interact with one another in complex ways. For example, the naturally occurring El Nino and La Nina phenomenon showcases an intricate relationship between the atmosphere and ocean in the equatorial Pacific, with impacts at a global level. Sea surface temperature is a critical variable, connecting the atmosphere and the ocean; scientists study changes in global sea surface temperature patterns to understand, and predict, future ocean conditions. Since sea surface height measurements yield critical information about the depth of the subsurface temperatures (in general, warm water expands and cold water contracts), they too provide key information about the ocean, such as the onset, maintenance, and dissipation of El Nino and La Nina.
Physical changes often drive biological changes in the ocean. The deeper thermocline curtails the usually vigorous upwelling of deep-ocean nutrients to the surface, causing phytoplankton concentrations.
The opposite phase, La Nina, is characterized by strong trade winds that cause upwelling to intensify in the eastern Pacific. More intense upwelling generally coincides with higher phytoplankton concentrations. El Nino-induced decreases in phytoplankton biomass, which is the base of the food web, have severe impacts further up the food chain. Major fishery collapses have occurred during El Nino years, and this has impactions for larger marine animals that depend on this food supply. However, the La Nina events that often follow have the opposite effect: stronger east-to-west trade winds increase nutrient upwelling which fertilize surface waters, leading to large phytoplankton blooms. These blooms are accompanied by increases in fish populations. Combining physical and biological observational capabilities enables scientists to achieve a better understanding of the ocean.
The Ocean, You, and NASA
Earth’s ocean provides essential goods and services to humankind, called ecosystem services. These services include seafood, medicine, energy sources (from oil and gas, wind, and waves), storm protection (by way of coastal barriers such as mangroves, marshes, and coral reefs) detoxification (by trapping sediment and nutrients in estuaries), marine transportation and trade, recreational and educational resources, among many others.
Humans are directly impacted by changes in ecosystem services on a range of scales. A rise in sea level can increase the flood potential for entire countries, states, cities, and even individual homes. Decline in ocean productivity can have global and regional impacts on food supply. Beach closures and fish kills that result from polluted waters or harmful algal blooms negatively impact the regional economy and recreation potential.
NASA has the ability to observe and detect changes in the ocean (and on Earth as a whole) on a variety of spatial and temporal scales. This allows scientists to conduct research on the causes and consequences of those changes, which uniquely positions the Agency to improve life on our planet.
Future Earth-observing satellite missions, such as the Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) and Surface Water and Ocean Topography (SWOT) missions, are schedule to launch in the 2020-22 timeframe. The PACE mission will deliver the most comprehensive global combined ocean-atmosphere measurements in NASA history. Not only will PACE monitor the health of our ocean and its living marine resources, it will provide extensive measurements of aerosols (tiny airborne particles) and clouds. The SWOT mission brings together international communities whose focus is to better understand Earth’s ocean and terrestrial surface waters, and the interplay between them. These and other missions will ultimately unveil a variety of new products to aid our understanding of the atmosphere, land, and ocean and their roles in Earth’s changing climate for many years to come.
Source: National Aeronautics and Space Administration (NASA), US.