It is believed that the ocean carries between one third to one half of the heat transferred from the Earth's equator to its poles. In fact, it is the existence of this flux that keeps the mid-latitude regions of the Earth habitable. The ocean is the major sink for the constantly increasing human production of carbon dioxide and other "greenhouse gases." Analyses of cores from the ocean floor strongly suggest that during the distant past, when the Earth's climate was very different, the ocean circulation was also radically different.
Therefore, more information is urgently needed on oceanic circulation and changes and the oceanic carbon cycle. Present models of the ocean are inaccurate owing to uncertainties concerning the governing physics, chemistry, and biology, and also due to an inadequate ability to describe the instantaneous state of the system. ENVISAT offers the continuous, global observations of the oceans required to provide data sets for enhanced ocean modelling.
The first observations of ocean colour from space were carried out from 1978 to 1986 by the experimental Coastal Zone Colour Scanner (CZCS) aboard NASA's Nimbus-7 satellite. This instrument provided global and regional data sets which yielded a wealth of new information about the distribution and seasonal variability of primary productivity in oceans. During the period 1986 to 1996, the orderly development of ocean-colour science was hindered by the lack of an operational satellite sensor for the production of ocean-colour data. Although considerable energy was devoted to analysing the results of the then-defunct CZCS, there was no opportunity to conduct new studies in which observations from space could be matched with in situ data observations in near real time.
The picture began to improve in March 1996, when India launched the German sensor MOS (Modular Optoelectronic Scanner). Although this device does not provide global coverage, it was important as the first source of new ocean-colour data after a gap of ten years. In August 1996, Japan launched the Japanese sensor OCTS (Ocean Colour and Temperature Scanner) and the French sensor POLDER (POLarization and Directionality of the Earth’s Reflectance) on the ADEOS (ADvanced Earth Observation Satellite) mission. This was a very powerful combination, which operated until June 1997, until failure of the satellite's solar panel terminated the mission. In August 1997, the USA launched the SeaWiFS (Sea-viewing Wide Field-of-view Sensor), which has been in operation since September 1997. This instrument provides complete global coverage of the oceans about every two days if cloud free.
Over and above these sensors, a number of others have been launched (MISR, MODIS, OCI, OCM, OSMI, GLI, POLDER-2) other are planned for launch in the near future. Recent experience has emphasised that a certain controlled redundancy is essential if we are to enjoy an unbroken stream of ocean-colour data into the future.
IOCCG (International Ocean-Colour Coordinating Group) groups together information relative to the various missions and instruments enumerated here above (http://www.ioccg.org/sensors_ioccg.html).
Information may also be directly found at the following URLs: