Contribuer à un océan plus propre   The Braer oilspill disaster, Shetland Islands, Scotland, January 1993.   Ship dumping sewage in the North Sea.   The biggest contributor to oil pollution in the world's oceans is operational discharges from tankers. That is oil dumped during cleaning operations. The amount of oil introduced in the sea by such operations is equivalent to one full tanker disaster every week.   In a radar image the ocean is not blue but black and white and may also appear to have a lot of strange features. As a general rule, the brighter the sea surface, the rougher the sea (because wind speed is higher). But oil on the water dampens the rough surface and it becomes smooth again, and dark, as is shown in the image.   Like its predecessor ERS-1 (launched in July 1991 by Ariane 4, the ERS-2 satellite (launched on 21 April 1995 also by Ariane 4) monitors the ground day and night under all weather conditions thanks to its powerful sharp-eyed, cloud-piercing radars. ERS-2 also carries an instrument to help monitor the ozone layer.