The Earth's oceans play a vital role in the carbon cycle, making it imperative that we understand marine biological activity enough to predict how our planet will react to the extra 25 000 million tonnes of carbon dioxide humans are pumping into the atmosphere annually.

The colour of oceanic seawater depends largely on the number of microscopic phytoplankton, marine plants that live in the well-lit surface layer. Just like land-based plants, phytoplankton accumulate carbon dioxide during photosynthesis and store it in their tissues, making them potentially important carbon sinks. While phytoplankton themselves are individually microscopic, the chlorophyll they collectively contain colours the ocean's waters, which provides a means of detecting these tiny organisms from space with dedicated ocean colour sensors.

To support ocean carbon cycle research, ESA's GlobColour project has merged 55 terabytes of data from three state-of-the-art instruments aboard different satellites, including MERIS aboard ESA's Envisat, MODIS aboard NASA's Aqua and SeaWiFS aboard GeoEye's Orbview-2, to produce a 10-year dataset of global ocean colour stretching to 2007.

"I am quite impressed by the work ESA has done so far within GlobColour," said Dr Cyril Moulin of the International Ocean Carbon Coordination Project (IOCCP). "This 10-year dataset is going to be very useful for carbon studies and global modelling."

The ocean colour datasets are freely available to the public via the GlobColour website. A new web interface, Hermes, is available which allows users to select a time period, spatial region and product type. Based on this input, the system extracts the appropriate ocean colour products for users to download.

By combining observations from multiple sensors, GlobColour brings several benefits over existing products, such as better sampling of the daily variability, smaller errors because of the larger amount of data and reduced instrumental biases.

To guarantee the data set is of good quality, the data have undergone an intensive validation process by comparing measurements from in-situ buoys. The conclusion was that the error statistics of the merged data are better than data from the three individual sensors.

In addition to aiding carbon cycle research, ocean colour data can provide oceanographers with the information they need to monitor the state of the oceans for other applications, such as for the fisheries and aquaculture industries.

GlobColour, part of ESA's Data User Element (DUE), will begin providing near-real time ocean colour observations to support this type of operational oceanography from mid-2008.

This service will continue well into the future, thanks to the European Commission (EC), who will continue production of the GlobColour time series from 2009 as part of the Marine Core Service of the GMES (Global Monitoring for Environment and Security) initiative.

"We need to sustain an international effort to make sure we can link one satellite dataset to another to build the long-time series that we need to distinguish change from cycles, and GlobColour is definitely a significant step in that perspective," said Dr James Yoder, Chair of the International Ocean-Colour Coordinating Group (IOCCG).

Marine Core Service and GMES
The Marine Core Service will deliver systematic reference information on the state of the global ocean and European Union seas by providing observational and model data, real-time predictions and ocean scenario simulations.

GMES - a joint initiative of the EC and ESA - responds to Europe's needs for geo-spatial information services by bringing together the capacity of Europe to collect and manage data and information on the environment and civil security, for the benefit of European citizens.