Chlorophyll is a pigment found in all plants on land and in water; it gives plants their characteristic green colour. In the open ocean, chlorophyll is found in tiny single-celled free-floating organisms collectively known as phytoplankton. Many thousands of species of differing shapes and sizes make up phytoplankton. Phytoplankton are responsible for producing some 50 gigatonnes of organic carbon per year globally, an amount comparable to net production by land based plants.

Phytoplankton use chlorophyll to capture light for photosynthesis, in which carbon dioxide and water are combined to produce organic matter and oxygen. In the field of ecology, this process is referred to as primary production, since the organic material produced by phytoplankton serves as food for larger organisms at sea.

Phytoplankton are important players in the global carbon cycle. The chlorophyll they contain is a measure of the abundance of phytoplankton in the water

In a changing climate, it is important to know how the concentration and distribution of chlorophyll might be changing in response to changes in available light or nutrients or ambient temperature, and whether these have an impact on organisms higher up the food webs. We are also interested to know whether the timings of major events in the seasonal cycle of phytoplankton might change. Potential changes can affect the marine food chain. Finally it is important to know whether the primary production by phytoplankton is changing.

There are many methods for measuring chlorophyll concentration. For this work, it was quantified using satellite imagery. Because chlorophyll is a pigment, it absorbs light, changing the colour of water according to the amount and type of phytoplankton present. Although individual phytoplankton cells are very small, they are sufficiently abundant in the sun-lit layers of the surface ocean for the changes in ocean colour to be detectable at the level of an orbiting satellite.

Ocean-colour changes detected by satellites are mapped as chlorophyll concentration using optical models and validated by comparison with direct ship-based measurements. Although satellite measurement isn’t perfect (satellites can’t see through clouds and other coloured material in the water can affect the measurement), it does allow high frequency measurement (one measurement every few days) around the globe, to a high spatial resolution (about 4 km²) in the example provided here. Further, combined data from multiple satellites can generate long time series, which are important for demonstrating any changing pattern over time.

The data presented here are global-scale fields of chlorophyll concentration derived from ocean-colour sensors. Data from multiple sources (SeaWiFS and MODIS-A sensors from NASA and the MERIS sensor from European Space Agency) have been combined and validated against ship-based measurements to produce a time series from late 1997 to mid 2012. This product was generated through the Ocean Colour Climate Change Initiative of the European Space Agency (www.esa-oceancolour-cci.org).

In addition, for the TWAP Project, two test products have been generated using the OC-CCI data, to illustrate applications of the data: primary production and indicators of phenology.

Primary production was computed on a monthly basis, using chlorophyll fields, information on light available at the sea surface (obtained from NASA) and a model of primary production. Implementation of the model requires assignment of some parameters, which were estimated based on available ship-based measurements.

Indicators of phenology (the time of occurrence of peak values in chlorophyll in any given year in any location; the time when the chlorophyll concentration started to increase beyond a background value; and the duration of the peak, as represented by half width of peak at half height of peak) were also computed for each year, and for every one-degree grid. These characteristics of phenology were computed using five-day average data for each grid.