This WP will compare simulated marine biogeochemistry of importance for climatic impacts and feedbacks with relevant observations. The WP has been designed to be an integral part of the development cycle of marine biogeochemical models. It will perform an evaluation of improved marine biogeochemical processes across a hierarchy of configurations, including offline, forced (interfaced with OCMIP) and coupled mode (results of full ESM simulations) and feed results back to WP2 in order to guide further model development. Finally, we will develop new skill assessment metrics for both global and regional improvements in marine biogeochemistry. Suitable new top-down and key process-oriented diagnostics and performance metrics will be integrated into ESMValTool.