Since the beginning of the industrial revolution humans have released approximately 500 billion metric tons of carbon into the atmosphere from burning fossil fuels, cement production and land-use changes. About 30% of this carbon dioxide has been taken up (or absorbed) by the oceans. The oceanic uptake of carbon dioxide leads to a change in marine carbonate chemistry resulting in a decrease of seawater pH and carbonate ion concentration, a situation which is commonly called 'Ocean Acidification' (OA). To date, the majority of the scientific studies into the potential impacts of OA and efforts for monitoring the effects of OA have focussed on the use of models and in situ studies (such as buoys, research cruises and lab or field based mesocosm studies). Space observations from satellite Earth observation (EO) have yet to be fully exploited and could play an important role in this area of science through providing quasi-synoptic, reproducible and calibrated measurements for investigating processes on global scales Pathfinders-OA is a feasibility study to evaluate the role that satellite EO can play in supporting and expanding OA research We are achieving this by bringing together multidisciplinary expertise and capability in:

• Marine carbonate chemistry (in situ and numerical modelling)
• Marine EO (active and passive sensors)
• Algorithm development and validation
• Efficient data processing

Pathfinders-OA is exploiting Earth Observation (EO) data to quantify parameters required for OA research. It is implementing existing algorithms and generating new ones in five geographical regions (global, Arctic seas, Bay of Bengal, the Greater Caribbean and the Amazon Plume). The project is investigating which algorithms perform best in each region and validating the algorithms using in situ data and model output. The result will be a set of regional algorithms and monthly datasets of EO derived pH and aragonite saturation state (Ωar) together with an estimate of their associated uncertainties.