The changing ocean and freshwater CO₂ system

On a global scale, ocean carbon dioxide levels and therefore the rate of surface ocean acidification (lowering of pH) is generally set by how much anthropogenic CO₂ humans emit to the atmosphere. Under human influence, the oceans will experience not just acidification but also warming, enhanced stratification, and deoxygenation. Under IPCC RCP8.5, a future greenhouse gas concentration trajectory that reflects very limited emission abatement, ocean CO₂ levels are expected to reach as high as 900 μatm by the end of century, lowering surface average pH by about 0.4 units and amplifying the seasonality of surface PCO₂ by up to 10-fold. However, on a local scale, the rate, magnitude, and variability of future acidification is not just a function of atmospheric CO₂ levels, but how chemical, biological and physical processes in the ocean mediate the uptake of anthropogenic CO₂. Such local processes can strongly modulate the overall long-term anthropogenic acidification from CO₂ uptake. Key regions where local processes modulate ocean acidification include biologically productive waters, intense mixing/upwelling zones, regions of recent sea-ice melt and coastal oceans. Coastal regions in particular, with large biological productivity or eutrophication have been shown to accelerate the magnitude and onset of anthropogenic ocean acidification on the regional scale. For inland freshwater systems (lakes and rivers), although data are sparse, CO₂ is generally supersaturated and subject to large variability via unique local conditions (e.g., runoff, eutrophication, local pollution etc.), implying that local processes will dominate the CO₂ conditions in the future. The large variability makes detection of acidification in inland waters a challenge. Understanding how local conditions and processes modulate the long-term rate of anthropogenic acidification will be important in order to predict the likely levels of CO₂ exposure that aquatic organisms will encounter in the future.