Modified future diurnal variability of the global surface ocean CO 2 system.

Our understanding of how increasing atmospheric CO 2 and climate change influences the marine CO 2 system and in turn ecosystems has increasingly focused on perturbations to carbonate chemistry variability. This variability can affect ocean-climate feedbacks and has been shown to influence marine ecosystems. The seasonal variability of the ocean CO 2 system has already changed, with enhanced seasonal variations in the surface ocean pCO 2 over recent decades and further amplification projected by models over the 21st century. Mesocosm studies and CO 2 vent sites indicate that diurnal variability of the CO 2 system, the amplitude of which in extreme events can exceed that of mean seasonal variability, is also likely to be altered by climate change. Here, we modified a global ocean biogeochemical model to resolve physically and biologically driven diurnal variability of the ocean CO 2 system. Forcing the model with 3-h atmospheric outputs derived from an Earth system model, we explore how surface ocean diurnal variability responds to historical changes and project how it changes under two contrasting 21st-century emission scenarios. Compared to preindustrial values, the global mean diurnal amplitude of pCO 2 increases by 4.8 μatm (+226%) in the high-emission scenario but only 1.2 μatm (+55%) in the high-mitigation scenario. The probability of extreme diurnal amplitudes of pCO 2 and [H + ] is also affected, with 30- to 60-fold increases relative to the preindustrial under high 21st-century emissions. The main driver of heightened pCO 2 diurnal variability is the enhanced sensitivity of pCO 2 to changes in temperature as the ocean absorbs atmospheric CO 2 . Our projections suggest that organisms in the future ocean will be exposed to enhanced diurnal variability in pCO 2 and [H + ], with likely increases in the associated metabolic cost that such variability imposes.