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

Our understanding of how increasing atmospheric CO and climate change influences the marine CO 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 system has already changed, with enhanced seasonal variations in the surface ocean pCO over recent decades and further amplification projected by models over the 21st century. Mesocosm studies and CO vent sites indicate that diurnal variability of the CO 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 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 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 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 diurnal variability is the enhanced sensitivity of pCO to changes in temperature as the ocean absorbs atmospheric CO . Our projections suggest that organisms in the future ocean will be exposed to enhanced diurnal variability in pCO and [H ], with likely increases in the associated metabolic cost that such variability imposes.