Modified future diurnal variability of the global surface ocean CO₂ system

Our understanding of how increasing atmospheric CO₂ and climate change influences the marine CO₂ 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 CO₂ system has already changed, with enhanced seasonal variations in the surface ocean pCO₂ over recent decades and further amplification projected by models over the 21st century. Mesocosm studies and CO₂ vent sites indicate that diurnal variability of the CO₂ 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 CO₂ 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 pCO₂ 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 pCO₂ 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 pCO₂ diurnal variability is the enhanced sensitivity of pCO₂ to changes in temperature as the ocean absorbs atmospheric CO₂ . Our projections suggest that organisms in the future ocean will be exposed to enhanced diurnal variability in pCO₂ and [H⁺ ], with likely increases in the associated metabolic cost that such variability imposes.