This study presents a compilation of high-resolution, continuous time series of upper ocean pH data collected across a variety of marine ecosystems, from polar to tropical, open-ocean to coastal, kelp forest to coral reef. The data reveal significant variability in pH, with standard deviations ranging from 0.004 to 0.277 and pH ranges spanning 0.024 to 1.430 units. The observed variability is site-specific, with diel, semi-diurnal, and stochastic patterns of varying amplitudes. These pH signatures indicate current exposure levels to both high and low dissolved CO₂, often showing that resident organisms are already experiencing pH regimes not predicted until 2100. The data provide a first step toward understanding the biophysical link between environmental pH exposure history and physiological resilience of marine organisms to seawater CO₂ fluctuations. Knowledge of spatial and temporal variation in seawater chemistry allows for improved design of ocean acidification (OA) experiments, enabling testing of organisms with a priori expectations of their tolerance based on natural exposure ranges. This hypothesis-testing can deepen understanding of OA effects. These and similar comparative time series can guide management efforts to identify marine habitats that can serve as refugia to acidification and those particularly vulnerable to future ocean change. The study highlights the natural variability in seawater pH across different marine ecosystems, showing that many sites exhibit greater variability than open ocean and Antarctic sites. The data reveal that pH fluctuations can be extreme, sometimes daily, and may occur in locations not typically associated with such conditions. The study also emphasizes the importance of continuous long-term observations to understand the biophysical link between natural variation and physiological capacity in marine organisms. The findings suggest that while open ocean areas exhibit stable pH conditions, other ecosystems, such as coastal upwelling, estuarine, and kelp forest areas, experience significant pH variability. These variations can influence the physiological responses, resilience, and adaptation potential of marine organisms. The study also highlights the importance of considering carbonate chemistry variability in experiments and models aimed at understanding the impacts of acidification. The data collected provide a critical step in understanding the consequences of ocean change by linking present-day pH exposures to organismal tolerance and ecological change in marine ecosystems.This study presents a compilation of high-resolution, continuous time series of upper ocean pH data collected across a variety of marine ecosystems, from polar to tropical, open-ocean to coastal, kelp forest to coral reef. The data reveal significant variability in pH, with standard deviations ranging from 0.004 to 0.277 and pH ranges spanning 0.024 to 1.430 units. The observed variability is site-specific, with diel, semi-diurnal, and stochastic patterns of varying amplitudes. These pH signatures indicate current exposure levels to both high and low dissolved CO₂, often showing that resident organisms are already experiencing pH regimes not predicted until 2100. The data provide a first step toward understanding the biophysical link between environmental pH exposure history and physiological resilience of marine organisms to seawater CO₂ fluctuations. Knowledge of spatial and temporal variation in seawater chemistry allows for improved design of ocean acidification (OA) experiments, enabling testing of organisms with a priori expectations of their tolerance based on natural exposure ranges. This hypothesis-testing can deepen understanding of OA effects. These and similar comparative time series can guide management efforts to identify marine habitats that can serve as refugia to acidification and those particularly vulnerable to future ocean change. The study highlights the natural variability in seawater pH across different marine ecosystems, showing that many sites exhibit greater variability than open ocean and Antarctic sites. The data reveal that pH fluctuations can be extreme, sometimes daily, and may occur in locations not typically associated with such conditions. The study also emphasizes the importance of continuous long-term observations to understand the biophysical link between natural variation and physiological capacity in marine organisms. The findings suggest that while open ocean areas exhibit stable pH conditions, other ecosystems, such as coastal upwelling, estuarine, and kelp forest areas, experience significant pH variability. These variations can influence the physiological responses, resilience, and adaptation potential of marine organisms. The study also highlights the importance of considering carbonate chemistry variability in experiments and models aimed at understanding the impacts of acidification. The data collected provide a critical step in understanding the consequences of ocean change by linking present-day pH exposures to organismal tolerance and ecological change in marine ecosystems.