Using present day observations to detect when ocean acidification exceeds natural variability of surface seawater Ωaragonite

One of the major challenges to assessing the impact of ocean acidification on marine life is the need to better understand the magnitude of long-term change in the context of natural variability. High-frequency moored observations can be highly effective in defining interannual, seasonal, and subseasonal variability at key locations. Here we present monthly aragonite saturation state (Ω_aragonite) climatology for 15 open ocean, coastal, and coral reef locations using 3-hourly moored observations of surface seawater pCO₂ and pH collected together since as early as 2009. We then use these present day surface mooring observations to estimate pre-industrial variability at each location and compare these results to previous modeling studies addressing global-scale variability and change. Our observations suggest that open oceans sites, especially in the subtropics, are experiencing Ω_aragonite values throughout much of the year which are outside the range of pre-industrial values. In coastal and coral reef ecosystems, which have higher natural variability, seasonal patterns where present day Ω_aragonite values exceeding pre-industrial bounds are emerging with some sites exhibiting subseasonal conditions approaching Ω_aragonite = 1. Linking these seasonal patterns in carbonate chemistry to biological processes in these regions is critical to identify when and where marine life may encounter Ω_aragonite values outside the conditions to which they have adapted.