Autonomous seawater <i>p</i>CO<sub>2</sub> and pH time series from 40 surface buoys and the emergence of anthropogenic trends

Abstract

Abstract. Ship-based time series, some now approaching over 3 decades long, are critical climate records that have dramatically improved our ability to characterize natural and anthropogenic drivers of ocean carbon dioxide (CO2) uptake and biogeochemical processes. Advancements in autonomous marine carbon sensors and technologies over the last 2 decades have led to the expansion of observations at fixed time series sites, thereby improving the capability of characterizing sub-seasonal variability in the ocean. Here, we present a data product of 40 individual autonomous moored surface ocean pCO2 (partial pressure of CO2) time series established between 2004 and 2013, 17 also include autonomous pH measurements. These time series characterize a wide range of surface ocean carbonate conditions in different oceanic (17 sites), coastal (13 sites), and coral reef (10 sites) regimes. A time of trend emergence (ToE) methodology applied to the time series that exhibit well-constrained daily to interannual variability and an estimate of decadal variability indicates that the length of sustained observations necessary to detect statistically significant anthropogenic trends varies by marine environment. The ToE estimates for seawater pCO2 and pH range from 8 to 15 years at the open ocean sites, 16 to 41 years at the coastal sites, and 9 to 22 years at the coral reef sites. Only two open ocean pCO2 time series, Woods Hole Oceanographic Institution Hawaii Ocean Time-series Station (WHOTS) in the subtropical North Pacific and Stratus in the South Pacific gyre, have been deployed longer than the estimated trend detection time and, for these, deseasoned monthly means show estimated anthropogenic trends of 1.9±0.3 and 1.6±0.3 µatm yr−1, respectively. In the future, it is possible that updates to this product will allow for the estimation of anthropogenic trends at more sites; however, the product currently provides a valuable tool in an accessible format for evaluating climatology and natural variability of surface ocean carbonate chemistry in a variety of regions. Data are available at https://doi.org/10.7289/V5DB8043 and https://www.nodc.noaa.gov/ocads/oceans/Moorings/ndp097.html (Sutton et al., 2018).

Observing Changes in Ocean Carbonate Chemistry: Our Autonomous Future

“…Small systematic biases in estimated pCO 2 can have a large impact when scaled up to global air-sea flux estimates. The average difference between shipboard underway pCO 2 and float-estimated pCO 2 is 3.7 μatm pCO 2 (n = [35][36][37][38][39] [99,100]. However, it is unclear whether or not this represents a real systematic bias in the float estimates, or an artifact due to small sample size and spatiotemporal variability (± 1 day and ± 25 km) between float and ship measurements.…”

“…Box boundaries that are directly adjacent to one another (i.e., the upper boundaries of Profiling floats, Decadal Hydrographic Survey, and Volunteer Observing Ships) indicate the same temporal or spatial boundary but are offset for clarity in situ calibrations using traceable standard gases to achieve climate-quality pCO 2 data when combined with careful preand post-deployment quality control and calibration [33]. MAPCO 2 systems are deployed in a global network at over 40 locations, ranging from open ocean sites to the coastal ocean, with the oldest records spanning up to 15 years (www.pmel.noaa.gov/co2/, [35,36]). These moorings record year-round observations at most sites, characterizing subseasonal to interannual, as well as regional variability of pCO 2 in an array that can be used to understand the largescale processes that influence air-sea carbon dioxide fluxes.…”

An Enhanced Ocean Acidification Observing Network: From People to Technology to Data Synthesis and Information Exchange

“…Many international or local climate assessments require climate quality data both in the open ocean and coastal seas (Karl et al, 2010). The inherent variability in coastal areas results in more years of climate-quality data being required to observe trends (Sutton et al, 2018) compared to the open ocean. Uses such as monitoring for aquaculture and biological experiments, or for interpretations of local mechanisms underlying temporal and spatial variation can be served by either weather-quality data or climate-quality data.…”

“…The observing network should be optimally configured to meet modeling community needs and be fit to purpose. As detailed in the GOA-ON requirements , the purpose can include detection of OA, whereby assets should be deployed where anticipated time of emergence (ToE) of an OA signal above background natural variability occurs within a few decades in terms of biogeochemical changes, and within perhaps several decades in the case of ecological monitoring (Sutton et al, 2018). This detection requires "climate-quality" data, which involves a more stringent accuracy and precision than may be needed for some applications ( Table 1).…”

A Surface Ocean CO2 Reference Network, SOCONET and Associated Marine Boundary Layer CO2 Measurements

“…that these measurements can differ from the MBL reference data product in annual mean and seasonal variability due to local and regional effects (Northcott et al, 2019;Sutton et al, 2019). Although the uncertainty associated with pCO 2a is often not considered to be significant compared to other sources of uncertainty in Eq.…”