Natural variability and anthropogenic change revealed by moored time series observations of pCO2 and pH

Chair: James Orr

Adrienne J. Sutton(1,2,) Christopher L. Sabine(1,) Richard A. Feely(1)

1
NOAA Pacific Marine Environmental Laboratory, Seattle, WA, 98115, USA
2 University of Washington, Joint Institute for the Study of the Atmosphere and Ocean, Seattle, WA, 98195, USA

Background
One of the major challenges to assessing present day and future impacts of ocean acidification is the need to better understand anthropogenic change in the context of natural variability of carbonate chemistry. Temporally-dense observations, such as those on moorings, can be highly effective in defining interannual, seasonal, and subseasonal variability at key locations and assessing the underlying trends.

Methods
Here we present time series of carbonate chemistry at mooring sites in open ocean regions, U.S. coastal waters, and Caribbean and Pacific coral systems using 3-hourly observations of surface seawater pCO2 and pH collected as early as 1997 and 2009, respectively. We use monthly climatologies from the observations to calculate deseasonalized trends in pCO2, pH, and aragonite saturation state (Ωaragonite) and assess the factors affecting the detection of trends at these locations.

Findings
Our observations suggest that the rate of change in carbonate chemistry at most open ocean sites is generally consistent with the change expected from equilibration with atmospheric increases of 2 µatm yr-1. Due to the length of the time series (≤ 18 years), these rates encompass both anthropogenic change and decadal modes of variability, such as the Pacific Decadal Oscillation, which we find has had a significant influence in the Equatorial Pacific. Observed rates of change at coastal and coral reef mooring locations, while less robust due to large interannual variability, are highly variable and range from no significant change to rates of change three times that of atmospheric increases. Strong seasonal patterns also emerge with some sites exhibiting subseasonal conditions approaching Ωaragonite = 1.

Conclusions
These results highlight the considerable variability in ocean carbonate chemistry. Moored observations can resolve these broad time scales of variability and change at key locations and are a powerful tool for identifying biases in less temporally-resolved data products and models.