Reviewer Comment on essd-2021-328

doi:10.5194/essd-2021-328-rc2

Abstract

Abstract. A common strategy for calculating the direction and rate of carbon dioxide gas (CO2) exchange between the ocean and atmosphere relies on knowledge of the partial pressure of CO2 in surface seawater (pCO2(sw)), a quantity that is frequently observed by autonomous sensors on ships and moored buoys, albeit with significant spatial and temporal gaps. Here we present a monthly gridded data product of pCO2(sw) at 0.25â latitude by 0.25â longitude resolution in the northeastern Pacific Ocean, centered on the California Current System (CCS) and spanning all months from January 1998 to December 2020. The data product (RFR-CCS; Sharp et al., 2022; <a href="https://doi.org/10.5281/zenodo.5523389">https://doi.org/10.5281/zenodo.5523389</a>) was created using observations from the most recent (2021) version of the Surface Ocean CO2 Atlas (Bakker et al., 2016). These observations were fit against a variety of collocated and contemporaneous satellite- and model-derived surface variables using a random forest regression (RFR) model. We validate RFR-CCS in multiple ways, including direct comparisons with observations from sensors on moored buoys, and find that the data product effectively captures seasonal pCO2(sw) cycles at nearshore sites. This result is notable because global gridded pCO2(sw) products do not capture local variability effectively in this region, suggesting that RFR-CCS is a better option than regional extractions from global products to represent pCO2(sw) in the CCS over the last 2Â decades. Lessons learned from the construction of RFR-CCS provide insight into how global pCO2(sw) products could effectively characterize seasonal variability in nearshore coastal environments. We briefly review the physical and biological processes â acting across a variety of spatial and temporal scales â that are responsible for the latitudinal and nearshore-to-offshore pCO2(sw) gradients seen in the RFR-CCS reconstruction of pCO2(sw). RFR-CCS will be valuable for the validation of high-resolution models, the attribution of spatiotemporal carbonate system variability to physical and biological drivers, and the quantification of multiyear trends and interannual variability of ocean acidification.

Highlights

The concentration of carbon dioxide gas (CO2) in Earth’s atmosphere has rapidly increased from about 280 parts per million in 1750 to over 400 parts per million today (Joos and Spahni, 2008; Dlugokencky and Tans, 2019)
85 Here, we present a reconstruction of pCO2(sw) (1998–2020) in a broad region of the Northeast Pacific (NEP) that includes the California Current System (CCS), surrounding open-ocean regions, and the highly variable continental shelf of the North American west coast spanning from southern Alaska to Baja California
RFR-CCS was constructed from pCO2(sw) observations in the Surface Ocean CO2 Atlas version 2021 (Bakker et al, 2016), which were related to predictor variables (Table 1) using a random forest regression approach

Summary

Introduction

The concentration of carbon dioxide gas (CO2) in Earth’s atmosphere has rapidly increased from about 280 parts per million in 1750 to over 400 parts per million today (Joos and Spahni, 2008; Dlugokencky and Tans, 2019). This rise in CO2 concentration is a direct result of human activities such as fossil fuel combustion, deforestation, and agriculture (Ciais et al, 30 2014; Friedlingstein et al, 2020). A portion of anthropogenic CO2 (~25%) dissolves directly into the ocean (Friedlingstein et al, 2020), mitigating its warming potential. A primary method for calculating the amount of CO2 transferred to the ocean requires knowing the difference between the partial pressure of CO2 in the atmosphere and surface seawater

Methods

Results

Conclusion