Quantifying the Atmospheric CO2 Forcing Effect on Surface Ocean pCO2 in the North Pacific Subtropical Gyre in the Past Two Decades

Shuangling Chen,Chuanmin Hu,Fei Chai,Adrienne J Sutton

doi:10.3389/fmars.2021.636881

Abstract

Despite the well-recognized importance in understanding the long term impact of anthropogenic release of atmospheric CO2 (its partial pressure named as pCO2air) on surface seawater pCO2 (pCO2sw), it has been difficult to quantify the trends or changing rates of pCO2sw driven by increasing atmospheric CO2 forcing (pCO2swatm_forced) due to its combination with the natural variability of pCO2sw (pCO2swnat_forced) and the requirement of long time series data records. Here, using a novel satellite-based pCO2sw model with inputs of ocean color and other ancillary data between 2002 and 2019, we address this challenge for a mooring station at the Hawaii Ocean Time-series Station in the North Pacific subtropical gyre. Specifically, using the developed pCO2sw model, we differentiated and separately quantified the interannual-decadal trends of pCO2swnat_forced and pCO2swatm_forced. Between 2002 and 2019, both pCO2sw and pCO2air show significant increases at rates of 1.7 ± 0.1 μatm yr–1 and 2.2 ± 0.1 μatm yr–1, respectively. Correspondingly, the changing rate in pCO2swnat_forced is mainly driven by large scale forcing such as Pacific Decadal Oscillation, with a negative rate (-0.5 ± 0.2 μatm yr–1) and a positive rate (0.6 ± 0.3 μatm yr–1) before and after 2013. The pCO2swatm_forced shows a smaller increasing rate of 1.4 ± 0.1 μatm yr–1 than that of the modeled pCO2sw, varying in different time intervals in response to the variations in atmospheric pCO2. The findings of decoupled trends in pCO2swatm_forced and pCO2swnat_forced highlight the necessity to differentiate the two toward a better understanding of the long term oceanic absorption of anthropogenic CO2 and the anthropogenic impact on the changing surface ocean carbonic chemistry.

Highlights

Since industrialization, the global ocean has been a major sink of the increasing atmospheric CO2, absorbing ∼25% of anthropogenic CO2 in recent years (Sabine et al, 2004a; Friedlingstein et al, 2019; Gruber et al, 2019)
The Random Forest based Regression Ensemble (RFRE) model is more sensitive to changes in sea surface temperature (SST) and pCO2air than to changes in Chla (Figure 5)
With nearly 20 years of satellite data records, it would be straightforward to extend the current study to other oceanic regions to investigate the interannual-decadal surface pCO2sw dynamics by differentiating the atmospheric forcing and natural forcing effects toward a better understanding of the ocean in absorbing anthropogenic CO2 and its impact on the surface ocean carbonate chemistry

Summary

Introduction

The global ocean has been a major sink of the increasing atmospheric CO2, absorbing ∼25% of anthropogenic CO2 in recent years (Sabine et al, 2004a; Friedlingstein et al, 2019; Gruber et al, 2019). The resulting ocean acidification has great potential to degrade marine ecosystems and marine biota, the calcifying organisms such as shellfish and corals (Widdicombe and Spicer, 2008; Doney, 2010; Fabricius et al, 2011; Dickinson et al, 2012; Chan and Connolly, 2013; Davis et al, 2017). Both impacts are closely related to the sustainable development of the marine biota and ecology. Due to the sparse measurements of chemical tracers in space and time, there is still significant uncertainty in the long-term accumulation rates of anthropogenic CO2 and the potential of the ocean in continued absorption of anthropogenic CO2, making it important to investigate the anthropogenic CO2 variabilities at the sea surface

Methods

Results

Discussion

Conclusion