Abstract
AbstractThe North Atlantic Ocean is a region of intense uptake of atmospheric CO2. To assess how this CO2 sink has evolved over recent decades, various approaches have been used to estimate basin‐wide uptake from the irregularly sampled in situ CO2 observations. Until now, the lack of robust uncertainties associated with observation‐based gap‐filling methods required to produce these estimates has limited the capacity to validate climate model simulated surface ocean CO2 concentrations. After robustly quantifying basin‐wide and annually varying interpolation uncertainties using both observational and model data, we show that the North Atlantic surface ocean fugacity of CO2 (fCO2−ocean) increased at a significantly slower rate than that simulated by the latest generation of Earth System Models during the period 1992–2014. We further show, with initialized model simulations, that the inability of these models to capture the observed trend in surface fCO2−ocean is primarily due to biases in the models' ocean biogeochemistry. Our results imply that current projections may underestimate the contribution of the North Atlantic to mitigating increasing future atmospheric CO2 concentrations.
Highlights
About 50% of the carbon dioxide (CO2) emitted from fossil fuel and land‐use change activities is absorbed each year by the natural terrestrial and marine sinks, in similar proportions (Le Quéré et al, 2018)
The Multiple linear regression (MLR) over 5° latitude band widths shows that while the method is robust in the subtropical North Atlantic, the range of uncertainties increases as we move northward (Figure S4), suggesting that the proxy variables used within the MLR might miss some information regarding the long‐term change in the subpolar region and/or that not enough observations exist at high latitudes to detect a well‐delimited signal
Since questions asked by policy makers relate to how the system is changing, which are answered by using Earth System Models (ESMs), it is important to assess how these models simulate change
Summary
About 50% of the carbon dioxide (CO2) emitted from fossil fuel and land‐use change activities is absorbed each year by the natural terrestrial and marine sinks, in similar proportions (Le Quéré et al, 2018). Rödenbeck et al, 2015) or are too broad to allow the Global Biogeochemical Cycles detection of significant temporal trends in surface fCO2−ocean (Jones et al, 2015, 2019) This lack of well‐ delimited interpolation uncertainty on observational‐based products limits our ability to understand the marine carbon response to increasing atmospheric CO2 and climate change, and inhibits our evaluation of surface CO2 concentrations simulated by climate models. By making use of the strengths of both observational and model data, here we robustly quantify basin‐wide interpolation uncertainties of North Atlantic surface fCO2−ocean from 1992 to 2014, which allows us to (1) determine whether the change in the surface ocean CO2 concentrations is significant, and (2) robustly compare the observation‐ based results with those simulated by the current generation of Earth System Models (ESMs) and identify potential shortcomings in those models
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