Abstract
Abstract. Oceanic dissolved inorganic carbon (TC) is the largest pool of carbon that substantially interacts with the atmosphere on human timescales. Oceanic TC is increasing through uptake of anthropogenic carbon dioxide (CO2), and seawater pH is decreasing as a consequence. Both the exchange of CO2 between the ocean and atmosphere and the pH response are governed by a set of parameters that interact through chemical equilibria, collectively known as the marine carbonate system. To investigate these processes, at least two of the marine carbonate system's parameters are typically measured – most commonly, two from TC, total alkalinity (AT), pH, and seawater CO2 fugacity (fCO2; or its partial pressure, pCO2, or its dry-air mole fraction, xCO2) – from which the remaining parameters can be calculated and the equilibrium state of seawater solved. Several software tools exist to carry out these calculations, but no fully functional and rigorously validated tool written in Python, a popular scientific programming language, was previously available. Here, we present PyCO2SYS, a Python package intended to fill this capability gap. We describe the elements of PyCO2SYS that have been inherited from the existing CO2SYS family of software and explain subsequent adjustments and improvements. For example, PyCO2SYS uses automatic differentiation to solve the marine carbonate system and calculate chemical buffer factors, ensuring that the effect of every modelled solute and reaction is accurately included in all its results. We validate PyCO2SYS with internal consistency tests and comparisons against other software, showing that PyCO2SYS produces results that are either virtually identical or different for known reasons, with the differences negligible for all practical purposes. We discuss insights that guided the development of PyCO2SYS: for example, the fact that the marine carbonate system cannot be unambiguously solved from certain pairs of parameters. Finally, we consider potential future developments to PyCO2SYS and discuss the outlook for this and other software for solving the marine carbonate system. The code for PyCO2SYS is distributed via GitHub (https://github.com/mvdh7/PyCO2SYS, last access: 23 December 2021) under the GNU General Public License v3, archived on Zenodo (Humphreys et al., 2021), and documented online (https://pyco2sys.readthedocs.io/en/latest/, last access: 23 December 2021).
Highlights
The ocean absorbs about a quarter of the anthropogenic carbon dioxide (CO2) currently emitted each year (Friedlingstein et al, 2020)
As the original CO2SYS software is so well-established in the research field, we provide a relatively brief summary of the components of PyCO2SYS that are identical to CO2SYS-MATLAB in Sect. 2, before describing the areas where PyCO2SYS differs in more detail in Sect
Pressure is in decibars and represents the hydrostatic pressure exerted by the overlying water column, consistent with typical oceanographic conductivity– temperature–depth (CTD) measurement reporting
Summary
The ocean absorbs about a quarter of the anthropogenic carbon dioxide (CO2) currently emitted each year (Friedlingstein et al, 2020). Removing CO2 from the atmosphere reduces the impact of these emissions on Earth’s climate. CO2 uptake causes seawater pH and calcium carbonate mineral saturation states ( ) to decline through a process termed ocean acidification, which has adverse effects on some marine species and ecosystems (Doney et al, 2009). Exchange of CO2 between the atmosphere and ocean, and the biogeochemical consequences of this process, are governed by a series of equilibrium chemical reactions and parameters collectively known as the marine carbonate system (Millero, 2000).
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