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PyCO2SYS: marine carbonate system calculations in Python

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PyCO2SYS is a Python implementation of CO2SYS, based on the MATLAB v2.0.5 but also including the updates made for MATLAB CO2SYS v3 as well as some additional related calculations. PyCO2SYS solves the full marine carbonate system from the values of any two of its parameters.

Every combination of input parameters has been tested, with differences in the results small enough to be attributable to floating point errors and iterative solver endpoint differences (i.e. negligible). See the scripts in validate to see how and check this for yourself, and their discussion in the online docs. Please let us know ASAP if you discover a discrepancy that we have not spotted!

Documentation is available online at

There are also some usage examples that you can either download or run live in your web browser (with no Python installation required) at PyCO2SYS-examples.


A paper describing PyCO2SYS is freely available:

Humphreys, M. P., Lewis, E. R., Sharp, J. D., and Pierrot, D. (2022). PyCO2SYS v1.8: marine carbonate system calculations in Python. Geoscientific Model Development 15, 15-43. doi:10.5194/gmd-15-15-2022.

Please also specify the full version number that you used. You can find this in Python with:

import PyCO2SYS as pyco2

As per the instructions in the the CO2SYS-MATLAB repo, you should also consider citing the original work by Lewis and Wallace (1998).


If you manage Python with conda, we recommend that you first install NumPy, pandas and xarray into the environment where PyCO2SYS is to be installed with conda.

Then, you can install from the Python Package Index:

pip install PyCO2SYS

Update an existing installation:

pip install PyCO2SYS --upgrade --no-cache-dir


Documentation for the current release, based on the main branch, is available at and at

Basic use

The only function you need is pyco2.sys. To solve the marine carbonate system from two of its parameters (par1 and par2), just use:

import PyCO2SYS as pyco2
results = pyco2.sys(par1, par2, par1_type, par2_type, **kwargs)

The keys to the results dict are described in the documentation. Arguments should be provided as scalars or NumPy arrays in any mutually broadcastable combination. A large number of optional kwargs can be provided to specify everything beyond the carbonate system parameters — read the docs!.

You can also look at the examples Notebooks that you can try out without needing to install anything on your computer.


PyCO2SYS is maintained by Dr Matthew Humphreys at the NIOZ (Royal Netherlands Institute for Sea Research) with the support of the main developers of all previous versions of CO2SYS.

Contributions are welcome - please check the guidelines before setting to work.


PyCO2SYS is licensed under the GNU General Public License version 3 (GPLv3).

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