Introduction

Here we give a brief introduction to openscm_units.

The unit registry

openscm_units.unit_registry extends Pint’s default unit registry by adding simple climate modelling related units. We’ll spare the details here (they can be found in our documentation), but the short idea is that you can now do all sorts of simple climate modelling related conversions which were previously impossible.

import traceback

import pandas as pd
import seaborn as sns
from pint.errors import DimensionalityError

import openscm_units
from openscm_units import unit_registry

print(f"You are using openscm_units version {openscm_units.__version__}")

You are using openscm_units version 0.5.4

Basics

openscm_units.unit_registry knows about basic units, e.g. ‘CO2’.

unit_registry("CO2")

1 CO2

Standard conversions are now trivial.

unit_registry("CO2").to("C")

0.2727272727272727 C

emissions_aus = 0.34 * unit_registry("Gt C / yr")
emissions_aus.to("Mt CO2/yr")

1246.666666666667 CO2 megametric_ton/a

Contexts

In general, we cannot simply convert e.g. CO\(_2\) emissions into CH\(_4\) emissions.

try:
    unit_registry("CH4").to("CO2")
except DimensionalityError:
    traceback.print_exc(limit=0, chain=False)

pint.errors.DimensionalityError: Cannot convert from 'CH4' ([methane]) to 'CO2' ([carbon])

However, a number of metrics exist which do allow conversions between GHG species. Pint plus OpenSCM’s inbuilt metric conversions allow you to perform such conversions trivially by specifying the context keyword.

with unit_registry.context("AR4GWP100"):
    ch4_ar4gwp100_co2e = unit_registry("CH4").to("CO2")

ch4_ar4gwp100_co2e

25.0 CO2

Gas mixtures

Some gases (mainly, refrigerants) are actually mixtures of other gases, for example HFC407a (aka R-407A). In general, they can be used like any other gas. Additionally, openscm_units provides the ability to split these gases into their constituents.

emissions = 20 * unit_registry("kt HFC407a / year")

with unit_registry.context("AR4GWP100"):
    print(emissions.to("Gg CO2 / year"))

42140.0 CO2 * gigagram / year

unit_registry.split_gas_mixture(emissions)

[4.0 <Unit('HFC32 * kt / year')>,
 8.0 <Unit('HFC125 * kt / year')>,
 8.0 <Unit('HFC134a * kt / year')>]

Building up complexity

openscm_units is meant to be a simple repository which does one thing, but does it well. We encourage you to use it wherever you like (and if you do please let us know via the issue tracker). As an example of something we can do, we can quickly see how GWP100 has changed between assessment reports.

units_of_interest = ["CO2", "CH4", "N2O", "HFC32", "CFC11"]
metrics_of_interest = ["SARGWP100", "AR4GWP100", "AR5GWP100"]
data = {
    "unit": [],
    "metric": [],
    "value": [],
}
for metric in metrics_of_interest:
    with unit_registry.context(metric):
        for unit in units_of_interest:
            data["unit"].append(unit)
            data["metric"].append(metric)
            data["value"].append(unit_registry(unit).to("CO2").magnitude)

data = pd.DataFrame(data)

sns.catplot(
    data=data,
    x="metric",
    y="value",
    kind="bar",
    col="unit",
    col_wrap=5,
    sharey=False,
)

<seaborn.axisgrid.FacetGrid at 0x7f56983c5a60>