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CO₂e calculation

A CO₂ equivalent (CO₂e) is a metric used to compare the emissions from various greenhouse gases based on their impact on global warming. For this, the amounts of other gases are converted to the amount of CO₂ that would have the same impact on global warming (over a 100-year period).

The formula used for the calculation of the carbon footprint (CO₂e) of one Nextflow task is based on the methodology introduced by Green Algorithms, which provides a standardized way to estimate the carbon footprint of computational tasks:

\[ \begin{equation*} \text{CO₂e (g)} = t \times \left( n_c \times u_c \times P_c + n_m \times P_m \right) \times PUE \times CI \end{equation*} \]

where

\[ \begin{aligned} t & = \text{runtime of the computation (h)} \\ n_c & = \text{number of cores} \\ u_c & = \text{core usage factor (between 0 and 1)} \\ P_c & = \text{power draw per core (W)} \\ n_m & = \text{size of memory available (GB)} \\ P_m & = \text{power draw of memory (W, per GB)} \\ PUE & = \text{Power Usage Effectiveness of the data center} \\ CI & = \text{carbon intensity of energy production, which is the amount of CO₂e emitted per kWh of energy, depending on the energy mix of a region} \end{aligned} \]

For the final CO₂e estimation of a pipeline run, the values of all cached and completed tasks are summed up. This includes failed tasks as well.

Note

The usage of GPUs is not yet supported.

Additional equivalences

Let \( E_{\mathrm{CO_2e}} \) denote the total emitted CO₂ equivalent (in grams) for the computation or pipeline run. This value is used in the following equivalence calculations:

Tree sequestration time

This equates to the time a tree needs to bind the same amount of carbon from the atmosphere. It is estimated to be on average \(10\)-\(11~\mathrm{kg}\) per year (= \(917~\mathrm{g}\) per month).

\[ t_{\text{tree}} = \frac{E_{\mathrm{CO_2e}}}{917~\mathrm{gCO_2e}/\mathrm{month}} \]

Car kilometers

The car kilometers are defined as the distance an average car would need to travel to emit the same amount of CO₂. An average European car emits \(175~\mathrm{gCO_2e}\) per km.

\[ d_{\text{car}} = \frac{E_{\mathrm{CO_2e}}}{175~\mathrm{gCO_2e}/\mathrm{km}} \]

Flights London-Paris

A flight between London and Paris is estimated to emit \(50\,000~\mathrm{g}\) of CO₂ equivalents.

\[ n_{\text{L-P}} = \frac{E_{\mathrm{CO_2e}}}{50\,000~\mathrm{gCO_2e}} \]

The value is given as a percentage when less than one flight is equivalent to the emitted CO₂.

References

Green Algorithms: Quantifying the Carbon Footprint of Computation
Lannelongue, L., Grealey, J., Inouye, M.,
Adv. Sci. 2021, 2100707. https://doi.org/10.1002/advs.202100707