H2 effects on CH4 lifetime

[kdorheim]

NOTE: this PR is targeting the dev-h2 branch

This branch implements the H2 effects on CH4, it will close #749. We have added equation (5) into Hector to account for how $H_2$ interacts with OH, decreasing the amount of OH that can interact with $[CH_4]$. We derived the value for the $c_{H_2}$ from Bertagni et al. 2022.

Since we don't have $H_2$ emissions for the historical or future periods, we shouldn't see any changes in Hector results (Leeyabot or the old new tests). In order to get a feel for how this development affects Hector results I've included some results from emission impulse runs.

This first figure shows results from three Hector runs, a constant emission-driven run, and then two impulse runs varying in magnitude. It is intended to show how this model development cascades throughout the model. The pulse of $H_2$ emissions causes $\tau_{OH}$ (methane's lifetime with respect to OH) to change, which causes $[CH_4]$ to change, which is what we were expecting. Since in Hector the trop. $O_3$ burden and strat. $H_2O$ vapor radiative forcing computations are proportional to $[CH_4]$ the $H_2$ emissions pulse affects these variables as well.

This second figure demonstrates the sensitivity of impulse response to our new parameter $c_{H_2}$. As we would hope, increasing and decreasing this parameter changes the system response.

In this third figure, we compare the RF response from unit emission of $CO_2$ and $H_2$ 100 years after the emissions impulse, which can be used to calculate the GWP100. Hector's GWP100 for H2 ~ 5.17 is consistent with the Sand et al. results included in Supplementary Table 6. Sand et al. found the GWP100 of hydrogen due to methane ranges from 4.9 to 5.7 with a model mean of 5.1, which is not too far off what we are getting from Hector at the moment!

References

Bertagni, M. B., Pacala, S. W., Paulot, F., & Porporato, A. (2022). Risk of the hydrogen economy for atmospheric methane. Nature Communications, 13(1), 7706.

Sand, M., Skeie, R. B., Sandstad, M., Krishnan, S., Myhre, G., Bryant, H., Derwent, R., Hauglustaine, D., Paulot, F., Prather, M., & Stevenson, D. (2023). A multi-model assessment of the Global Warming Potential of hydrogen. Communications Earth & Environment, 4(1), 1–12.

Materials Used

PR-758.zip

[github-actions]

Differences in Hector outputs

Hello, this is leeyabot!

The current pull request's outputs do not differ from 3.1.1 (d931a00).

[kdorheim]

Some specific questions for @ssmithClimate & @bpbond

• COEFF_H2() - do we like this function name for the H2 coefficient to be added to the OH lifetime equation? Do we think it is worth having it exposed as an R function? Or should it be something users would need to change in the ini file? Should we expose the other coefficient R bindings as well?
• Right now in the oh_component.cpp all of the coefficients in what corresponds to equations (1-5) are undefined unitvals, is that okay? Or do we want to define them as something?

[kdorheim]

@bpbond just a heads up I did increase this for my own sanity purposes related to the gcam-hector coupling...

[kdorheim]

@bpbond is this the correct usage of the const double?

[kdorheim]

@bpbond do we have strong feelings on the use of e as a variable name?

[github-actions]

Differences in Hector outputs

Hello, this is leeyabot!

The current pull request's outputs do not differ from 3.1.1 (d931a00).