In this commentary, we provide additional context for Ocko and Hamburg (2022) related to the climate consequences of replacing fossil fuels with clean hydrogen alternatives. We first provide a tutorial for the derivations of underlying differential equations that describe the radiative forcing of hydrogen emissions, which differ slightly from equations relied on by previous studies. Ocko and Hamburg (2022) defined a metric based on time-integrated radiative forcing from continuous emissions. To complement their analysis, we further present results for temperature and radiative forcing over the next centuries for unit pulse and continuous emissions scenarios. Our results are qualitatively consistent with previous studies, including Ocko and Hamburg (2022). Our results clearly show that for the same quantity of emissions, hydrogen shows a consistently smaller climate impact than methane. As with other short-lived species, the radiative forcing from a continuous emission of hydrogen is proportional to emission rates, whereas the radiative forcing from a continuous emission of carbon dioxide is closely related to cumulative emissions. After a cessation of hydrogen emissions, the Earth cools rapidly, whereas after a cessation of carbon dioxide emissions, the Earth continues to warm somewhat and remains warm for many centuries. Regardless, our results support the conclusion of Ocko and Hamburg (2022) that, if methane were a feedstock for hydrogen production, any possible near-term consequences will depend primarily on methane leakage and secondarily on hydrogen leakage.

In a recent paper, Ocko and Hamburg (2022) examined the climate consequences of replacing fossil fuel technologies with clean hydrogen alternatives. The paper accounted for a range of hydrogen and methane emission rates for two types of clean hydrogen production pathways, i.e., green hydrogen produced via renewables and water, and blue hydrogen produced via steam methane reforming with carbon capture, usage, and storage (CCUS). They calculated the time-integrated radiative forcing using equations derived recently for hydrogen based on chemistry–climate modeling experiments (Warwick et al., 2022). Ocko and Hamburg (2022) found that high emission rates of hydrogen could diminish net climate benefits of clean hydrogen technologies, and high emission rates of methane might lead to net climate disbenefits for blue hydrogen in the near term (e.g., 20-year timescale).

Here, we provide context for understanding the results of Ocko and Hamburg (2022) in three different ways: (1) we present equations underlying the time evolution of hydrogen and its radiative and thermal consequences and solve them analytically for unit pulse and continuous hydrogen emissions scenarios; (2) we present global mean temperature and radiative forcing in the time domain covering 500 years; and (3) we examine three scenarios, including a unit pulse, a limited duration (square wave), and a continuous emissions framework. Our aim here is to complement Ocko and Hamburg (2022), which emphasizes the near term, with an analysis that places greater emphasis on long-term outcomes using newly developed equations.

We derive and apply equations underlying the estimate of radiative forcing from hydrogen (H

The system that describes the radiative forcing from H

The change of

The presence of additional H

For the special case of a unit pulse perturbation of H

The indirect radiative forcing from a unit pulse emission of H

For a 1

Equation (7) is the response to a unit pulse emission of H

Considering a continuous unit emission scenario where

In a linear system, the time-integrated radiative forcing from a unit pulse emission to some time horizon

Ocko and Hamburg (2022) used a metric that is equal to the time-integrated radiative forcing of continuous emissions to time horizon

Comparing Eq. (12) with Eq. (7a), we can see that the CAGWP metric is equivalent to the AGWP metric, except that the radiative forcing at time 0 is
weighted by

Expanding Eq. (12), we have

Equations (10) and (13) consider continuous emissions through the whole period. Equations considering a continuous emission to time

Here, we show radiative forcing and time-integrated radiative forcing functions for

The AGWP for a unit pulse emission is

Radiative forcing for continuous emissions of

And the corresponding CAGWP is

For a linear system, the absolute global temperature change potential (AGTP), defined as the change of global mean surface temperature realized at a given
time horizon from a pulse or continuous emissions of any gas

In Eq. (22),

Uncertainty in the temperature response function is shown in Sect. S3.

As in Ocko and Hamburg (2022), we focus on comparing the climate impact of replacing fossil fuel technologies with clean hydrogen alternatives. Climate impacts from hydrogen or fossil fuels are the summation of climate impacts of one or more components in a linear system (Sect. S4).

In this commentary, we analyze the climate impact per 1

We focus our discussions on three H

Climate impact from emissions of respective species. Radiative forcing

We first examine the time-evolving climate impact from emissions of carbon dioxide (

Figure 1 shows the climate impact of individual species under various emission scenarios. Results showing ratios of CH

The climate impact of CH

For 0.01

Radiative forcing from consumption of green hydrogen (H

Same as Fig. 2 but for the global mean temperature response. Figures showing only 100-year results are plotted in Fig. S14.

Under the low-leakage scenario (i.e., 1 % H

Under the high-leakage scenario, the additional leakage of H

In our central cases, we do not include CH

Finally, Ocko and Hamburg (2022) quantified the net climate benefits of consuming H

The radiative forcing calculation presented here is a linear approximation, with radiative forcing increasing linearly with concentration, when in
fact absorption bands become increasingly saturated at higher concentrations, and this results in less sensitivity at higher concentrations. The
radiative forcing calculation assumes an unchanging background atmospheric composition, whereas it is likely that the climate impact of an emission
will depend on the background climate state (Duan et al., 2019; Robrecht et al., 2019). For instance, the indirect radiative forcing of hydrogen (H

Many important uncertainties persist. For example, we considered the chemical adjustment to radiative forcing for CH

Ocko and Hamburg (2022) propose a metric, which we call CAGWP, that involves the integral of radiative forcing for continuous emissions, which
differs from the standard GWP metric based on a unit emission of 1

There are different motivations for reducing warming at various timescales. One motivation is to avoid near-term climate damage that might come, for example, from increasing storm or drought intensity. Another motivation is to avoid long-term climate damage that might come, for example, from the melting of the large ice sheets (Pattyn et al., 2018) or from making parts of the tropics effectively uninhabitable (Dunne et al., 2013; Sun et al., 2019). Decision-making can balance near-term and long-term risks and identify opportunities to address both kinds of risk simultaneously.

Different climate forcing agents differ in their degrees of reversibility. To a close approximation, on the timescale of decades or more, temperature
change from CH

Considering how different market sizes would affect the overall impact of H

Our analysis confirms the results of Ocko and Hamburg (2022) under consistent assumptions but complements their presentation with additional
uncertainty analysis and a longer-term perspective. While we confirm the results presented in Ocko and Hamburg (2022), it is clear that over longer
time horizons (e.g., 100 years), substituting blue or green hydrogen (H

We have developed a tutorial for the derivations of underlying differential equations that describe the radiative forcing of H

Consideration of CH

Ocko and Hamburg (2022) propose that the climate impact of blue and green H

We emphasize that to attain near-term climate benefits from “blue” H

Scripts used to derive equations presented in this analysis are written in Wolfram Mathematica and are available online at

All parameter values used to evaluate the climate impact of different species in this paper have been taken directly from previous studies, which are listed and cited in the paper (e.g., Ocko and Hamburg, 2022; Myhre et al., 2013; Gasser et al., 2017).

The supplement related to this article is available online at:

LD and KC designed the simulations, developed the equations, and did the calculations. LD prepared the initial paper and both of them reviewed and edited the paper.

In the interest of transparency, we would like to point out that Ken Caldeira is an employee of a non-profit organization that funds early commercial demonstration projects related to clean alternatives that can displace carbon-intensive technologies, and this can include clean hydrogen to decarbonize industry. In the further interest of transparency, note that Lei Duan is a consultant for a for-profit entity that has no known investments related to clean hydrogen.

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

This work is supported by a gift from Gates Ventures LLC to the Carnegie Institution for Science. The authors thank Leslie Willoughby for language polishing.

This paper was edited by Andreas Engel and reviewed by two anonymous referees.