19 Dec 2022
19 Dec 2022
Status: this preprint is currently under review for the journal ACP.

Comment on “Climate consequences of hydrogen emissions” by Ocko and Hamburg (2022)

Lei Duan1,2 and Ken Caldeira1,3 Lei Duan and Ken Caldeira
  • 1Carnegie Institution for Science, Stanford, California, USA
  • 2Orca Sciences LLC, Kirkland, Washington, USA
  • 3Breakthrough Energy LLC, Kirkland, Washington, USA

Abstract. 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. To develop a better understanding of the climate impact from atmospheric hydrogen additions, we first provide a step-by-step tutorial for the derivations of underlying differential equations that describe radiative forcing of hydrogen emissions, which differ slightly from equations relied on by previous studies. Ocko and Hamburg (2022) used a time-integrated metric on radiative forcing and considered a continuous emission scenario, while we present both the time-evolving radiative forcing and global mean temperature response results under a unit pulse and continuous emissions scenarios. Our analysis covers timescales of 500 years and results on short-term timescales (e.g., 20 years) are qualitatively consistent with previous studies. Some qualitative results are clear: radiative forcing from hydrogen emission is smaller compared to the same quantity of methane emission, both of which decay with time and show less long-term influence than carbon dioxide. On the time scale of a few decades, the radiative forcing from a continuous emission of hydrogen or methane 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 clean hydrogen consumption, the earth cools rapidly, whereas after a cessation of carbon dioxide emissions, the earth continues to warm somewhat and remains warm for many centuries. These longer-term differences may be important to consider in a policy context. Hydrogen leakage has the potential to reduce near-term climate benefits of hydrogen use, but methane is likely to play a more substantial role. In our analysis, consideration of methane emission associated with fossil fuel combustion is a critical factor for determining the relative short-term climate benefits of clean hydrogen alternatives. In the main cases, consideration of methane leakage substantially increases the climate impacts of fossil fuels and could result in net climate benefits for blue hydrogen even in the near-term. 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 critically on the issue of methane leakage.

Lei Duan and Ken Caldeira

Status: open (until 22 Feb 2023)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • AC1: 'Comment on acp-2022-810', Lei Duan, 27 Dec 2022 reply
  • RC1: 'Comment on acp-2022-810', Anonymous Referee #1, 05 Jan 2023 reply

Lei Duan and Ken Caldeira

Lei Duan and Ken Caldeira


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Short summary
Ocko and Hamburg (2022) emphasizes the short-term climate impact of hydrogen, and we present an analysis that places greater focus on long-term outcomes. We have derived equations that describe the time-evolving impact of hydrogen, and shown that higher methane leakage is primarily responsible for the warming potential of blue hydrogen, while hydrogen leakage plays a less critical role. Fossil fuels show more prominent longer-term climate impacts than clean hydrogen under all emission scenarios.