Comment on acp-2022-810

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> 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.