Climate consequences of hydrogen emissions
Authors: Ocko Ilissa B. , Hamburg Steven P. 
Published: July 2022

Abstract. Given the urgency to decarbonize global energy systems,
governments and industry are moving ahead with efforts to increase
deployment of hydrogen technologies, infrastructure, and applications at an
unprecedented pace, including USD billions in national incentives and direct
investments. While zero- and low-carbon hydrogen hold great promise to help
solve some of the world's most pressing energy challenges, hydrogen is also
an indirect greenhouse gas whose warming impact is both widely overlooked
and underestimated. This is largely because hydrogen's atmospheric warming
effects are short-lived – lasting only a couple decades – but standard
methods for characterizing climate impacts of gases consider only the
long-term effect from a one-time pulse of emissions. For gases whose impacts
are short-lived, like hydrogen, this long-term framing masks a much stronger
warming potency in the near to medium term. This is of concern because
hydrogen is a small molecule known to easily leak into the atmosphere, and
the total amount of emissions (e.g., leakage, venting, and purging) from existing
hydrogen systems is unknown. Therefore, the effectiveness of hydrogen as a
decarbonization strategy, especially over timescales of several decades,
remains unclear. This paper evaluates the climate consequences of hydrogen
emissions over all timescales by employing already published data to assess
its potency as a climate forcer, evaluate the net warming impacts from
replacing fossil fuel technologies with their clean hydrogen alternatives,
and estimate temperature responses to projected levels of hydrogen demand.
We use the standard global warming potential metric, given its acceptance to
stakeholders, and incorporate newly published equations that more fully
capture hydrogen's several indirect effects, but we consider the effects of
constant rather than pulse emissions over multiple time horizons. We account
for a plausible range of hydrogen emission rates and include methane
emissions when hydrogen is produced via natural gas with carbon capture, usage, and storage (CCUS) (“blue”
hydrogen) as opposed to renewables and water (“green” hydrogen). For the
first time, we show the strong timescale dependence when evaluating the
climate change mitigation potential of clean hydrogen alternatives, with the
emission rate determining the scale of climate benefits or disbenefits. For
example, green hydrogen applications with higher-end emission rates (10 %)
may only cut climate impacts from fossil fuel technologies in half over the
first 2 decades, which is far from the common perception that green
hydrogen energy systems are climate neutral. However, over a 100-year
period, climate impacts could be reduced by around 80 %. On the other
hand, lower-end emissions (1 %) could yield limited impacts on the climate
over all timescales. For blue hydrogen, associated methane emissions can
make hydrogen applications worse for the climate than fossil fuel
technologies for several decades if emissions are high for both gases; however, blue hydrogen yields climate benefits over a 100-year period. While more work is needed to
evaluate the warming impact of hydrogen emissions for specific end-use cases
and value-chain pathways, it is clear that hydrogen emissions matter for the
climate and warrant further attention from scientists, industry, and
governments. This is critical to informing where and how to deploy hydrogen
effectively in the emerging decarbonized global economy.