CMI Annual Report 2021

industrial level because most of current and near-term future
H 2
production comes from steam methane reforming.
Model results show that H 2
emission pulses cause a small,
transient growth of tropospheric CH 4
that slowly decays in a
few decades. Moreover, the replacement of fossil-fuel energy
with renewable or low-carbon hydrogen can have very different
consequences for tropospheric CH 4
. This depends on the H 2
production method and the amount of H 2
lost to the
atmosphere (Figure 5.2). For renewable H 2
, tropospheric CH 4
would decrease due to the fossil-fuel displacement only if the
rate of H 2
leakages is kept below a critical rate. This critical
rate is around 8%, but with an uncertainty interval of between
5 and 11% related to how OH consumption is partitioned
among the tropospheric gases and how much of atmospheric
H 2
is consumed by soil bacteria. For blue H 2
, not only would
the CH 4
emissions increase, but the combination of CH 4
and H 2
leakages may have undesired consequences for the
tropospheric burden of CH 4
(Figure 5.2).
Figure 5.2a-d.
Scenarios of a
hydrogen-based
economy. ΔS is the
source variation
(panels a and c).
Δ[CH 4
] and Δ[H 2
]
are the consequent
variation in
tropospheric
concentrations
(panels b and d).
(a-b) Changes in H 2
and CH 4
sources
(a) and tropospheric
concentrations
(b) as a function of
the H 2
leakage rate
and the green H 2
replacement of fossil
fuels energy (%).
(c-d) Changes
in H 2
and CH 4
sources (c) and CH 4
concentration (d)
for a 15% fossil-fuel
replacement with
green or blue H 2
with
different CH 4
and H 2
leakage rates.
These preliminary results call for more precise estimates of
future H 2
leakage rates and additional analyses with high
resolution three-dimensional atmospheric chemistry models.
Carbon Mitigation Initiative Twenty-first Year Report 2021
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