Hydrogen as an Indirect Greenhouse Gas

Via Frank Smith: Richard Derwent has written, (in "Global Warming Consequences of a Future Hydrogen Economy", pp 25-33, Issues in Environmental Science and Technology, No. 20, Transport & the Environment, editors R.E.Hester and R.M.Harrison, The Royal Society of Chemistry 2004 (ISBN 0-85404-295-4; ISSN 1350-7583)), that complete replacement of the fossil fuel based energy system by hydrogen would require 2500 Tg (teragrams) per year and assuming a leakage rate of 1 %, i.e of 25 Tg.y-1, there would be a global warming effect equivalent to 150 Tg of CO2. [1 Tg = 1 million metric tons] The following discussion describes where some of these numbers come from and what they mean.  

Some facts about atmospheric hydrogen: Mixing ratio 510 ppb ± 10 ppb, depending on the hemisphere. Atmospheric lifetime about 2.5 years Global burden in atmosphere = about 180 Tg

The Global Warming Potential of a gas is defined as the mass emission of carbon dioxide that produces the same long-term commitment to global warming as a 1 kg release of that gas.

Many different trace gases may induce global tropospheric warming through the "Greenhouse Effect". The future commitment to global warming from the instantaneous release of such gases depends on three factors;

1. The different infrared absorption properties of each trace gas.
2. The different atmospheric lifetimes which govern the long-term behaviour of each gas.
3 The influence of each trace gas on the concentration of other greenhouse gases.

Some radiatively active gases are highly stable in the atmosphere and may linger for centuries. There is a significant long-term commitment to global warming from the continued atmospheric release of a number of trace gases, in addition to carbon dioxide, including nitrous oxide (N2O) and the chlorofluorocarbons. Methane (CH4), on the other hand, has an atmospheric lifetime of only a decade and so what is released today will disappear within a few decades.

Historically, carbon dioxide and water vapour have contributed most to the "Greenhouse Effect". The increasing concentrations of carbon dioxide since the industrial revolution are likely to have contributed more to global warming than have the changes in any other trace gases over this same period. However, some of these other trace gases exhibit a much greater efficiency for the absorption of long wavelength radiation compared with carbon dioxide. As a result over the last few decades, the balance between the effects of carbon dioxide and the effects of the other radiatively active gases may have changed. Recent analyses have indicated that the other radiatively active gases may now jointly contribute as much annually to global warming as does carbon dioxide (V.Ramanathan,R.J.Cicerone, H.B.Singh, and J.T.Kiehl,J.Geophysical Research, 1985, Vol.90, p 5547-5566).

Derwent estimates the concentration-time behaviour of a trace gas by solving the differential equation for a well-mixed box representing the atmosphere:

dc/dt = - (c/τ) where c is the average volume mixing ratio, t is time and τ is the lifetime of the trace gas.

Then c = Co e-(t/τ) where Co is the intial concentration at t = 0. Integrating this equation over the time horizon, th, the total commitment to elevated concentrations of that trace gas, ΔC becomes

ΔC = integral from 0 to th of Co e-(t/τ)

For 1 kg release of a trace gas of molecular weight MW , then ΔC = (τ/MW) . (MWair/5.136 X 10+18) X 10+6 X (1 - exp -(th/τ)) ppm years, where MWair is the molecular weight of the atmosphere, 28.96 g. mol-1

For trace gases which may exert an indirect "Greenhouse Effect" due to tropospheric ozone, it's necessary to calculate ΔW, the global warming commitment in oC years.

ΔW = a . ΔC

where ΔC is the commitment to elevated concentrations in ppm years and a is the warming contribution of ozone in oC per ppm.

These equations come from R.G.Derwent, AERE Harwell Report R 13716, Trace Gases and their Relative Contribution to the Greenhouse Effect, Jan 1990.

Tropospheric ozone precursor species include nitrogen oxides (NOx, meaning NO and NO2), methane (CH4), organic compounds, hydrogen and carbon monoxide (CO). Each of these gases may potentially be an indirect greenhouse gas because their emissions may influence the tropospheric distribution of ozone. These ozone precursor species also control the tropospheric distribution of hydroxyl radicals. This distribution in turn controls the lifetime and hence global scale build-up of methane (D.H.Ehhalt, Tellus, 1974 Vol. 26, p 58.).

Because hydrogen reacts with tropospheric hydroxyl (OH) radicals, emissions of hydrogen to the atmosphere perturb the distributions of methane and ozone, the second and third most important greenhouse gases after carbon dioxide. The methane concentration is increased above what it would have been by the removal of OH radicals by the reaction:

OH + H2 --> H + H2O

This in turn diminishes the destruction of methane by the reaction:

OH + CH4 --> CH3 + H2O

The mechanism whereby the ozone concentration is affected is not perfectly clear from the quoted review paper. Quoting the author directly,

"As a result of the increase in the atmospheric burden of hydrogen following the emission pulse, adjustments followed on in the concentrations of all the major tropospheric free radical species and ultimately in tropospheric ozone."

Hydrogen is therefore an indirect greenhouse gas with a global warming potential of 5.8 over a 100-year time period. A future hydrogen economy would therefore have greenhouse consequences and would not be free from climate perturbations. The contribution by hydrogen of excess methane to the global warming potential was calculated to be 3.4, while its contribution to the excess ozone was 2.4. (See R.G.Derwent, W.J.Collins, C.E.Johnson & D.S.Stevenson, Climate Change, 2001, Vol. 49, p 463, in which the authors have quantified the importance of hydrogen as a greenhouse gas.)

In the figure illustrating the computer modelling using the global Lagrangian chemistry transport model STOCHEM (M.G.Sanderson, W.J.Collins, R.G.Derwent & C.E.Johnson, J.Atmos.Chem., 2003, Vol.46, p15.), a pulse of 40 Tg H2 is injected into the atmosphere and comparison made between its behaviour over a four-year period and the baseline fluctuations previously established. The hydrogen-excess (maximum 36.5 Tg at 1.5 months) decayed away exponentially, the OH decreased to a minimum at about 6 months, from which it slowly recovered in fits and starts. The growth of methane mirrors the decline of H2, rising to a maximum 5.5 Tg at about 4 years before decaying with the 12 year time constant associated with the methane adjustment time. The excess O3 peaks (at 0.55 Tg) after six months and declines with an e-folding time of 2.5 years.

A 1 Tg H2 emission pulse produced a time-integrated methane radiative forcing calculated as 0.35 mW.m-2 year over a 100-year period. The time-integrated ozone radiative forcing was calculated as 0.25 mW.m-2 year over a 100 year time horizon.

The fossil fuel system currently in use is assumed to emit 23 000 Tg annually of CO2. To replace it completely would require annual production of H2 at 2500 Tg. Leakage of 1 % of this (25 Tg H2.y-1) would impact the climate at 0.6 % of the current fossil-based system; leakage of 10 % (250 Tg H2.y-1) at 6 % of the current system.

Table 1 of this paper reviews (from 4 author groups) what is known about present hydrogen sources (roughly one quarter man-made, one quarter from biomass burning, a quarter from methane oxidation and another quarter from oxidation of organic compounds) and sinks. Sources are estimated at around 80 Tg.y-1 and sinks from 74 to 98 Tg.y-1, of which soil uptake is thought to be the largest single sink.

(Richard Derwent's address is: 18 Kingsland Grange, NEWBURY, Berkshire, RG14 6LH, England)

Created: 2005-06-14 02:38:09 by Arthur Smith