Up, Up and Away, my beautiful, my beautiful balloon.
-The Fifth Dimension, 1967
Hydrogen, like helium, is lighter than air. You can fill balloons with hydrogen, and they will rise and float away. Hydrogen is in vogue as an alternative fuel for the transportation sector. You can burn hydrogen and it produces only water vapor. Hydrogen would seem to have little direct impact Earth’s climate or its ozone layer.
But, hydrogen carries a dirty secret. The easiest way to make hydrogen is from fossil fuels, especially natural gas. With this approach, producing hydrogen is responsible for carbon dioxide added to Earth’s atmosphere. Only if the hydrogen is made without fossil fuels (e.g., solar) is it really clean.
As a small molecule, hydrogen is easily lost to leaks during storage and transport. It is of more than passing interest that the hydrogen content of the atmosphere has increased 70% since the late 1800s, as determined by levels of hydrogen gas trapped in buried bubbles in Antarctic ice. When the concentration of an atmospheric gas is changing, we must look to both changing sources and changing sinks for an explanation. It is easy to jump to the conclusion that human production of hydrogen as fuel may be responsible, but the increase started long before our interest in using hydrogen as an alternative fuel for automobiles.
Hydrogen is produced from natural processes, such as the oxidation of methane in the atmosphere, as well as human sources. Hydrogen is consumed in a variety of reactions, especially in soils and sediments by microbes that consume hydrogen as an energy source for metabolism. It seems unlikely that we have reduced the consumption of hydrogen by soil microbes; changes in sources seem more likely.
If it does not meet another fate, hydrogen released to Earth’s atmosphere will eventually escape to space. On average, a hydrogen molecule (H2) spends about 2 years in the atmosphere. The balance between the production of hydrogen and its destruction determines the steady-state concentration of hydrogen in the atmosphere, now about 550 parts per billion (ppb).
The implications of rising levels of hydrogen need to be considered carefully because hydrogen in the atmosphere destroys a molecule known as the hydroxyl radical that otherwise destroys methane—a gas that contributes to greenhouse warming of our climate. Currently, there is much controversy about why methane concentrations in the atmosphere are rising, and loss of hydroxyl radicals may be part of the cause.
Hydrogen is often touted as the alternative to powering transportation systems using fossil fuels, which add carbon dioxide to the atmosphere. This is an interesting proposition. Being odorless, colorless, and at trace concentrations, hydrogen is easy to overlook, but any constituent that has changed 70% in concentration in Earth’s atmosphere within the last 150 years is worthy of our attention. This is how trouble begins.
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