Hydrogen (H₂) holds promise as an alternative energy source that will play an important role in decarbonizing the global energy system, potentially fueling our industrial production, homes, commerce and infrastructure one day.

However, scientists warn there is reason to be cautiously optimistic. H2 interacts with methane, ozone (pollution) and water vapor in the atmosphere in ways that may warm the climate, thereby potentially negating its benefits.

A new study co-led by an Auburn University researcher and published in the esteemed scientific journal, Nature, suggests embracing a "H2 economy" will require a deeper scientific understanding of the global H2 cycle to support a climate-safe, sustainable H2 economy.

Unlike greenhouse gases, including carbon dioxide and methane, H2 itself does not trap heat in Earth's atmosphere. Through interactions with other gases, however, H2 indirectly heats the atmosphere roughly 11 times faster than carbon dioxide during the first 100 years after release, and approximately 37 times faster during the first 20 years.

"This indirect warming raises concerns about the climate consequences of potential H2 leakage, and highlights that the climate benefits of a future H2 economy will depend on minimizing leakage through the H2 value chain and reducing natural gas (methane) emissions," said Zutao Ouyang, leading author of the study and assistant professor of ecosystem modeling in Auburn's College of Forestry, Wildlife and Environment (CFWE).

To assess the impact of this leakage, Ouyang worked with an international consortium of scientists known as the Global Carbon Project, to develop the first comprehensive accounting of global H2 sources and sinks to assess changes in atmospheric H2 and its climate consequences.

The study, "The Global H2 budget," published today, is co-led by Ouyang and Rob Jackson, Stanford University professor and chair of the Global Carbon Project, in collaboration with researchers from approximately 30 international institutions from France, Australia, China, Japan, United Kingdom, Norway and Austria.

"The team collected direct measurements of H2 in the atmosphere, along with the most comprehensive ever data collection and modeling to estimate the major sources and sinks of H2 and produce a first-of-its-kind global picture," said Pep Canadell, the executive director of Global Carbon Project, and chief research scientist at CSIRO Environment, Australia.

After, they projected future H2 emissions, removals and atmospheric levels under different scenarios of the Intergovernmental Panel on Climate Change (IPCC), using a simplified Earth system model to estimate how these changes could affect the climate.

Since H2 production began to increase 30 years ago, the researchers estimate H2 concentrations in the atmosphere increased by about 70% from preindustrial times through 2003, then briefly stabilized before picking up again around 2010. The authors found this increase is largely due to leakage from increased H2 production and, surprisingly and less known, due to the formation of H2 that results from the oxidation of increasing methane emissions from human activities.

Methane (CH4) emissions — especially from fossil fuel extraction, distribution and use — have an important, but often under-appreciated, influence on atmospheric H2. Methane and H2 share the same pathway by which they are cleaned (oxidized) in the atmosphere, and because methane oxidation itself produces H2, this creates feedback that can raise atmospheric H2 concentrations when methane emissions increase; further competing for the detergent that cleans the atmosphere from methane.

Though the overall climate effects are relatively small currently, these complex interactions have the potential to undermine the positive effects of H2.

"The largest source of H2 in the world is the oxidation of methane in our atmosphere," Jackson said. "But methane and H2 also compete for atmospheric cleansing detergents. This competition extends methane's lifecycle in the presence of H2, thereby increasing indirect climate warming. More H2 means more methane, and more methane means more H2."

The authors agree, if a H2 economy is to become widely accepted, the unintended warming driven interactions between H2 and methane must be reduced to build trust in H2 as a viable decarbonization pathway.

"By quantifying the previously unaccounted warming feedback between H2 and methane that are missing in current climate projections, we hope to improve future climate scenarios and support decision-makers in minimizing both economic losses and climate risks associated with H2 leakage," Ouyang said.

With this research in hand, policymakers, industries and researchers are equipped with the quantitative evidence needed to set safe H2 and methane leakage thresholds, craft effective regulations and prioritize cost-efficient mitigation strategies.

"The significance of this international study cannot be overstated," said Janaki Alavalapati, the Emmett F. Thompson Dean of the CFWE. "The team's comprehensive assessment of H2 sources and sinks, and their associated warming impacts, can help ensure that the global expansion of H2 aligns with a climate-safe and sustainable energy transition."

The research study was funded by the Gordon and Betty Moore Foundation, Stanford Doerr School of Sustainability, Stanford's Global Methane Office and Auburn's CFWE.