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Hydrogen is quickly gaining attention as a "clean" fuel that can support a transition to a decarbonized energy system. Given the urgency to decarbonize global energy systems, governments and industry are moving ahead with efforts to increase hydrogen technologies, infrastructure, and applications at an unprecedented pace, including 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 a short-lived indirect greenhouse gas whose warming impact is not well-characterized. There are multiple areas of uncertainty. To date, hydrogen's warming effects have been primarily characterized using the GWP-100 metric—which is misleading for short-lived gases, such as hydrogen, as it obscures impacts on shorter timescales. Furthermore, hydrogen is a small molecule known to easily leak into the atmosphere; however, the total amount of leakage in current hydrogen systems remains unknown, with the analytical capacity to accurately measure leakage in situ largely unavailable. Therefore, the net climate benefit of a future hydrogen economy is unknown over the near to medium term. This paper explores the climate implications of hydrogen leakage over all timescales by assessing the change in cumulative radiative forcing from replacing fossil fuel systems with hydrogen applications and estimating temperature responses to leakage using a plausible range of hydrogen leak rates and the latest estimate of hydrogen's radiative efficiency. We also consider the climate impacts from methane leakage when the hydrogen is produced via natural gas with CCUS ('blue' hydrogen) as opposed to renewables and water ('green' hydrogen); both are considered "clean". We find that the climate consequences of hydrogen applications relative to their fossil fuel counterparts strongly depend on time horizon and leakage rate, with vastly different climate outcomes in the near- vs. long-term and for best- vs. worst-case leak rates. For example, worst-case hydrogen leak rates could yield a near-doubling in radiative forcing relative to fossil fuel counterparts in the first five years following the technology switch, but an 80 % decrease in radiative forcing over the following 100 years after deployment. On the other hand, best-case hydrogen leak rates could yield an 80 % decrease in radiative forcing in the first five years. Simple estimates of temperature responses to a 10 % hydrogen leakage rate (a high but plausible level) suggest a theoretical maximum contribution of around a quarter of a degree (C) in 2050 if hydrogen replaces the entire fossil fuel energy system, and at least a tenth of a degree (C) in 2050 if hydrogen accounts for more than half of final energy demand. Thus, a greater understanding of hydrogen's warming impacts at different possible leakage rates is critical to inform where and how to deploy hydrogen effectively in the emerging decarbonized global economy.

Net zero greenhouse gas targets have become a central element for climate action. However, most company and government pledges focus on the year that net zero is reached, with limited awareness of how critical the emissions pathway is in determining the climate outcome in both the near- and long-term. Here we show that different pathways of carbon dioxide and methane—the most prominent long-lived and short-lived greenhouse gases, respectively—can lead to nearly 0.4 °C of warming difference in midcentury and potential overshoot of the 2 °C target, even if they technically reach global net zero greenhouse gas emissions in 2050. While all paths achieve the Paris Agreement temperature goals in the long-term, there is still a 0.2 °C difference by end-of-century. We find that early action to reduce both emissions of carbon dioxide and methane simultaneously leads to the best climate outcomes over all timescales. We therefore recommend that companies and countries supplement net zero targets with a two-basket set of interim milestones to ensure that early action is taken for both carbon dioxide and methane. A one-basket approach, such as the standard format for Nationally Determined Contributions, is not sufficient because it can lead to a delay in methane mitigation.