Methane mitigation is essential for addressing climate change, but the value of rapidly implementing available mitigation measures is not well understood. In this paper, we analyze the climate benefits of fast action to reduce methane emissions as compared to slower and delayed mitigation timelines. We find that the scale up and deployment of greatly underutilized but available mitigation measures will have significant near-term temperature benefits beyond that from slow or delayed action. Overall, strategies exist to cut global methane emissions from human activities in half within the next ten years and half of these strategies currently incur no net cost. Pursuing all mitigation measures now could slow the global-mean rate of near-term decadal warming by around 30%, avoid a quarter of a degree centigrade of additional global-mean warming by midcentury, and set ourselves on a path to avoid more than half a degree centigrade by end of century. On the other hand, slow implementation of these measures may result in an additional tenth of a degree of global-mean warming by midcentury and 5% faster warming rate (relative to fast action), and waiting to pursue these measures until midcentury may result in an additional two tenths of a degree centigrade by midcentury and 15% faster warming rate (relative to fast action). Slow or delayed methane action is viewed by many as reasonable given that current and on-the-horizon climate policies heavily emphasize actions that benefit the climate in the long-term, such as decarbonization and reaching net-zero emissions, whereas methane emitted over the next couple of decades will play a limited role in long-term warming. However, given that fast methane action can considerably limit climate damages in the near-term, it is urgent to scale up efforts and take advantage of this achievable and affordable opportunity as we simultaneously reduce carbon dioxide emissions.

Both 20- and 100-year time scales should always be reported

Anthropogenic aerosols comprise optically scattering and absorbing particles, with the principal concentrations being in the Northern Hemisphere, yielding negative and positive global mean radiative forcings, respectively. Aerosols also influence cloud albedo, yielding additional negative radiative forcings. Climate responses to a comprehensive set of isolated aerosol forcing simulations are investigated in a coupled atmosphere–ocean framework, forced by preindustrial to present-day aerosol-induced radiative perturbations. Atmospheric and oceanic climate responses (including precipitation, atmospheric circulation, atmospheric and oceanic heat transport, sea surface temperature, and salinity) to negative and positive particulate forcings are consistently anticorrelated. The striking effects include distinct patterns of changes north and south of the equator that are governed by the sign of the aerosol forcing and its initiation of an interhemispheric forcing asymmetry. The presence of opposing signs of the forcings between the aerosol scatterers and absorbers, and the resulting contrast in climate responses, thus dilutes the individual effects of aerosol types on influencing global and regional climate conditions. The aerosol-induced changes in the variables also have a distinct fingerprint when compared to the responses of the more globally uniform and interhemispherically symmetric well-mixed greenhouse gas forcing. The significance of employing a full ocean model is demonstrated in this study by the ability to partition how individual aerosols influence atmospheric and oceanic conditions separately.

Abstract. Given the urgency to decarbonize global energy systems, governments and industry are moving ahead with efforts to increase deployment of hydrogen technologies, infrastructure, and applications at an unprecedented pace, including USD 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 an indirect greenhouse gas whose warming impact is both widely overlooked and underestimated. This is largely because hydrogen's atmospheric warming effects are short-lived – lasting only a couple decades – but standard methods for characterizing climate impacts of gases consider only the long-term effect from a one-time pulse of emissions. For gases whose impacts are short-lived, like hydrogen, this long-term framing masks a much stronger warming potency in the near to medium term. This is of concern because hydrogen is a small molecule known to easily leak into the atmosphere, and the total amount of emissions (e.g., leakage, venting, and purging) from existing hydrogen systems is unknown. Therefore, the effectiveness of hydrogen as a decarbonization strategy, especially over timescales of several decades, remains unclear. This paper evaluates the climate consequences of hydrogen emissions over all timescales by employing already published data to assess its potency as a climate forcer, evaluate the net warming impacts from replacing fossil fuel technologies with their clean hydrogen alternatives, and estimate temperature responses to projected levels of hydrogen demand. We use the standard global warming potential metric, given its acceptance to stakeholders, and incorporate newly published equations that more fully capture hydrogen's several indirect effects, but we consider the effects of constant rather than pulse emissions over multiple time horizons. We account for a plausible range of hydrogen emission rates and include methane emissions when hydrogen is produced via natural gas with carbon capture, usage, and storage (CCUS) (“blue” hydrogen) as opposed to renewables and water (“green” hydrogen). For the first time, we show the strong timescale dependence when evaluating the climate change mitigation potential of clean hydrogen alternatives, with the emission rate determining the scale of climate benefits or disbenefits. For example, green hydrogen applications with higher-end emission rates (10 %) may only cut climate impacts from fossil fuel technologies in half over the first 2 decades, which is far from the common perception that green hydrogen energy systems are climate neutral. However, over a 100-year period, climate impacts could be reduced by around 80 %. On the other hand, lower-end emissions (1 %) could yield limited impacts on the climate over all timescales. For blue hydrogen, associated methane emissions can make hydrogen applications worse for the climate than fossil fuel technologies for several decades if emissions are high for both gases; however, blue hydrogen yields climate benefits over a 100-year period. While more work is needed to evaluate the warming impact of hydrogen emissions for specific end-use cases and value-chain pathways, it is clear that hydrogen emissions matter for the climate and warrant further attention from scientists, industry, and governments. This is critical to informing where and how to deploy hydrogen effectively in the emerging decarbonized global economy.

Food consumption is a major source of greenhouse gas (GHG) emissions, and evaluating its future warming impact is crucial for guiding climate mitigation action. However, the lack of granularity in reporting food item emissions and the widespread use of oversimplified metrics such as CO2 equivalents have complicated interpretation. We resolve these challenges by developing a global food consumption GHG emissions inventory separated by individual gas species and employing a reduced-complexity climate model, evaluating the associated future warming contribution and potential benefits from certain mitigation measures. We find that global food consumption alone could add nearly 1 °C to warming by 2100. Seventy five percent of this warming is driven by foods that are high sources of methane (ruminant meat, dairy and rice). However, over 55% of anticipated warming can be avoided from simultaneous improvements to production practices, the universal adoption of a healthy diet and consumer- and retail-level food waste reductions.