Abstract
According to the latest report from the intergovernmental panel on climate change (IPCC), currently, global warming due to methane (CH4) alone is about 0.5°C while due to carbon dioxide (CO2) alone is about 0.75°C. As CH4 emissions will continue growing, in order to limit warming to 1.5°C, some of the most effective strategies are rapidly reducing CH4 emissions and developing large scale CH4 removal methods. The aim of this review article is to summarise and propose possible methods for atmospheric CH4 removal, based on the hydroxyl radical (°OH), which is the principal natural sink of many gases in the atmosphere and on many water surfaces. Inspired by mechanisms of °OH generation in the atmosphere and observed or predicted enhancement of °OH by climate change and human activities, we proposed several methods to enhance the °OH sink by some physical means using water vapour and artificial UV radiation. © 2022 Society of Chemical Industry and John Wiley & Sons, Ltd.
Generated Summary
This review article explores methods for atmospheric methane (CH4) removal by enhancing the natural hydroxyl radical (OH) sink, a key atmospheric oxidizing agent. Inspired by observed and predicted enhancements of OH due to climate change and human activities, the study proposes various methods to enhance the OH sink. These methods leverage water vapor and artificial UV radiation to address the increasing CH4 emissions, contributing to global warming.
Key Findings & Statistics
- Global warming due to methane (CH4) alone is about 0.5°C.
- Global warming due to carbon dioxide (CO2) alone is about 0.75°C.
- CH4 has a GWP 27–35 times higher than that of CO2.
- CH4 has a GWP 84 times higher than that of CO2 over 20 years.
- CH4 contributes 0.5°C of warming, next to the highest contribution of 0.75°C from CO2.
- The tropospheric CH4 concentration has grown by nearly 2.6 times over its pre-industrial level.
- In a baseline scenario, warming due to CH4 alone can be as high as 0.9°C, ranging from 0.75°C to 1.5°C by the end of the century.
- CH4 emissions from the fossil fuel industry are 25–40% higher than previous estimates.
- Human-caused CH4 emissions should be reduced by 45% in this decade to limit warming.
- More than 100 countries signed the Global Methane Pledge committing to reduce anthropogenic CH4 emissions by 30% in 2030 compared to the 2020 level.
- The annual global mean °OH concentration would increase by 12.5% in a warmer climate with doubled tropospheric CO2.
- Evapotranspiration amounts to 14.7 km³ which benefits the southwestern United States by water vapor export.
- Irrigation can increase relative humidity by 9–20%.
- The daily amount of °OH generated by a DET can be approximately 1.724 x 107 mol.
Other Important Findings
- The hydroxyl radical (°OH) is the principal natural sink of many gases in the atmosphere.
- Water vapor and UV radiation are essential for °OH production in the troposphere.
- Increasing humidity in hot, dry regions could enhance the °OH sink.
- Artificial UV radiation can be used to generate O3 and °OH in specific locations or times.
- Offshore wind turbines can power UV lamps for artificial UV radiation.
- Methods for CH4 removal provide great benefits for regional surface O3 reduction.
- Water vapor is a key link between physical climate and °OH.
- Artificial UV is only proposed above oceans, or at altitude in locations that do not require adding additional water vapor.
Limitations Noted in the Document
- The study acknowledges potential high costs and externalities of the proposed strategies.
- Careful assessments (e.g., techno-economic analysis, life cycle assessment) are needed.
- The study notes the great diversity among different atmospheric chemistry models, which predict °OH variability, distribution, and trends.
- Life cycle assessments will be performed to assess if the methods have overall negative emissions, or if the production and deployment of the devices proposed generate more GHGs than they remove.
Conclusion
The review emphasizes that strategies to remove atmospheric CH4 by enhancing the OH sink have the potential to mitigate climate change. The study proposes strategies that can enhance OH atmospheric concentrations through interventions like humidity adjustments and artificial UV radiation. The authors stress the need for further research into the risks, impacts, costs, and public acceptance of these strategies. The review also suggests that to meet the Paris Agreement targets, large-scale CH4 removal methods, combined with GHG mitigation and CO2 removal, are essential. The authors highlight the diversity in atmospheric chemistry models as a major knowledge gap that should be addressed for better evaluation of the proposed schemes. Further research, including techno-economic and life cycle assessments, is crucial to determine the feasibility and effectiveness of these methods in achieving negative emissions. The study highlights that the success of these strategies depends on addressing the significant variations in OH concentrations predicted by different atmospheric chemistry models. Ultimately, the review underscores the urgency of developing negative emissions technologies to combat climate change, with a focus on CH4 removal as a critical approach. These technologies, along with broader GHG mitigation and CO2 removal efforts, are essential to slow down warming and meet the targets of the Paris Agreement.