Abstract
Methane’s contribution to radiative forcing is second only to that of CO2. Though previously neglected, methane is now gaining increasing public attention as a GHG. At the recent COP26 in Glasgow, 105 countries signed “the methane pledge” committing to a 30% reduction in emissions from oil and gas by 2030 compared to 2020 levels. Removal methods are complementary to such reduction, as they can deal with other sources of anthropogenic emissions as well as legacy emissions already accumulated in the troposphere. They can also provide future insurance in case biogenic emissions start rising significantly. This article reviews proposed methods for atmospheric methane removal at a climatically significant scale. These methods include enhancement of natural hydroxyl and chlorine sinks, photocatalysis in solar updraft towers, zeolite catalyst in direct air capture devices, and methanotrophic bacteria. Though these are still at an early stage of development, a comparison is provided with some carbon dioxide removal methods in terms of expected costs. The cheapest method is potentially enhancement of the chlorine natural sink, costing as little as $1.6 per ton CO2-eq, but this should be carried out over remote areas to avoid endangering human health. Complementarity with methane emissions reduction is also discussed.
Generated Summary
This journal article reviews methods proposed for atmospheric methane removal at a climatically significant scale. The study compares various methods, including enhancing natural hydroxyl and chlorine sinks, photocatalysis in solar updraft towers, zeolite catalysts in direct air capture devices, and methanotrophic bacteria. The research explores these methods’ development stages and provides a comparison with some carbon dioxide removal methods in terms of expected costs. The focus is on technologies to reduce atmospheric methane concentrations, addressing the increasing importance of methane as a greenhouse gas, and the need for action to mitigate its effects. The study also discusses the potential of these methods in the context of global warming and climate change mitigation strategies.
Key Findings & Statistics
- The atmospheric stock of carbon dioxide (CO2) in the atmosphere has increased by about 50% since preindustrial time (417 vs 278ppm), while that of methane (CH4) has more than doubled (1879 vs 722 ppb) [1].
- At the recent COP26 in Glasgow, 105 countries signed “the methane pledge” committing to a 30% reduction in emissions from oil and gas by 2030 compared to 2020 levels [3].
- The annual increase in atmospheric CH4 in 2020 was the largest recorded since 1983 [16].
- Since the preindustrial era, tropospheric concentrations of CO2 and CH4 have increased by 47% and 167%, respectively [1].
- The IPCC [18] predicts that, over the next 10 to 20 years, CH4 and CO2 will have similar global warming impacts, as measured by heat absorbed (global warming potential, GWP) and temperature rise (Global Temperature change Potential, GTP).
- The GWP of CH4 is nearly 28 times higher than that of CO2 on a 100-year basis, and 84 times higher on a 20-year basis [18].
- Methane featured in the agenda of the recent U.S. Leaders’ Climate Summit.
- China’s biggest gas and oil producer is targeting a 50% reduction in CH4 emission intensity by 2025 [10].
- Most anthropogenic CH4 emissions come from agriculture and waste management, which together constitute 60% of anthropogenic and 38% of total emissions.
- CH4 emissions from oil and gas industries represent about 33% of anthropogenic emissions and about 17-19% of total CH4 emissions, with extraction, processing and distribution accounting for about 2/3 and coal mining for 1/3 [26].
- The GWP of CH4 is nearly 28 times higher than that of CO2 on a 100-year basis, and 84 times higher on a 20-year basis [18].
- The cheapest method is potentially enhancement of the chlorine natural sink, costing as little as $1.6 per ton CO2-eq
Other Important Findings
- Methane is now gaining increasing public attention as a GHG.
- Removal methods are complementary to such reduction, as they can deal with other sources of anthropogenic emissions as well as legacy emissions already accumulated in the troposphere.
- They can also provide future insurance in case biogenic emissions start rising significantly.
- The oxidation reaction is exothermic and exergonic, and as such requires no minimum energy input once the activation energy is overcome.
- Atmospheric natural self-cleansing and volatile organic compound (VOC) removal is mainly due to hydroxyl radicals [62].
- Over polluted coastal areas, the chlorine sink destroys up to 11% of CH4 [71].
- The main benefit of returning to CH4 pre-industrial levels will be to reduce global warming by up to about 0.5°C [103], which can help reduce a temperature overshoot above 2°C by mid-century [104].
- The expression “at a climatically significant scale” often appears in discussions about GHG removal, but lacks precise definition.
Limitations Noted in the Document
- The article mentions that these methods are still at an early stage of development.
- The study acknowledges that the proposed methods for atmospheric methane removal seem even more technologically challenging because they need to deplete CH4 already released to the atmosphere, where it has been diluted about 0.5 million times in an air volume of some 1.4 billion cubic kilometers.
- The relatively short life cycle of atmospheric CH4 (about 10 years) means that removal would have to be ongoing to reduce the concentration by a target amount.
- The study does not provide any specific details about how to process the very large volume of air in the atmosphere for the methods like, bio-inspired aqueous-phase catalytic oxidation, bio-reactors containing methanogens, enzymatic systems, and catalysts made of precious metals.
- The article notes that it is yet too early in the development process to have accurate cost estimates for CH4 removal directly in the troposphere, so initial estimations have been provided.
Conclusion
The article emphasizes the increasing importance of addressing methane (CH4) emissions, which have more than doubled since the preindustrial era. The focus on CH4 has grown, with international agreements like “The Global Methane Pledge” aiming for significant reductions in emissions. The study reviews various methods for removing atmospheric methane, including enhancing natural sinks like hydroxyl radicals and chlorine atoms, as well as technologies like photocatalysis and direct air capture. The article highlights the potential of these methods to complement emission reduction efforts and address both anthropogenic and legacy emissions. The review underscores that while the technologies are at an early stage, they offer potential solutions for future challenges. The article concludes that, despite the challenges, accelerating the removal of CH4 has the potential to reduce global warming and offers co-benefits for agriculture, human health, and the economy. The focus is to reduce global warming by up to 0.5°C, which can help reduce a temperature overshoot above 2°C by mid-century. The authors note that the criteria of “at a climatically significant scale” may lead to too many options being dismissed, and too few remaining, while the scale of the global warming problem requires a large portfolio of methods and technologies to be developed.