Abstract
• Action on methane is the easiest and faster way to slow down global warming. • At COP26 105 countries signed ‘the methane pledge’ for -30% CH 4 emissions by 2030. • CH 4 removal (remediation) is complementary with reducing CH 4 emissions (mitigation). • Atmospheric CH 4 removal methods are described and early costs estimates provided. • Co-benefits include–Human health, agriculture, economy, and ozone layer recovery. Methane's contribution to radiative forcing is second only to that of CO 2 . 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 CO 2 -eq, but this should be carried out over remote areas to avoid endangering human health. Complementarity with methane emissions reduction is also discussed.
Highlights
A spotlight on methane versus carbon dioxide While the atmospheric stock of carbon dioxide (CO2) in the atmosphere has increased by about 50% since preindustrial time (417 vs 278 ppm), that of methane (CH4) has more than doubled (1879 vs 722 ppb) [1]
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
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
Summary
A spotlight on methane versus carbon dioxide While the atmospheric stock of carbon dioxide (CO2) in the atmosphere has increased by about 50% since preindustrial time (417 vs 278 ppm), that of methane (CH4) has more than doubled (1879 vs 722 ppb) [1]. The importance of CH4 as a greenhouse gas has been known about for many years, as reflected by the Kyoto protocol of 1997 [2], until recently public attention focused mainly on CO2. In November 2021, at the UN Climate Change Conference (COP 26) held in Glasgow, 105 participating countries signed “The Global Methane Pledge” committing to a 30% reduction in emissions from oil and gas by 2030 relative to 2020 [3]. The new contribution of Working Group 1 to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC WR1 AR6) released on August 2021 [4] highlights the need to quickly reduce global CH4 emissions to slow warming [5, 6] and “buy us time” [7]
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