Perspectives on removal of atmospheric methane

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.