Methane gas emissions could have a larger warming effect on climate than has been previously thought. A recent study has found that the interactions between greenhouse gas (GHG) emissions and aerosols in the atmosphere can change the impact of various emissions, and that mitigation policies should take these effects into account. Aerosols, or tiny air-borne particles, enter the atmosphere from natural sources, such as volcanoes and sea-spray but are also generated by humans, for example, through industrial pollution. Aerosols directly affect the climate by reflecting or absorbing the sun's radiation; they also indirectly influence climate by changing the reflective properties of clouds. In this study, the researchers used a model to investigate the complex interactions between GHGs and aerosols in the atmosphere. In particular, the researchers considered the impact on climate warming when the gases methane, carbon monoxide and nitrogen oxides compete with aerosols for hydroxyl in the atmosphere. Hydroxyl is a naturally occurring molecule that plays a key role in atmospheric oxidation chemistry and removing pollutants from the atmosphere. Hydroxyl also plays an important role in the production of sulphate aerosols, which scatter the sun's radiation, creating a cooling effect. Methane is the second largest contributor to historical warming after carbon dioxide and is emitted from wetlands, herbivores, landfills and during the exploration for fossil fuels. Carbon monoxide and nitrogen oxides emissions come mainly from combustion processes, such as from burning fossil fuels. The simulation results suggest that interactions between various atmospheric components can significantly influence the warming potential of individual GHGs. It was found that the relative warming caused by methane over a 100-year timescale is approximately 10% greater than commonly recognised, because previous estimates did not include interactions between aerosols and oxidants, such as hydroxyl. The impact of methane warming increases to between 20 to 40% when the indirect effects of aerosols on clouds are also considered. Chemical reactions of methane in the atmosphere consume hydroxyl that might otherwise be used to produce cooling sulphate aerosols. It is estimated that methane emissions reduce the concentration of hydroxyl by 26% and consequently sulphate aerosols by 11%. Similarly, carbon monoxide emissions increase warming by reducing hydroxyl by about 13% and consequently, sulphate aerosols by 9%. In contrast, the results suggest that nitrogen oxides emissions cause substantial cooling over all time scales, when the complex effects with aerosols are included.


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