[A-CG35] Global Carbon Cycle Observation and Analysis

[ACG35-07] The Impact of Trend and Variability on the Hydroxyl Radical (OH) in the Global Methane Budget

*Dmitry Belikov¹, Prabir Patra^2,1, Naoko Saitoh¹, Naveen Chandra² (1.Center for Environmental Remote Sensing (CEReS), Chiba University, 2.Research Institute for Global Change (RIGC), Japan Agency for Marine-Earth Science and Technology (JAMSTEC))

Keywords:methane, inverse modeling, greenhouse gases, climate change

Methane (CH₄) is a significant contributor to the anthropogenically produced greenhouse effect in the Earth’s atmosphere. Its concentration in the troposphere has more than doubled since the pre-industrial era, mainly due to human activities. This has led to a 0.62 W/m² increase in radiative forcing from 1750 to 2011. Although total global CH₄ emissions are constrained reasonably well by atmospheric data, estimates of emissions from different source sectors or at the regional scale can vary by up to a factor of two. Despite a clear understanding of the long-term trends and drivers of CH₄ growth, interannual variations are still not comprehended well, mainly because of the uncertainties in CH₄ sink due to hydroxyl radical (OH). The global OH levels estimated by two methods (bottom-up by chemistry-climate models and top-down using Ch3CCl3) differ in terms of magnitude, trends, and inter-annual variations, leading to varying CH₄ sink estimates. To improve the understanding of CH₄ emissions and their impact on the atmosphere, it is crucial to better incorporate 3D simulations from atmospheric chemistry models in atmospheric inversions.
Here we quantified the impact of OH on top-down estimates of CH₄ emissions during 2001-2021 using the JAMSTEC’s Model for Interdisciplinary Research on Climate (MIROC, version 4.0) based atmospheric chemistry-transport model (referred to as MIROC4-ACTM) and atmospheric measurements from about 60 sites. This work aims to better understand the production and loss processes of OH and quantitatively assess their influence on the temporal changes in the CH₄ lifetime and the global CH₄ budget on a decadal scale. We analyze the trends and year-to-year variation of CH₄ emissions estimated using different OH fields and then assess the contribution of various chemical processes to the OH budget by quantifying the main OH production and loss processes. Preliminary analysis suggests that the anthropogenic emissions are better estimated (at lower interannual variability) for the regions dominated by human activities, and we also find the varied magnitude of interannual variations due to natural climate variations such as the El Nino Southern Oscillation. Further sensitivity experiments are performed for checking the ability of our inverse modeling system to derive regional emission trends.