Methane Emissions in a Chemistry‐Climate Model: Feedbacks and Climate Response

Abstract

Understanding the past, present, and future evolution of methane remains a grand challenge. Here we have used a hierarchy of models, ranging from simple box models to a chemistry-climate model (CCM), UM-UKCA, to assess the contemporary and possible future atmospheric methane burden. We assess two emission data sets for the year 2000 deployed in UM-UKCA against key observational constraints. We explore the impact of the treatment of model boundary conditions for methane and show that, depending on other factors, such as CO emissions, satisfactory agreement may be obtained with either of the CH 4 emission data sets, highlighting the difficulty in unambiguous choice of model emissions in a coupled chemistry model with strong feedbacks. The feedbacks in the CH 4-CO-OH system, and their uncertainties, play a critical role in the projection of possible futures. In a future driven by large increases in greenhouse gas forcing, increases in tropospheric temperature drive, an increase in water vapor, and, hence, [OH]. In the absence of methane emission changes this leads to a significant decrease in methane compared to the year 2000. However, adding a projected increase in methane emissions from the RCP8.5 scenario leads to a large increase in methane abundance. This is modified by changes to CO and NOx emissions. Clearly, future levels of methane are uncertain and depend critically on climate change and on the future emission pathways of methane and ozone precursors. We highlight that further work is needed to understand the coupled CH 4-CO-OH system in order to understand better future methane evolution. Plain Language Summary Methane is an important greenhouse gas and needs to be modeled accurately to understand climate change. We use a combination of modeling approaches to investigate how we could model methane in present day conditions and in the future. We used a simple box model to explore how methane behaves in the atmosphere and interacts with other important species such as OH and CO. We then used a more complex 3-D model of the whole atmosphere employing a more physically realistic way to treat methane emissions in our model than is normally done. We show that the choice of emissions data set is difficult but that good agreement with observations is possible. The simple model shows how some of the difficulty arises: The different components of the system interact nonlinearly. We go on to use this 3-D modeling approach to study methane levels in future climate. We show that not only methane emissions but also other factors are important to methane levels, again agreeing with the broad conclusions of our simple model. Methane in the future is highly uncertain and depends on climate, emissions, and the interactions between them, mediated by key chemical species, CO, methane, and OH.

Description and Evaluation of an Emission‐Driven and Fully Coupled Methane Cycle in UKESM1

“…Heimann et al. (2020) previously reported a methane interhemispheric gradient of 104 ppb with a global mean methane surface mole fraction of 1590 ppb for the period 2000–2005, calculated from their BASE experiment, which is comparable to our emission‐driven simulation (cf. Heimann et al., 2020, Section 3.2).…”

“…Recently, Heimann et al. (2020) showed that increased CO and methane emissions could each at least partially resolve the negative bias in carbon monoxide, but they also argue that the negative bias in CO could be due to missing higher VOC sources. However, without targeted sensitivity experiments it is difficult to conclusively attribute these biases to primary emissions of CO or to secondary in situ CO production from chemical oxidation of methane and non‐methane volatile organic compounds (NMVOC) in our model.…”

Recent advances in satellite mapping of global air quality: evidences during COVID-19 pandemic

“…Natural sources include emission from agricultural lands, wetlands, marshy areas and enteric fermentation in ruminant animals. Anthropogenic sources include emission from fossil fuel extraction, landfill and various other anthropogenic activities (Janssens-Maenhout et al 2019;Heimann et al 2020). Most of the CH 4 emissions, almost to the tune of 60%, happen in the northern hemisphere, however the extended lifetime to inter-hemispheric transport lessens the north to south (N:S) latitudinal gradient of CH 4 upto 100 ppb (Heimann et al 2020).…”

Impact of interannual and multidecadal trends on methane-climate feedbacks and sensitivity

“…Slower oxidation of CH 4 leads to lower δ 13 CH 4 by extending the 12 CH 4 lifetime more than that of 13 CH 4 [1][2][3][4][5][6][7]23,24 . Biogenic emissions from wetlands and permafrost 5,6,[25][26][27][28][29][30][31][32][33] and atmospheric methane lifetime 10,[20][21][22][23][34][35][36][37][38][39][40] are major methaneclimate feedbacks, while other feedback processes, such as wildfires 36,[41][42][43][44] and natural thermogenic emissions 45 , are considered secondary 6,7 . The methane-lifetime feedback contributes the highest uncertainty in estimates of feedback strength 7,40 .…”

“…Nevertheless, exceptions apply. For example, with respect to LSAT, the atmospheric sink provides secondary positive feedback: higher CH 4 emissions, with positive feedback via biomass burning 41,42,44 , result in increased atmospheric carbon monoxide (CO), reacting with atmospheric • OH to decrease its concentration, hence extending the CH 4 lifetime 22,36,37 . Similarly, increased emissions of biogenic volatile organic compounds (BVOCs) result in positive feedback by limiting • OH 40 .…”