Future methane, hydroxyl, and their uncertainties: key climate and emission parameters for future predictions

Abstract

Accurate prediction of future methane abundances following a climate scenario requires understanding the lifetime changes driven by anthropogenic emissions, meteorological factors, and chemistry-climate feedbacks. Uncertainty in any of these influences or the underlying processes implies uncertainty in future abundance and radiative forcing. We simulate methane lifetime in three chemical transport models (CTMs) – UCI CTM, GEOS-Chem, and Oslo CTM3 – over the period 1997–2009 and compare the models' year-to-year variability against constraints from global methyl chloroform observations. Using sensitivity tests, we find that temperature, water vapor, stratospheric ozone column, biomass burning and lightning NO<sub>x</sub> are the dominant sources of interannual changes in methane lifetime in all three models. We also evaluate each model's response to forcings that have impacts on decadal time scales, such as methane feedback, and anthropogenic emissions. In general, these different CTMs show similar sensitivities to the driving variables. We construct a parametric model that reproduces most of the interannual variability of each CTM and use it to predict methane lifetime from 1980 through 2100 following a specified emissions and climate scenario (RCP 8.5). The parametric model propagates uncertainties through all steps and provides a foundation for predicting methane abundances in any climate scenario. Our sensitivity tests also enable a new estimate of the methane global warming potential (GWP), accounting for stratospheric ozone effects, including those mediated by water vapor. We estimate the 100-yr GWP to be 32, which is 25% larger than past assessments

Key drivers of ozone change and its radiative forcing over the 21st century

“…Likewise, zonal and annual mean stratospheric ozone agrees fairly well with satellite 10 estimates in the Southern Hemisphere (SH) and low latitudes (±30 DU), but larger deviations are found at mid-and high latitudes in the Northern Hemisphere (NH), a discrepancy also apparent in the models of the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP) (Iglesias-Suarez et al, 2016). The tropospheric ozone budget (production, loss, dry deposition, stratospheric input), 15 burden and lifetime for the CNTRL simulation (see Table 2 and Fig.…”

“…Fuglestvedt et al, 1999;Wild and Prather, 2000;Holmes et al, 2013). The simulations considered here neglect this feedback by imposing fixed and uniform lower boundary conditions for methane of 1748 and 3744 ppbv for years 2000 and 2100 respectively.…”

“…We calculate a value of f of 1.43 which is at the mid-upper end of the literature range (1.19-1.53) (Prather et al, 2001;Stevenson et al, 2013;Voulgarakis et al, 2013) and within 7 % of the observationally constrained best estimate of 1.34 (Holmes et al, 2013). 10…”

“…Impacts of future climate change on ozone are robust across a number of 25 modelling studies and multi-model activities (Kawase et al, 2011;Arblaster et al, 2014;Banerjee et al, 2016;Iglesias-Suarez et al, 2016).…”

How well do integrated assessment models represent non-CO2 radiative forcing?

“…Note however, that the ACCMIP range is defined by only two models that recorded a long-term projection (LMDzORINCA and GISS-E2-R), and should only serve as an indication of the trend. This projection corresponds with a study of future uncertainties in methane abundance, which concludes that the methane concentration in 2100 in RCP8.5 is 3990 ± 330 ppb (Holmes et al 2013). Compared to this range, the projections of MERGE, MERGE_ETL and MAGICC5.3 are slightly high and those of FUND slightly low.…”