Reactive greenhouse gas scenarios: Systematic exploration of uncertainties and the role of atmospheric chemistry

Abstract

[1] Knowledge of the atmospheric chemistry of reactive greenhouse gases is needed to accurately quantify the relationship between human activities and climate, and to incorporate uncertainty in our projections of greenhouse gas abundances. We present a method for estimating the fraction of greenhouse gases attributable to human activities, both currently and for future scenarios. Key variables used to calculate the atmospheric chemistry and budgets of major non-CO 2 greenhouse gases are codified along with their uncertainties, and then used to project budgets and abundances under the new climate-change scenarios. This new approach uses our knowledge of changing abundances and lifetimes to estimate current total anthropogenic emissions, independently and possibly more accurately than inventorybased scenarios. We derive a present-day atmospheric lifetime for methane (CH 4 ) of 9.1 AE 0.9 y and anthropogenic emissions of 352 AE 45 Tg/y (64% of total emissions). For N 2 O, corresponding values are 131 AE 10 y and 6.5 AE 1.3 TgN/y (41% of total); and for HFC-134a, the lifetime is 14.2 AE 1.5 y. Citation: Prather, M. J., C. D. Holmes, and J. Hsu (2012), Reactive greenhouse gas scenarios: Systematic exploration of uncertainties and the role of atmospheric chemistry, Geophys.

The global methane budget 2000–2012

“…A series of experiments were conducted by several chemistryclimate models and chemistry transport models participating in the Atmospheric Chemistry and Climate Model In- tercomparison Project (ACCMIP) to study the long-term changes in atmospheric composition between 1850 and 2100 . For the year 2000, the multimodel mean (14 models) global mass-weighted OH tropospheric concentration is 11.7 ± 1.0 × 10 5 molec cm −3 (range 10.3-13.4 × 10 5 molec cm −3 , Voulgarakis et al, 2013), consistent with the estimates of Prather et al (2012) at 11.2 ± 1.3 × 10 5 molec cm −3 . However, it is worth noting that, in the ACCMIP estimations, the differences in global OH are larger between models than between pre-industrial, present and future emission scenario simulations.…”

“…inland waters, geological). The uncertainty in the global methane chemical loss by OH, the predominant sink, is estimated between 10 % (Prather et al, 2012) and 20 % (Kirschke et al, 2013), implying a similar uncertainty in global methane emissions as other sinks are much smaller and the atmospheric growth rate is well defined . Globally, the contribution of natural emissions to the total emissions is reasonably well quantified by combining lifetime estimates with reconstructed preindustrial atmospheric methane concentrations from ice cores (e.g.…”

“…CH 4 has a short lifetime in the atmosphere (∼9 years for the modern inventory; Prather et al, 2012), and a stabilization or reduction of CH 4 emissions leads rapidly to a stabilization or reduction of methane radiative forcing. Reduction in CH 4 emissions is therefore an effective option for climate change mitigation.…”

Earth System Chemistry integrated Modelling (ESCiMo) with the Modular Earth Submodel System (MESSy) version 2.51

“…Observation-based estimates derived from methyl chloroform abundance are generally longer than the model-based estimates (e.g. Prather et al, 2012;Prinn et al, 2005, with 11.2 ± 1.3 and 10.2 (+0.9/−0.7) years, respectively). The wide range of lifetime estimates is mainly caused by different methods of calculation and applied weighting (Lawrence et al, 2001), whereas varying included vertical layers due to different tropopause heights have a minor impact (see also O'Connor et al, 2014).…”

Soil Methanotrophy Model (MeMo v1.0): a process-based model to quantify global uptake of atmospheric methane by soil

“…Anthropogenic activities during the last 200 years have increased the concentration of CH 4 in the atmosphere from pre-industrial era levels of approximately 710 parts per billion (ppb) to the current mixing ratio of approximately 1800 ppb (Etheridge et al, 1998;Kirschke et al, 2013). The atmospheric lifetime of CH 4 is 9.1 ± 0.9 years (Prather et al, 2012) and most CH 4 is consumed in the troposphere via oxidation by OH radicals, which represents ∼ 90 % of the global CH 4 sink (Prather et al, 2012;Ciais et al, 2013). Soil bacteria known as methanotrophs consume ∼ 9 to 10 % of atmospheric CH 4 and a further ∼ 1 % is oxidized by reaction with chlorine radicals from sea salt in the marine boundary layer (Allan et al, 2007;Ciais et al, 2013).…”