Orbital and millennial-scale features of atmospheric CH4 over the past 800,000 years

Differences between the last two glacial maxima and implications for ice-sheet, δ18O, and sea-level reconstructions

“…Ice-sheet nucleation may, in addition, depend on chaotic aspects of the weather/climate system; for example, successive winters with heavy snowfall may e almost randomly e cause some locations to receive an initial snow cover with enough volume and albedo feedback to ensure its survival and subsequent growth potential (e.g., Oglesby, 1990). Finally, modelling studies (e.g., Abe-Ouchi et al, 2013) indicate that glacial culminations like the PGM and LGM reflect the outcome of temporal developments in forcings and feedbacks through the preceding glacial cycle that include insolation (e.g., Laskar et al, 2004;Colleoni et al, 2011), CO 2 and CH 4 concentrations (Monnin et al, 2001;Loulergue et al, 2008; Waelbroeck et al, 2002;Rohling et al, 2009Rohling et al, , 2014Elderfield et al, 2012;Grant et al, 2014), and also in state variables such as surface and deep-sea temperature (e.g., Stenni et al, 2010;Elderfield et al, 2012;Parrenin et al, 2013;Rohling et al, 2012Rohling et al, , 2014Martínez-Botí et al, 2015;Snyder, 2016a,b). Climate simulations by Colleoni et al (2014) suggest that orbital and greenhouse-gas changes for the penultimate glacial cycle were more favourable for glacial inception over Eurasia than over North America, relative to the last glacial cycle.…”

“…This probabilistic analysis accounts for (i) chronological and measurement uncertainties for CO 2 (Monnin et al, 2001(Monnin et al, , 2004Schmitt et al, 2012;Schneider et al, 2013;Landais et al, 2013;Ahn and Brook, 2014) and/or CH 4 time series (Loulergue et al, 2008), and (ii) uncertainties associated with conversion of CO 2 and/or CH 4 to DF CO2 , DF CH4 , and DF GHG . Input data for the Monte Carlo routines are the ice-core 'gas ages' with uncertainties of the AICC2012 chronology Bazin et al, 2013) and CO 2 and/or CH 4 data with analytical uncertainties (Monnin et al, 2001(Monnin et al, , 2004Loulergue et al, 2008;Schmitt et al, 2012;Schneider et al, 2013;Landais et al, 2013;Ahn and Brook, 2014). Each data point was separately and randomly sampled n times within its uncertainties and converted to DF values, using the equations of K€ ohler et al…”

“…For this analysis, we performed Monte-Carlo-style probabilistic assessments (n ¼ 10,000 simulations) based on the uncertainties associated with the various records. For CO 2 , CH 4 , and Antarctic temperature records (Monnin et al, 2001(Monnin et al, , 2004Loulergue et al, 2008;Lourantou et al, 2010;Stenni et al, 2010;Schmitt et al, 2012;Schneider et al, 2013;Landais et al, 2013;Parrenin et al, 2013;Ahn and Brook, 2014), we account for uncertainties associated with chronology (AICC 2012, Veres et al, 2013;Bazin et al, 2013) and proxy measurement in each record to determine the 68% (16 th e84th percentile) and 95% (2.5 th e97.5th percentile) probability intervals, the median (50 th percentile), and the probability maximum (modal value) with its 95% probability interval (e.g., Grant et al, 2012;Rohling et al, 2014;Marino et al, 2015). We probabilistically calculate greenhouse gas (GHG) components of Earth's radiative balance (DF CO2 , DF CH4 , and DF GHG ) from ice-core time series of CO 2 and/or CH 4 , referencing radiative forcing estimates to the values at AD1000 (cf.…”

Deglacial ice sheet meltdown: orbital pacemaking and CO<sub>2</sub> effects

“…The coupled model is driven by orbital parameter changes (Berger, 1978), and reconstructed atmospheric concentrations of CO 2 (Lüthi et al, 2008), CH 4 (Loulergue et al, 2008), and N 2 O (Schilt et al, 2010).…”

Errors in greenhouse forcing and soil carbon sequestration estimates in freshwater wetlands: a comment on Mitsch et al. (2013)

“…Importantly, peatlands comprise a huge global C pool (Bridgham et al 2006;Yu 2012) that is critically important to protect because oxidation of that resource due to drainage will likely lead to a large positive radiative forcing, even after accounting for reduction in CH 4 emissions. Abundant evidence suggests that wetlands were an important driver of past glacial/interglacial cycles (Chappellaz et al 1993a, b;Blunier et al 1995;Loulergue et al 2008) and will likely remain an important feedback to future anthropogenic climate change. For all of these reasons, accurately assessing the C and radiative balances of wetlands remains an important research priority.…”