Understanding the glacial methane cycle

Abstract

Atmospheric methane (CH4) varied with climate during the Quaternary, rising from a concentration of 375 p.p.b.v. during the last glacial maximum (LGM) 21,000 years ago, to 680 p.p.b.v. at the beginning of the industrial revolution. However, the causes of this increase remain unclear; proposed hypotheses rely on fluctuations in either the magnitude of CH4 sources or CH4 atmospheric lifetime, or both. Here we use an Earth System model to provide a comprehensive assessment of these competing hypotheses, including estimates of uncertainty. We show that in this model, the global LGM CH4 source was reduced by 28–46%, and the lifetime increased by 2–8%, with a best-estimate LGM CH4 concentration of 463–480 p.p.b.v. Simulating the observed LGM concentration requires a 46–49% reduction in sources, indicating that we cannot reconcile the observed amplitude. This highlights the need for better understanding of the effects of low CO2 and cooler climate on wetlands and other natural CH4 sources.

Emerging role of wetland methane emissions in driving 21st century climate change

“…However, due to the paucity of data, wetland CH 4 feedbacks were not fully assessed in the Intergovernmental Panel on Climate Change Fifth Assessment Report. The degree to which future expansion of wetlands and CH 4 emissions will evolve and consequently drive climate feedbacks is thus a question of major concern. Here we present an ensemble estimate of wetland CH 4 emissions driven by 38 general circulation models for the 21st century.…”

“…As the second most important anthropogenic greenhouse gas in the atmosphere after CO 2 , CH 4 is strongly associated with climate feedbacks. However, due to the paucity of data, wetland CH 4 feedbacks were not fully assessed in the Intergovernmental Panel on Climate Change Fifth Assessment Report.…”

“…Wetland methane (CH 4 ) emissions are the largest natural source in the global CH 4 budget, contributing to roughly one third of total natural and anthropogenic emissions. As the second most important anthropogenic greenhouse gas in the atmosphere after CO 2 , CH 4 is strongly associated with climate feedbacks.…”

Reduced cooling following future volcanic eruptions

“…HadGEM2-ES has been used extensively in the Coupled Model Intercomparison Project phase 5 (CMIP5) , for understanding past changes in climate (Hopcroft and Valdes 2015b), aerosols (Booth et al 2012;Hopcroft et al 2015) and atmospheric CH 4 (Hopcroft et al 2017), and for long-term future projections (Caesar et al 2013). A high-top version of HadGEM2-ES (extending much further into the stratosphere) was not used as it shows little difference in the volcanic eruption response (Marshall et al 2009), and is significantly more computationally expensive.…”

Cold-seep ostracods from the western Svalbard margin: direct palaeo-indicator for methane seepage?

“…It is important to understand causes behind changes in the activity of release of methane in the geological past because methane is a ∼ 25 times more powerful greenhouse gas than carbon dioxide, and it constitutes an important factor in regional and global climate change (Nisbet and Chappellaz, 2009;Consolaro et al, 2015;Hopcroft et al, 2017). Reconstructions of deep-sea seep activities in the geological past have often been based on δ 13 C values measured in foraminiferal shells, but the signals are often caused by secondary mineralization of diagenetic carbonate, making inferences about timing of seepage events difficult (Uchida et al, 2008;Consolaro et al, 2015;Sztybor and Rasmussen, 2017a).…”