The methane budget at the last glacial maximum
March 2017 - A study published last month in Nature Communications, led by the University of Bristol in collaboration with the Met Office Hadley Centre, examines the cause of the observed methane increase between the last glacial maximum and the late Holocene.
Methane (CH4) is a potent greenhouse gas, contributing about 20% of the net human radiative forcing over the historical period, making it the 2nd largest contributor to climate change after carbon dioxide.
Concentration increases over the past 200 years have been attributed to increased emissions caused by human activities, although over the last decade it is thought natural sources may also have had an influence (Schaefer et al., 2016).
While the driving factors behind the increase over the past 200 years are unequivocal, different hypotheses exist to explain the increase between the last glacial maximum (LGM), 21000 years ago, and the late Holocene period between 800-1600 AD when CH4 concentrations increased from 375 parts per billion (ppb) to 680 ppb.
Some studies indicate that 50-100% of the increase was actually due to changes in the CH4 lifetime in the atmosphere while others indicate that it was solely driven by changes in the strength of natural CH4 emission sources such as wetlands.
The cause of the increase in atmospheric methane from the last ice age to the onset of industrialization remains unclear. A new study sheds light on this issue, showing that changes in the strength of natural sources of methane, such as wetlands, contributed to the increase.
A recent study, led by the University of Bristol in collaboration with the Met Office Hadley Centre, examines the cause of the observed CH4 increase between the LGM and the late Holocene using the comprehensive Met Office Hadley Centre Earth system model (Collins et al., 2011).
This model includes a near-complete inventory of the major natural CH4 emission sources (wetlands, peatlands, biomass burning, oceans, termites and hydrates), atmospheric processes, and sinks, including aspects which affect CH4 lifetime (O’Connor et al., 2014).
Results, shown in Figure 1 below, indicate that although there are large contributing factors affecting the CH4 lifetime (such as biogenic volatile organic compounds or non-CH4 sources and climate), the net effect of these is that the CH4 lifetime at the LGM is longer than during the late Holocene period by only 2-8%.
Figure 1: Summary of the contributions of different processes to the CH4 budget at the LGM relative to the late Holocene period.
Consequently, this study concludes that changes in CH4 lifetime are only a minor contributing factor to the CH4 concentration increase between the LGM and the late Holocene. Therefore the observed increase was largely driven by changes in the strength of natural sources, particularly wetlands, although we cannot fully reconcile the observed increase.
This study, in particular, highlights the need for better understanding of the effects of climate and carbon dioxide on wetland and other natural CH4 sources so that robust and reliable future projections of CH4 emissions, CH4 lifetime, and CH4 concentration can be made.
Understanding these factors will help to inform estimates of the greenhouse gases we can emit whilst still maintaining a likely chance of limiting temperature rise below certain levels.
The paper “Understanding the glacial methane budget” was published in Nature Communications on 21st February 2017.
Collins, W. J. et al. 2011. Development and evaluation of an Earth-System model – HadGEM2. Geosci. Model Dev. 4: 1051–1075.
O’Connor, F. M. et al. 2014. Evaluation of the new UKCA climate-composition model – Part 2: The Troposphere. Geosci. Model Dev. 7: 41–91.
Schaefer, H. et al. 2016. A 21st century shift from fossil-fuel to biogenic methane emissions indicated by 13CH4. Science. 352: 80–84.