Invited reviewThe relative importance of methane sources and sinks over the Last Interglacial period and into the last glaciation
Introduction
Methane is the second most important anthropogenic greenhouse gas (GHG) after CO2, accounting for approximately one third of the radiative forcing of CO2. Around the 1980s, the observed growth rate of methane was as high as 16 ppbv (parts per billion by volume) per year, equivalent to a ∼1% increase every year. This growth was sustained until the mid 1990's and after a subsequent decade of near stability, atmospheric methane mixing ratio started to rise again in 2005. The atmospheric mixing ratio of methane is the result of the balance between sources and sinks. Natural sources are very diverse and present three different origins: biogenic (such as wetlands, fresh waters, ruminants, termites), thermogenic (such as natural gas) and pyrogenic (wildfires). Some sources are a mix of different origins, like hydrates, which have both a biogenic and/or thermogenic origin. However, wetlands dominate these natural emissions, representing around 60–80% of the total (Intergovernmental Panel on Climate Change Fifth Assessment Report, Chapter 6, Table 6.8). Methane is produced in soils by methanogenic archeae. The amount and quality of organic substrate, temperature, and oxic level are the main drivers for methanogenesis (Bridgham et al., 2013, and references therein). The methane produced, mostly in anoxic soil layers, can be oxidised in the upper oxic layers. Vegetation can also play a role in the transport and oxidation of methane. Methane emissions are therefore strongly ecosystem-dependant and a large spatial and temporal variability is observed (Jackowicz-Korczyński et al., 2010, Turetsky et al., 2014). Some processes related to methane production are still poorly understood (e.g. microbial community dynamics and the roles of vascular plants), and even the present day global emission rate is still largely uncertain (Kirschke et al., 2013). However, despite the complexity of the interplay of different factors, methane emissions do display a consistent response to temperature across ecosystems (Yvon-Durocher et al., 2014).
After a certain time (broadly defined the lifetime, τ), a fraction of the emitted methane is either oxidised in the troposphere or stratosphere, or oxidised at the surface. The lifetimes we discuss here can be calculated as the reciprocal of the observed (or calculated) rates of loss of the compounds and are termed e-folding lifetimes. The major oxidising species for many hydrogen bearing compounds is the hydroxyl radical (OH). OH is very short lived (τ < 0.5 s). OH is formed as a break down product from the photolysis of O3 and subsequent reaction with H2O. Given that the rate of photolysis of O3 depends strongly on the abundance of O3, and that the amount of water in the atmosphere can be strongly modified by temperature, there are many complexities and nuances that need to be included in models if they are to represent the oxidation of compounds by OH correctly. Indeed, in the recent Atmospheric Chemistry–Climate Modelling Intercomparison (ACCMIP) tropospheric OH was evaluated for the present day and changes from the PIH to the present, and for changes out to the year 2100 under a range of future emission and climate scenarios (Naik et al., 2013, Voulgarakis et al., 2013). The inclusion of photolysis algorithms that take into account the interactive changes in trace gases and aerosols is something that has only recently become common place within the 3D chemistry modelling community and even within ACCMIP several models used precalculated photolysis frequencies and assumed they would be valid under changes in composition. The reaction between OH and CH4 in the troposphere accounts for about 85% of CH4 removal. Stratospheric OH and tropospheric chlorine oxidation, together with soil oxidation, explain the remaining sinks. Owing to the complications with modelling OH, and the fact that the rate coefficient for the reaction between OH and CH4 is one of the most strongly temperature sensitive reactions that occurs in the atmosphere, modelling methane oxidation accurately is a major challenge. Because tropospheric methane oxidation represents a significant loss for tropospheric OH, methane produces a positive feedback on the climate system, as its lifetime increases with its mixing ratio (e.g. Prather, 2007). Also, methane is a precursor of tropospheric ozone and stratospheric water vapour, which are both GHGs. A breakdown of source and sink strengths in the recent past is presented in Table 1.
Both the sources and the sinks of methane are strongly climate-dependant. With respect to the sources, changes in hydrological regime can affect the global wetland extent (e.g. Bousquet et al., 2006), whilst temperature is the main driver for methane emissions from these wetlands (e.g. Christensen et al., 2003, Turetsky et al., 2014). Whilst with respect to the sinks, the rate of methane oxidation by hydroxyl radicals increases with temperature. In addition to this direct effect, several indirect effects take place, including the increase in OH concentration due to an increase in humidity and the temperature dependency of non-methane volatile organic compound (NMVOC) emissions. However, even for the recent past the interplay of these different components of the methane budget is still a great matter of debate (Kirschke et al., 2013). The investigation of past changes can inform our understanding of natural variations in the budget and the role of changing climate. In particular, air of past atmospheres has been trapped in bubbles in ice cores. Such a direct record of past atmospheric composition is available for the last 800,000 years (Loulergue et al., 2008). Methane and CO2 are usually stable in ice cores and the records they yield overlap with recent direct atmospheric measurements (e.g. MacFarling Meure et al., 2006), such as those made at Mauna Loa (Keeling et al., 1976).
As illustrated in Fig. 1, on the glacial–interglacial time scale, the methane atmospheric mixing ratio ranges from ∼320 ppbv during cold glacials to ∼780 ppbv during warm interglacials (Loulergue et al., 2008). However, the ice-core records show that great variations can also occur on shorter time scales. For example, the rise in temperature at the beginning of Dansgaard–Oeschger (DO) events are, most of the time, accompanied by an increase in methane over a century (Landais et al., 2004, Baumgartner et al., 2014, Rosen et al., 2014). A better investigation of such rapid changes in the global methane budget has been possible very recently thanks to the improvements in measuring techniques, which showed that the rate of methane increase could have reached 2.5 ppbv/yr at the beginning of DO events (Chappellaz et al., 2013, Rosen et al., 2014). Ice core records originating from the two major ice sheets (i.e. Antarctica and Greenland) provide coarse information of the latitudinal variations in the balance between methane sources and sinks, via the interpolar gradient in methane mixing ratio. This gradient has been mostly used to infer latitudinal shifts in methane sources during the Holocene and during DO events (Chappellaz et al., 1997, Dällenbach et al., 2000, Baumgartner et al., 2012).
In this paper, we review current knowledge of global methane dynamics during the Last Glacial–Interglacial cycle, that is changes in sources and sinks. In particular, we discuss the glacial–interglacial amplitude, the so-called Holocene anomaly, and the rapid rise in methane at the onset of Dansgaard–Oeschger events. Finally, we present a recent contribution to this topic in a relatively unexplored period, the Last Interglacial (LIG) (130–115 kaBP). This period of time presents a unique opportunity to study the methane global cycle in a warmer than today climate but with a similar geography. It is also something of a curiosity that, though the climate was warmer than in the PIH, the volume mixing ratio of methane in the atmosphere was no higher; this represents a departure from the first-order observation from the ice core records that methane mixing ratios generally increase (decrease) as the climate warms (cools).
Section snippets
Measuring past fluctuations in methane
The first major studies which intended to quantify the methane budget in the past were motivated by the availability of a comprehensive record of past fluctuations in deep ice core drillings. However, before the first military (Camp Century) and non-military (Dye 3) deep ice core drilling programs, scientists examined the air trapped in ice bubbles from different ice samples in ice sheets. For example, Robbins et al. (1973) gave estimates of past changes of both carbon monoxide and methane from
Modelling the last interglacial sinks and sources
In this section, we focus on the LIG period because of its apparent anomaly: the global climate was likely warmer, especially at high latitudes (Anderson et al., 2006), but the methane mixing ratio was very similar to the PIH levels (Loulergue et al., 2008). This period has not been investigated to date with comprehensive process-based models and so such analysis could give us important insights into methane source and sink responses to orbital forcing changes in a warmer climate than present.
Conclusions
In this paper, we have reviewed much of the recent work that has quantified the relative importance of changes in methane sources and sinks during the Last Glacial–interglacial cycle. Measuring techniques are constantly improving and accuracy of the palaeo records has increased, allowing high precision analysis of the interpolar gradient for example. Models of both atmospheric chemistry and surface processes have been applied over this time period. Most modelling studies have quantified the
Acknowledgements
We gratefully acknowledge I. Colin Prentice for his initial idea on this work. We also warmly acknowledge Antara Banerjee and Luke Abraham for having the UKCA-CheST model set up and running on HECToR supercomputer. We warmly thank Catherine Prigent for sharing her global inundation product. A.T.A., P.J.T and J.A.P. acknowledge NCAS climate for funding. A.T.A. was supported by a Herchel Smith fellowship. J.G.L. gratefully acknowledges support from the NERC funded CLAIRE-UK project. The research
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