Can the recent rise in methane emissions be explained by natural variability?

So far, we have looked at the ways of measuring methane, the methane cycle and the uncertainties in our understanding of this cycle. In the next blogs, we will move on and see how atmospheric methane levels have developed over time and where we are standing now.

Past methane concentrations have varied between glaciation and interglaciations from 350 to 800 ppbv in the last 650.000 years. That might seem like a big difference, but emissions in 2011 were estimated to be 1803 ppbv (!), according to the IPCC AR5.

Today we will look at the natural variability of these emissions. Can natural cycles and feedbacks explain the rise in emissions? The main methane source in the past were wetlands and the largest sink was oxidation by OH radicals. Therefore, the large variability of past methane is dependent on these elements.

Methane emissions by wetlands are related to changes in solar radiation. Increased orbital forcing intensifies tropical monsoons (because of the land-sea temperature gradient) and the humidity in the intertropical convergence zone. Both these factors result in more rain, therefore a larger area of tropical wetlands and more methane emissions. Additionally, more solar radiation can cause a retreat of the Arctic ice sheets, leading to exposure of high latitude wetlands and thus higher methane levels.

Loulergue et al. examined the influence of orbital forcing on methane emissions. They reconstructed methane emissions over the past 800.000 years using the EPICA Dome C ice core from Antartica and compared this record with the orbital components of methane emissions (fig. 1; fig. 2). They found that eccentricity (100 kyr) had the largest impact on methane emissions, but contribution of precession (19-23 kyr) is growing towards present. The bottom graph in figure 1b depicts the residual methane concentrations, not explained by orbital forcing.
However, the role of eccentricity in this time is highly debated. In his book 'Earth's Climate - Past and Future',  Ruddiman only mentions the role of precession. He also notes the absence of any methane variability at obliquity frequency in the Antartic Vostok ice record. Precession has the largest effect on the temperature in tropics and obliquity on higher latitudes. Therefore, the absence of obliquity suggests that the largest changes were caused by changes in the tropics.

Figure 1 | a)  The contribution of the 100kyr (eccentricity), 41 kyr (obliquity) and 23kyr &19kyr (precession) cycle over 0-400 kyr  and 400-800 kyr before present in the methane record of EPICA Dome C, according to Loulergue et al.. As shown, the largest contribution on methane emissions was by eccentricity in the period 400-800 kyr BP and by precession between 0-400 kyr BP. MTM stands for the multi-taper method. b) The seperated orbital component. Top graph shows the observed methane variation over the last 800.000 years, the red line shows the variation calculated by orbital fluxuations. Then from top to bottom: eccentricity, obliquity, precession and the residual methane variability. It should be noted that the amplitude of the residual is approximately the same as of the components.

As mentioned before, wetland emissions are not the only controlling factor in the methane variability. Valdes and others show that methane oxidation by OH radicals play a significant role as well. They suggest that the main driver of the 40% drop in methane concentrations during the Last Glacial Maximum (LGM) was caused by a reduced concentration of Volatile Organic Compounds (VOCs), amplifying the OH sink.

VOCs are compounds that evaporate at room temperature, including methane. Some VOCs are more reactive towards OH than methane, so an increased concentration of VOCs reduces the methane sink by OH radicals. A large flux of these VOCs comes from vegetation cover. The LGM knew less forest cover because of ice sheets and drier conditions. This decreased VOC concentration might have caused a 40% drop in methane concentrations during this period.

Because the extent of ice sheets, and therefore the vegetation cover, used to be controlled by solar radiation, the OH sink is also dependent on this variability.

The question is now: can the variability in solar radiation explain rising methane emissions? The answer is no. Solar radiation is dropping - scientists believe we should be heading towards a glaciation, according to the orbital parameters. But, of course, temperature is rising. This observation makes it very likely that human society is influencing methane variability.

Next week, we will explore the starting point from which human society has had an impact on the planet, and on methane emissions.

Figure 2 | The graphs of methane emissions, temperature anomalies and ice volume anomalies, according to the EPICA Dome C record, are remarkably similar. Because temperature and ice volume affect the extent of wetlands and the strength of the methane sink, temperature and ice volume are related to methane emissions.