Fossilised farts (and other agroGHGs)! Part 1: The natural sources of methane in the past.

Records of trapped gas (of which some are fossilised farts) come from ice cores. GISP2 and GRIP (central Greenland) and the Vostok cores (Antarctica) show GHG levels fluctuating for millennia; stadials and interstadials (Brook et al., 1996). Focusing on methane and nitrous oxide (nitrous oxide will become increasingly significant during the green revolution and greater use of fertilisers), the role which livestock domestication and increased production has played on levels of those gases has been increasing over the late Holocene, correlating with the increasing human population and complementarily food demand.

Butt (pardon the pun), there are a variety of natural and other anthropogenic process which produce CH­₄ (and N₂O). To add to the debate about when humans had a global effect on the planet, we must be able to distinguish the gases between the various human and natural process, and that’s  where the magic happens…err, the science I mean.

The earth has always been able to regulate the gases in the atmosphere thanks to dynamic equilibrium. A long time ago, in a land quite close to home, early Homo sapiens sapiens lived as hunter gatherers and there was no such thing as an economic crisis. Over the last 140 ka (Schlit et al., 2010), atmospheric concentrations of greenhouse gases has varied naturally (as they have done before human mastery of emitting vast amounts of GHGs) due to feedback processes which are triggered by either external (orbital cycles, volcanic eruptions) or internal (organisms, Dansgaard-Oeschger cycles, Heinrich events, oceans) processes (Brook et al. 1996).

In the prior blogs I (hope to) have shown the main anthropogenic sources of the GHGs methane and nitrous oxide (as well as CO­₂, but due to its greater abundance in the atmosphere and number of sources is hard to quantify what proportion is due to livestock processes). These two gases in particular are good indicators of livestock production and other agricultural process as they can have a signal specific to the process of formation and as natural methane is dependent on the precessional cycle of the earth (explained further in part 2) it is relatively easy to predict and categorise as it increases and decreases in line with the earth’s precession. The main sources of methane come from the humble bacteria. Decomposers, who (as the name suggests) decompose organic matter (dead plants, leaves, bodies…err cow poo or rice stalks, to name but a few) anaerobically (without oxygen) which produces methane.

The best way of removing oxygen from soil (where tonnes of organic matter meet with anaerobic decomposers) is through water-logging/increasing the water table. Like any other organic process involving enzymes; the warmer the environment, the greater the rate of the process occurs, ergo more methane.

More organic matter + higher temperatures + greater water logged soils + decomposer bacteria = a load of natural methane!

The greatest sources of natural methane are large areas on earth (where else?) where temperatures are sufficient to allow decomposition to occur, have large stores of organic carbon in the soil and are highly water-logged. The source of the organic material is the crux of the matter, if it comes from cows stomachs via the rectum (LOL) then increasing the cow population directly increases methane produced from anaerobic decomposition, as well as their farts of course.

Tropical/sub-tropical and boreal (peat land) wetlands are the largest sources of natural methane. Other natural sources include termites, wild fires, wild animals farting, methane hydrate release and oceans (Ruddiman et al., 2011). In the ice core records, the direct correlation between the CH₄ concentration and precessional cycle boils down to the northern hemisphere summer insolation. The precessional cycle dictates the amount of summer temperatures in each hemisphere, governing the seasonal intensity (temperatures/precipitation) and the monsoons. As the direction of the tilt of the earth towards the sun changes cyclically over 22-26 ka (Ruddiman et al., 2011), when the northern hemisphere (NH) is pointed towards the sun when at the same time the earth is closest (the perihelion) to the sun then the NH summer insolation is high, increasing temperatures and precipitation, as seen in speleothem records across the tropics (Burns, 2011). Due to the inter tropical convergence zone (ITCZ) shifting northwards, NH precipitation increases the water logged state of the tropical/sub-tropical rainforests and greater insolation heats and melts permafrost in the boreal band across North America and Eurasia; paving the way for those nasty bacteria to unlock the frozen carbon in the form of the by-product that is CH₄.

You see… all those old-fashioned climate sceptic and climate change deniers are right… it’s not the fault of humans; we just have to destroy all those evil decomposer-bacteria that produce this bad gas, get the Dettol at the ready!