January 27, 2017
Science Update is a monthly written series from Dr Paul Willis tackling some of the controversial topics in the public and aims to provide the current research behind these subjects.
This Science Update focuses on the often untold but important story of the greenhouse gas, methane. Jokes about farting cows aside, the impact of methane on climate change due to global warming has serious consequences for our future.
What is methane?
Methane is a small molecule consisting of a single carbon atom and four hydrogen atoms. At standard temperature and pressure it is a gas. Methane is a greenhouse gas 21 times more potent than carbon dioxide and comes from both natural sources (wetlands and decaying vegetation) and human activities (particularly agriculture, natural gas activities, and landfills). Slightly more than half of current atmospheric methane emissions are derived from human activities. It is the main component of natural gas.
Methane gas accounts for 20% of the total radiative forcing from greenhouse gases. This means it’s part of the atmospheric gases trapping the energy (and thus the heat) from the Sun that would otherwise be reflected back into space, i.e. the greenhouse effect. It’s also interesting to note that water vapour is by far the largest component of the greenhouse effect.
The methane concentration in Earth’s atmosphere has increased by about 150% since 1750. The current atmospheric methane concentration has never been as high during the past 420,000 years. A surge in methane levels has been detected over the past decade with a further acceleration during 2014-15. Concentrations of methane in the atmosphere during 2014-15 rose by more than 20 parts per billion (ppb), bringing the total to 1,830 ppb compared to ~750 ppb in 1750.
Atmospheric methane records
As explained by the CSIRO, methane is measured in parts per billion (ppb) molar. So, for example, the 2009 global mean level was about 1790 ppb. If you had a sample of air, made up of 1 billion molecules, 1790 of those would be methane molecules.
Atmospheric methane concentrations have been recorded by the CSIRO at Cape Grim in Tasmania since the late 1970s. This data shows a steady increase in that concentration with an annual fluctuation caused by release from wetlands, dependant on temperature and rainfall, destruction in the atmosphere by hydroxyl radicals and transport of methane around the globe from source regions.
Recordings for methane in the atmosphere at Cape Grim, Tasmania. Source: CSIRO
The longer term trends have been calculated for the last 2000 years from ice core data recovered from the Law Dome in Antarctica. This longer time frame shows a slight increase for the first 1800 years with a much more rapid increase in concentration since the beginning of the industrial revolution.
Atmospheric methane reconstructed for 2,000 years from Antarctic ice cores. Source: CSIRO
The Clathrate Gun
There are huge reserves of methane trapped in ice (a form known as clathrate or methane hydrate) across the artic regions of the world and in shallow, cold seas. If these areas warm, the ice can melt releasing the methane into the atmosphere. It is feared that this can happen quite rapidly creating a feedback loop of increased warming melting more ice, releasing more methane and increasing the warming; a scenario known as the Clathrate Gun Hypothesis. There is some evidence to suggest that large craters that have appeared across Siberia in recent years may have formed by the rapid degassing of the methane released from ice but still trapped in the soil.
Has this happened before?
It is generally agreed that the clathrate gun and rapid release of methane into the atmosphere was a crucial factor in the Permian Extinction. This was the largest extinction of all time happening around 252 million years ago and before the appearance of the first dinosaurs. Under the methane extinction scenario, the world warmed by around 2-3 degrees over two to three million years due to increased atmospheric carbon dioxide produced by massive volcanism known as the Russian Traps. This initial warming triggered the release of methane from clathrates resulting in a further increase in temperatures of around 5 degrees perhaps as rapidly as within a decade.
Some estimates place the increased warming as high as 16 degrees but the generally accepted figure is for a total of 8 degrees warming from the combined effects of volcanic CO2 and methane derived from clathrates. While geologically very quick, the duration of this increase can only be guessed. The radiometric dates give a duration of the extinction of around 60,000 years. While other scenarios have been put forward for the causes of the Permian Extinction that do not include methane, it is generally accepted that this gas was somehow implicated in the event.
In popular discussions around climate change and global warming, the effects of methane are largely ignored. But methane is a significant contributor to climate change and half of the methane added to the atmosphere is derived from human activities. There are vast reserves of methane trapped in ice that may escape into the atmosphere as the climate warms. If this were to happen on the same scale as the Permian Extinction, we can expect to see the massive loss of most species of animals and plants on Earth, including ourselves.
This article was originally posted on Australia’s Science Channel.