Methane (CH4) is a potent greenhouse gas, with ~86 times the warming power of carbon dioxide (CO2) for 2 decades. As it ages, methane oxidizes and loses hotness – but is still 26 times more warming than carbon dioxide for a century.
Light is visible energy. Sunlight provides ~400 watts – four 100-watt light bulbs worth – of energy per square meter.
The Sun also blazes with a variety of radiative energy at various wavelengths; but most of it is in the visible spectrum, which is why eyes are adapted to be receptive to this specific band. If we saw in another wavelength range, there wouldn’t be nearly as much to see.
When visible light tires it morphs into more heat than light; such is infrared.
Quantized energy from the Sun – photons – bounce around in the atmosphere, where they encounter molecules which absorb photonic energy. In doing so heat is shed.
Each type of greenhouse gas molecule only eats photons that are tuned to certain frequencies. “They can only absorb some frequencies,” American atmospheric chemist Neil Donahue elaborates. “Some molecules absorb quite a lot of different colors and frequencies, and some have very specific frequencies they absorb.”
“The 2 that do most of the work absorbing light in the air are carbon dioxide (CO2) and water (H2O), and they cover big chunks of the infrared spectrum. So where they’re present, heat doesn’t get very far,” Donahue continues.
CH4 gobbles photons at frequencies that H2O and CO2 largely ignore. That’s because the 4 hydrogen atoms dance around their carbon atom in a distinct way. “The frequency of the vibration of the molecule, which is exactly the same as the frequency of the light, that’s the thing that gets matched,” Donahue enthuses. As hydrogen is an especially jiggy atom, methane is an extremely warming greenhouse gas.
Water, carbon dioxide, and methane all cycle through the atmosphere, ocean, and soil. Molecules of greenhouse gases get absorbed in the sea, plants, or dirt, becoming what climatologists call carbon sinks.
Water is the predominant greenhouse gas. The water cycle is distinctive from those of carbon-based molecules. And, whereas water is not a pollutant, excessive carbon molecules in the atmosphere are, regardless of their atomic affinities (whether hydrogen or oxygen).
Sinks become sources when greenhouse molecules cut loose to bask in the sunlit atmosphere and heat the air.
Global warming has spelt polar melting. Glacial ice and permafrost soil have stores of greenhouse gases. Permafrost is especially rich in methane – but its old methane that readily loses the intensity of its warming power as it transforms into carbon dioxide. The hydrogen atoms go geriatric.
English environmental scientist Joshua Dean elaborates: “Old methane release occurs much slower than the pace of modern climate change. This is because methane is a rich source of energy within ecosystem food webs, particularly for microorganisms that consume this methane and release carbon dioxide. Thus, old methane is often rapidly consumed by microorganisms living in sediments, soils, and water, which convert it to carbon dioxide before it can be released to the atmosphere. Some methane will escape this process and, along with the carbon dioxide produced from these microorganisms, will still have the potential to influence global climate.”
CH4 emissions have contributed almost 1/4th of the cumulative warming from greenhouse gases since 1750. Although methane is far less abundant in the atmosphere than CO2, it absorbs thermal infrared radiation much more efficiently.
Global average methane concentrations in the atmosphere reached ~1875 parts per billion (ppb) at the end of 2019, more than 2 1/2 times preindustrial levels. The largest methane sources are man-made emissions from agriculture, waste, and the extraction and use of fossil fuels. Fracking – the use of which exploded this century – has vastly accelerated recent release. CH4 is naturally emitted from wetlands, freshwater systems, and geological sources.
Man-made methane emissions rose 12% in the past decade. Africa, the Middle East, China, South Asia and Oceania had the greatest increase: each contributing ~13 teragrams (Tg) per year. The United States sent up 5 Tg. Europe was the only region where CH4 emissions went down.
95% of the heat from greenhouse gases is absorbed in the ocean, which is a gigantic heat sink. (Atmospheric heating during the day soaks into the ocean at night, where it diffuses.) The marine heat equilibrates with the atmosphere over decades.
The atmospheric warming experienced now is the result of greenhouse gas pollution decades ago. Because the oceans absorb so much thermal energy, a mid-point estimate of the lag between emissions and atmospheric warming at a planetary scale is 40 years.
What that means is that the heat felt now dates to emissions in the 1980s. A look at the chart below shows why global warming is going to accelerate in the next few decades, regardless of what is done to ameliorate further pollution.
R.B. Jackson et al, “Increasing anthropogenic methane emissions arise equally from agricultural and fossil fuel sources,” Environmental Research Letters (15 July 2020).
Hiroko Tabuchi, “Global methane emissions reach a record high,” The New York Times (14 July 2020).
“Global methane emissions soar to record high,” ScienceDaily (14 July 2020).
Joshua F. Dean, “Old methane and modern climate change,” Science (21 February 2020).
M.N. Dyonisius et al, “Old carbon reservoirs were not important in the deglacial methane budget,” Science (21 February 2020).
Benjamin Hmiel et al, “Preindustrial 14CH4 indicates greater anthropogenic fossil CH4 emissions,” Nature (19 February 2020).
Janet Lee, “NASA flights detect millions of Arctic methane hotspots,” Phys.org (14 February 2020).
“Methane emitted by humans vastly underestimated,” ScienceDaily (19 February 2020).
Alan Marshall, “Climate change: the 40 year delay between cause and effect,” Skeptical Science (22 September 2010).
Ishi Nobu, “Fracking,” (3 December 2019).